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<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>Stone's Blog</title>
<link href="/atom.xml" rel="self"/>
<link href="http://yoursite.com/"/>
<updated>2021-09-06T03:30:08.430Z</updated>
<id>http://yoursite.com/</id>
<author>
<name>Purple M. Stone</name>
</author>
<generator uri="http://hexo.io/">Hexo</generator>
<entry>
<title>安装配置Go</title>
<link href="http://yoursite.com/2021/09/06/20210906_setup_%E5%AE%89%E8%A3%85%E9%85%8D%E7%BD%AEgo/"/>
<id>http://yoursite.com/2021/09/06/20210906_setup_安装配置go/</id>
<published>2021-09-06T03:31:09.000Z</published>
<updated>2021-09-06T03:30:08.430Z</updated>
<content type="html"><![CDATA[<p>按照微软教程安装即可：</p><p><a href="https://docs.microsoft.com/zh-cn/learn/paths/go-first-steps/" target="_blank" rel="noopener">https://docs.microsoft.com/zh-cn/learn/paths/go-first-steps/</a></p><p><strong>踩坑</strong></p><ul><li>国内下载Go链接:</li></ul><p><a href="https://golang.google.cn/dl/" target="_blank" rel="noopener">https://golang.google.cn/dl/</a></p><ul><li>VS Code 安装 Go插件失败解决：</li></ul><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">$ go env -w GO111MODULE=on //开启mod</span><br><span class="line">$ go env -w GOPROXY=https://goproxy.cn,direct //设置七牛云goproxy代理</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p>按照微软教程安装即可：</p>
<p><a href="https://docs.microsoft.com/zh-cn/learn/paths/go-first-steps/" target="_blank" rel="noopener">https://docs.mic
</summary>
<category term="Setup" scheme="http://yoursite.com/categories/Setup/"/>
<category term="setup" scheme="http://yoursite.com/tags/setup/"/>
</entry>
<entry>
<title>MySQL笔记(8)--存储过程(Stored Procedures)</title>
<link href="http://yoursite.com/2021/05/19/20210519_SQL008/"/>
<id>http://yoursite.com/2021/05/19/20210519_SQL008/</id>
<published>2021-05-19T15:00:09.000Z</published>
<updated>2021-05-20T11:46:43.151Z</updated>
<content type="html"><![CDATA[<h1 id="1-优点"><a href="#1-优点" class="headerlink" title="1 优点"></a>1 优点</h1><blockquote><p>Store and organize SQL<br>Faster execution<br>Data security</p></blockquote><h1 id="2-创建一个存储过程"><a href="#2-创建一个存储过程" class="headerlink" title="2 创建一个存储过程"></a>2 创建一个存储过程</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_clients()</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">SELECT</span> * <span class="keyword">FROM</span> clients;<span class="comment">-- body of procedure</span></span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;<span class="comment">-- 将分隔符定义回分号</span></span><br></pre></td></tr></table></figure><p>DELIMITER 重定义分隔符, 也就是将新分隔符包含的内容视为一个unit, 而不是将”;”结束的一个个语句视为unit.</p><p>调用存储过程:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CALL</span> get_clients()</span><br></pre></td></tr></table></figure><h1 id="3-删除存储过程"><a href="#3-删除存储过程" class="headerlink" title="3 删除存储过程"></a>3 删除存储过程</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_clients;</span><br></pre></td></tr></table></figure><p><strong>创建存储过程的基本框架</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_clients;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_clients()</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">SELECT</span> * <span class="keyword">FROM</span> clients;</span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><h1 id="4-参数"><a href="#4-参数" class="headerlink" title="4 参数"></a>4 参数</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_clients_by_state;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_clients_by_state</span><br><span class="line">(</span><br><span class="line"> state <span class="built_in">CHAR</span>(<span class="number">2</span>)</span><br><span class="line">)</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">SELECT</span> * <span class="keyword">FROM</span> clients c</span><br><span class="line"> <span class="keyword">WHERE</span> c.state = state;</span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>调用:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CALL</span> get_clients_by_state(<span class="string">'CA'</span>)</span><br></pre></td></tr></table></figure><h2 id="4-1-带默认值的参数"><a href="#4-1-带默认值的参数" class="headerlink" title="4.1 带默认值的参数"></a>4.1 带默认值的参数</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_clients_by_state;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_clients_by_state</span><br><span class="line">(</span><br><span class="line"> state <span class="built_in">CHAR</span>(<span class="number">2</span>)</span><br><span class="line">)</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">IF</span> state <span class="keyword">IS</span> <span class="literal">NULL</span> <span class="keyword">THEN</span><span class="comment">----------</span></span><br><span class="line"> <span class="keyword">SET</span> state = <span class="string">'CA'</span>;<span class="comment">----------</span></span><br><span class="line"><span class="keyword">END</span> <span class="keyword">IF</span>;<span class="comment">----------</span></span><br><span class="line"> </span><br><span class="line"> <span class="keyword">SELECT</span> * <span class="keyword">FROM</span> clients c</span><br><span class="line"> <span class="keyword">WHERE</span> c.state = state;</span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>调用：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CALL</span> get_clients_by_state(<span class="literal">NULL</span>)</span><br></pre></td></tr></table></figure><p><strong>IF-ELSE</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_clients_by_state;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_clients_by_state</span><br><span class="line">(</span><br><span class="line"> state <span class="built_in">CHAR</span>(<span class="number">2</span>)</span><br><span class="line">)</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">IF</span> state <span class="keyword">IS</span> <span class="literal">NULL</span> <span class="keyword">THEN</span></span><br><span class="line"> <span class="keyword">SELECT</span> * <span class="keyword">FROM</span> clients;</span><br><span class="line">ELSE</span><br><span class="line"> <span class="keyword">SELECT</span> * <span class="keyword">FROM</span> clients c</span><br><span class="line"> <span class="keyword">WHERE</span> c.state = state;</span><br><span class="line"><span class="keyword">END</span> <span class="keyword">IF</span>;</span><br><span class="line"> </span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>更简洁的写法：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_clients_by_state;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_clients_by_state</span><br><span class="line">(</span><br><span class="line"> state <span class="built_in">CHAR</span>(<span class="number">2</span>)</span><br><span class="line">)</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">SELECT</span> * <span class="keyword">FROM</span> clients c</span><br><span class="line"> <span class="keyword">WHERE</span> c.state = <span class="keyword">IFNULL</span>(state, c.state); </span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>IFNULL(state, c.state) 如果 state = NULL，则返回 c.state，而 c.state = c.state 永远为 TRUE</p><h2 id="4-2-参数验证"><a href="#4-2-参数验证" class="headerlink" title="4.2 参数验证"></a>4.2 参数验证</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> make_payment;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> make_payment(</span><br><span class="line"> invoice_id <span class="built_in">INT</span>,</span><br><span class="line"> payment_amount <span class="built_in">DECIMAL</span>(<span class="number">9</span>, <span class="number">2</span>),</span><br><span class="line"> payment_date <span class="built_in">DATE</span></span><br><span class="line">)</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">IF</span> payment_amount <= <span class="number">0</span> <span class="keyword">THEN</span></span><br><span class="line"> SIGNAL <span class="keyword">SQLSTATE</span> <span class="string">'22003'</span> <span class="comment">-- 错误码</span></span><br><span class="line"> <span class="keyword">SET</span> MESSAGE_TEXT = <span class="string">'Invalid payment amount'</span>;<span class="comment">-- 输出错误信息</span></span><br><span class="line"><span class="keyword">END</span> <span class="keyword">IF</span>;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">UPDATE</span> invoices i</span><br><span class="line"> <span class="keyword">SET</span></span><br><span class="line"> i.payment_total = payment_amount,</span><br><span class="line"> i.payment_date = payment_date</span><br><span class="line"><span class="keyword">WHERE</span> i.invoice_id = invoice_id;</span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><h2 id="4-3-输出参数"><a href="#4-3-输出参数" class="headerlink" title="4.3 输出参数"></a>4.3 输出参数</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_unpaid_invoices_for_client;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_unpaid_invoices_for_client(</span><br><span class="line"> client_id <span class="built_in">INT</span>,</span><br><span class="line"> <span class="keyword">OUT</span> invoices_count <span class="built_in">INT</span>,<span class="comment">-- ----------------</span></span><br><span class="line"> <span class="keyword">OUT</span> invoices_total <span class="built_in">DECIMAL</span>(<span class="number">9</span>, <span class="number">2</span>)<span class="comment">-- ----------------</span></span><br><span class="line">)</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">COUNT</span>(*), <span class="keyword">SUM</span>(invoice_total)</span><br><span class="line"><span class="keyword">INTO</span> invoices_count, invoices_total<span class="comment">-- ----------------</span></span><br><span class="line"> <span class="keyword">FROM</span> invoices i</span><br><span class="line"> <span class="keyword">WHERE</span> i.client_id = client_id</span><br><span class="line"> <span class="keyword">AND</span> payment_total = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>调用：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">set</span> @invoices_count = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">set</span> @invoices_total = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">call</span> sql_invoicing.get_unpaid_invoices_for_client</span><br><span class="line"> (<span class="number">3</span>, @invoices_count, @invoices_total);</span><br><span class="line"><span class="keyword">select</span> @invoices_count, @invoices_total;</span><br></pre></td></tr></table></figure><h1 id="5-变量"><a href="#5-变量" class="headerlink" title="5 变量"></a>5 变量</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- User or session variables</span></span><br><span class="line"><span class="keyword">set</span> @invoices_count = <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">-- Local variable</span></span><br><span class="line"><span class="comment">-- 在存储过程或函数中定义</span></span><br></pre></td></tr></table></figure><p><strong>Demo</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">PROCEDURE</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_risk_factor;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">PROCEDURE</span> get_risk_factor()</span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="comment">-- Local variables</span></span><br><span class="line"> <span class="keyword">DECLARE</span> risk_factor <span class="built_in">DECIMAL</span>(<span class="number">9</span>, <span class="number">2</span>) <span class="keyword">DEFAULT</span> <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">DECLARE</span> invoices_total <span class="built_in">DECIMAL</span>(<span class="number">9</span>, <span class="number">2</span>);</span><br><span class="line"> <span class="keyword">DECLARE</span> invoices_count <span class="built_in">INT</span>;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">COUNT</span>(*), <span class="keyword">SUM</span>(invoice_total)</span><br><span class="line"> <span class="keyword">INTO</span> invoices_count, invoices_total</span><br><span class="line"> <span class="keyword">FROM</span> invoices;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">SET</span> risk_factor = invoices_total / invoices_count * <span class="number">5</span>;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">SELECT</span> risk_factor;</span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><h1 id="6-函数"><a href="#6-函数" class="headerlink" title="6 函数"></a>6 函数</h1><blockquote><p>函数：只能返回单一值<br>存储过程：可以返回拥有多行多列的结果集</p><p>👉 函数属性</p><ul><li>DETERMINISTIC: 一样的输入总是会返回一样的输出</li><li>READS SQL DATA: 函数中有读sql数据的内容</li><li>MODIFIES SQL DATA：函数中有修改sql数据的内容 </li></ul></blockquote><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">FUNCTION</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> get_risk_factor_for_client;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">FUNCTION</span> get_risk_factor_for_client</span><br><span class="line">(</span><br><span class="line"> client_id <span class="built_in">INT</span></span><br><span class="line">) </span><br><span class="line"><span class="keyword">RETURNS</span> <span class="built_in">int</span></span><br><span class="line"><span class="keyword">READS</span> <span class="keyword">SQL</span> <span class="keyword">DATA</span></span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">DECLARE</span> risk_factor <span class="built_in">DECIMAL</span>(<span class="number">9</span>, <span class="number">2</span>) <span class="keyword">DEFAULT</span> <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">DECLARE</span> invoices_total <span class="built_in">DECIMAL</span>(<span class="number">9</span>, <span class="number">2</span>);</span><br><span class="line"> <span class="keyword">DECLARE</span> invoices_count <span class="built_in">INT</span>;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">COUNT</span>(*), <span class="keyword">SUM</span>(invoice_total)</span><br><span class="line"> <span class="keyword">INTO</span> invoices_count, invoices_total</span><br><span class="line"> <span class="keyword">FROM</span> invoices i</span><br><span class="line"> <span class="keyword">WHERE</span> i.client_id = client_id;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">SET</span> risk_factor = invoices_total / invoices_count * <span class="number">5</span>;</span><br><span class="line"> RETURN IFNULL(risk_factor, 0);</span><br><span class="line"><span class="keyword">END</span>$$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>使用函数：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> client_id,</span><br><span class="line"> <span class="keyword">name</span>,</span><br><span class="line"> get_risk_factor_for_client(client_id) <span class="keyword">AS</span> risk_factor</span><br><span class="line"><span class="keyword">FROM</span> clients</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<h1 id="1-优点"><a href="#1-优点" class="headerlink" title="1 优点"></a>1 优点</h1><blockquote>
<p>Store and organize SQL<br>Faster execution<br>Dat
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(9)--触发器(Triggers)</title>
<link href="http://yoursite.com/2021/05/19/20210520_SQL009/"/>
<id>http://yoursite.com/2021/05/19/20210520_SQL009/</id>
<published>2021-05-19T15:00:09.000Z</published>
<updated>2021-05-20T11:39:16.033Z</updated>
<content type="html"><![CDATA[<blockquote><p><strong>Trigger</strong>: A block of SQL code that automatically gets executed before or after an insert, update or delete statement.</p><p>To enhance data consistency</p></blockquote><h1 id="1-设置触发器"><a href="#1-设置触发器" class="headerlink" title="1 设置触发器"></a>1 设置触发器</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">DELIMITER $$</span><br><span class="line"></span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TRIGGER</span> payments_after_insert</span><br><span class="line"> <span class="keyword">AFTER</span> <span class="keyword">INSERT</span> <span class="keyword">ON</span> payments</span><br><span class="line"> <span class="keyword">FOR</span> <span class="keyword">EACH</span> <span class="keyword">ROW</span><span class="comment">-- 作用于受影响的每一行</span></span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">UPDATE</span> invoices</span><br><span class="line"> <span class="keyword">SET</span> payment_total = payment_total + NEW.amount</span><br><span class="line"> <span class="keyword">WHERE</span> invoice_id = NEW.invoice_id;</span><br><span class="line"><span class="keyword">END</span> $$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>可以触发除了 payments 之外的任何表的更新。NEW表示新 insert 的 payment (amount是payments 表的 column)。如果是DELETE，就用 OLD 表示删除的行。</p><h1 id="2-查看触发器"><a href="#2-查看触发器" class="headerlink" title="2 查看触发器"></a>2 查看触发器</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SHOW</span> <span class="keyword">TRIGGERS</span> <span class="keyword">LIKE</span> <span class="string">'payments%'</span><span class="comment">-- show filtered triggers</span></span><br></pre></td></tr></table></figure><h1 id="3-删除触发器"><a href="#3-删除触发器" class="headerlink" title="3 删除触发器"></a>3 删除触发器</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">TRIGGER</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> payments_after_insert</span><br></pre></td></tr></table></figure><h1 id="4-使用触发器进行审计"><a href="#4-使用触发器进行审计" class="headerlink" title="4 使用触发器进行审计"></a>4 使用触发器进行审计</h1><p><strong>使用触发器进行记录</strong></p><p>首先建立审计表：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_invoicing;</span><br><span class="line"></span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TABLE</span> payments_audit</span><br><span class="line">(</span><br><span class="line"> client_id <span class="built_in">INT</span> <span class="keyword">NOT</span> <span class="literal">NULL</span>,</span><br><span class="line"> <span class="built_in">date</span> <span class="built_in">DATE</span> <span class="keyword">NOT</span> <span class="literal">NULL</span>,</span><br><span class="line"> amount <span class="built_in">DECIMAL</span>(<span class="number">9</span>, <span class="number">2</span>) <span class="keyword">NOT</span> <span class="literal">NULL</span>,</span><br><span class="line"> action_type <span class="built_in">VARCHAR</span>(<span class="number">50</span>) <span class="keyword">NOT</span> <span class="literal">NULL</span>,</span><br><span class="line"> action_date DATETIME <span class="keyword">NOT</span> <span class="literal">NULL</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>假如有这两个触发器：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">TRIGGER</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> payments_after_insert;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"></span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TRIGGER</span> payments_after_insert</span><br><span class="line"> <span class="keyword">AFTER</span> <span class="keyword">INSERT</span> <span class="keyword">ON</span> payments</span><br><span class="line"> <span class="keyword">FOR</span> <span class="keyword">EACH</span> <span class="keyword">ROW</span></span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">UPDATE</span> invoices</span><br><span class="line"> <span class="keyword">SET</span> payment_total = payment_total + NEW.amount</span><br><span class="line"> <span class="keyword">WHERE</span> invoice_id = NEW.invoice_id;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">INSERT</span> <span class="keyword">INTO</span> payments_audit</span><br><span class="line"> <span class="keyword">VALUES</span> (NEW.client_id, NEW.date, NEW.amount, <span class="string">'Insert'</span>, <span class="keyword">NOW</span>());</span><br><span class="line"><span class="keyword">END</span> $$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">TRIGGER</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> payments_after_delete;</span><br><span class="line"></span><br><span class="line">DELIMITER $$</span><br><span class="line"></span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TRIGGER</span> payments_after_delete</span><br><span class="line"> <span class="keyword">AFTER</span> <span class="keyword">DELETE</span> <span class="keyword">ON</span> payments</span><br><span class="line"> <span class="keyword">FOR</span> <span class="keyword">EACH</span> <span class="keyword">ROW</span></span><br><span class="line"><span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">UPDATE</span> invoices</span><br><span class="line"> <span class="keyword">SET</span> payment_total = payment_total - OLD.amount</span><br><span class="line"> <span class="keyword">WHERE</span> invoice_id = OLD.invoice_id;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">INSERT</span> <span class="keyword">INTO</span> payments_audit</span><br><span class="line"> <span class="keyword">VALUES</span> (OLD.client_id, OLD.date, OLD.amount, <span class="string">'Delete'</span>, <span class="keyword">NOW</span>());</span><br><span class="line"><span class="keyword">END</span> $$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><p>然后触发事件发生：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> payments</span><br><span class="line"><span class="keyword">VALUES</span> (<span class="keyword">DEFAULT</span>, <span class="number">5</span>, <span class="number">3</span>, <span class="string">'2019-01-01'</span>, <span class="number">10</span>, <span class="number">1</span>)</span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DELETE</span> <span class="keyword">FROM</span> payments</span><br><span class="line"><span class="keyword">WHERE</span> payment_id = <span class="number">11</span></span><br></pre></td></tr></table></figure><p>刷新审计表可以看到：</p><div class="table-container"><table><thead><tr><th>client_id</th><th>date</th><th>amount</th><th>action_type</th><th>action_date</th></tr></thead><tbody><tr><td>5</td><td>2019-01-01</td><td>10.00</td><td>Insert</td><td>2021-05-20 19:12:31</td></tr><tr><td>5</td><td>2019-01-01</td><td>10.00</td><td>Delete</td><td>2021-05-20 19:13:31</td></tr></tbody></table></div><h1 id="5-事件"><a href="#5-事件" class="headerlink" title="5 事件"></a>5 事件</h1><blockquote><p><strong>Event</strong>: A task (or block of SQL code) that gets executed according to a <strong>schedule</strong>.</p></blockquote><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SHOW</span> <span class="keyword">VARIABLES</span> <span class="keyword">LIKE</span> <span class="string">'event%'</span><span class="comment">-- 查看命名中有event的变量</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SET</span> <span class="keyword">GLOBAL</span> event_scheduler = <span class="keyword">OFF</span><span class="comment">-- 关掉它</span></span><br></pre></td></tr></table></figure><p><strong>Demo</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">DELIMITER $$</span><br><span class="line"></span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">EVENT</span> yearly_delete_stale_audit_rows <span class="comment">-- 每年触发一次：yearly开头</span></span><br><span class="line"><span class="keyword">ON</span> SCHEDULE</span><br><span class="line"> EVERY <span class="number">1</span> <span class="keyword">YEAR</span> STARTS <span class="string">'2019-01-01'</span> ENDS <span class="string">'2029-01-01'</span></span><br><span class="line"><span class="keyword">DO</span> <span class="keyword">BEGIN</span></span><br><span class="line"> <span class="keyword">DELETE</span> <span class="keyword">FROM</span> payments_audit</span><br><span class="line"> <span class="keyword">WHERE</span> action_date < <span class="keyword">NOW</span>() - <span class="built_in">INTERVAL</span> <span class="number">1</span> <span class="keyword">YEAR</span>; <span class="comment">-- 删除所有超过一年的审计记录</span></span><br><span class="line"><span class="keyword">END</span> $$</span><br><span class="line"></span><br><span class="line">DELIMITER ;</span><br></pre></td></tr></table></figure><h1 id="6-查看，删除和更改事件"><a href="#6-查看，删除和更改事件" class="headerlink" title="6 查看，删除和更改事件"></a>6 查看，删除和更改事件</h1><p><strong>查看事件</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SHOW</span> <span class="keyword">EVENTS</span> <span class="keyword">LIKE</span> <span class="string">'yearly%'</span>;</span><br></pre></td></tr></table></figure><p><strong>删除事件</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">EVENT</span> <span class="keyword">IF</span> <span class="keyword">EXISTS</span> yearly_delete_stale_audit_rows;</span><br></pre></td></tr></table></figure><p><strong>更改事件</strong></p><p>ALTER EVENT：跟 CREATE EVENT 语法一样。</p><p>可以用来展示启用/禁用事件：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">ALTER</span> <span class="keyword">EVENT</span> yearly_delete_stale_audit_rows <span class="keyword">ENABLE</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">ALTER</span> <span class="keyword">EVENT</span> yearly_delete_stale_audit_rows <span class="keyword">DISABLE</span></span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<blockquote>
<p><strong>Trigger</strong>: A block of SQL code that automatically gets executed before or after an insert, update or delete s
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(7)--视图(Views)</title>
<link href="http://yoursite.com/2021/05/19/20210519_SQL007/"/>
<id>http://yoursite.com/2021/05/19/20210519_SQL007/</id>
<published>2021-05-19T13:00:09.000Z</published>
<updated>2021-05-19T14:27:44.886Z</updated>
<content type="html"><![CDATA[<h1 id="1-创建视图"><a href="#1-创建视图" class="headerlink" title="1 创建视图"></a>1 创建视图</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_invoicing;</span><br><span class="line"></span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">VIEW</span> sales_by_client <span class="keyword">AS</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> c.client_id,</span><br><span class="line"> c.name,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales</span><br><span class="line"><span class="keyword">FROM</span> clients c</span><br><span class="line"><span class="keyword">JOIN</span> invoices i <span class="keyword">USING</span> (client_id)</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id, <span class="keyword">name</span></span><br></pre></td></tr></table></figure><p>可以在”Views”看到这个视图.可以像使用table一样用视图.</p><h1 id="2-更改或删除视图"><a href="#2-更改或删除视图" class="headerlink" title="2 更改或删除视图"></a>2 更改或删除视图</h1><p><strong>删除视图</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DROP</span> <span class="keyword">VIEW</span> sales_by_client</span><br></pre></td></tr></table></figure><p><strong>更改视图</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_invoicing;</span><br><span class="line"></span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">OR</span> <span class="keyword">REPLACE</span> <span class="keyword">VIEW</span> sales_by_client <span class="keyword">AS</span> <span class="comment">-- create or replace</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> c.client_id,</span><br><span class="line"> c.name,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales</span><br><span class="line"><span class="keyword">FROM</span> clients c</span><br><span class="line"><span class="keyword">JOIN</span> invoices i <span class="keyword">USING</span> (client_id)</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id, <span class="keyword">name</span></span><br></pre></td></tr></table></figure><h1 id="3-可更新视图"><a href="#3-可更新视图" class="headerlink" title="3 可更新视图"></a>3 可更新视图</h1><p>如果视图中没有:</p><blockquote><p>DISTINCT<br>Aggregate Functions (MIN, MAX, SUM)<br>GROUP BY / HAVING<br>UNION</p></blockquote><p>就是可更新视图,就可以增删改:</p><p>比如:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">OR</span> <span class="keyword">REPLACE</span> <span class="keyword">VIEW</span> invoices_with_balance <span class="keyword">AS</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> invoice_id,</span><br><span class="line"> <span class="built_in">number</span>,</span><br><span class="line"> client_id,</span><br><span class="line"> invoice_total,</span><br><span class="line"> payment_total,</span><br><span class="line"> invoice_total - payment_total <span class="keyword">AS</span> balance,</span><br><span class="line"> due_date,</span><br><span class="line"> payment_date</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> (invoice_total - payment_total) > <span class="number">0</span></span><br></pre></td></tr></table></figure><p>invoices_with_balance 就是可更新视图,可以增删改:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DELETE</span> <span class="keyword">FROM</span> invoices_with_balance</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">1</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices_with_balance</span><br><span class="line"><span class="keyword">SET</span> due_date = <span class="keyword">DATE_ADD</span>(due_date, <span class="built_in">INTERVAL</span> <span class="number">5</span> <span class="keyword">DAY</span>)</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">2</span></span><br></pre></td></tr></table></figure><h1 id="4-WITH-OPTION-CHECK-子句"><a href="#4-WITH-OPTION-CHECK-子句" class="headerlink" title="4 WITH OPTION CHECK 子句"></a>4 WITH OPTION CHECK 子句</h1><p>上例中,进行如下更改:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices_with_balance</span><br><span class="line"><span class="keyword">SET</span> payment_total = invoice_total</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">2</span></span><br></pre></td></tr></table></figure><p>该视图中 invoice_id = 2 的行消失了(因为不再满足(invoice_total - payment_total) > 0的条件)</p><p>如果不想 UPDATE 或 DELETE 语句使得某些行从视图中删除, 就这样:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">OR</span> <span class="keyword">REPLACE</span> <span class="keyword">VIEW</span> invoices_with_balance <span class="keyword">AS</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> invoice_id,</span><br><span class="line"> <span class="built_in">number</span>,</span><br><span class="line"> client_id,</span><br><span class="line"> invoice_total,</span><br><span class="line"> payment_total,</span><br><span class="line"> invoice_total - payment_total <span class="keyword">AS</span> balance,</span><br><span class="line"> due_date,</span><br><span class="line"> payment_date</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> (invoice_total - payment_total) > <span class="number">0</span></span><br><span class="line"><span class="keyword">WITH</span> <span class="keyword">CHECK</span> <span class="keyword">OPTION</span><span class="comment">-- 增加这个子句,防止删除行</span></span><br></pre></td></tr></table></figure><p>后面再执行这样的:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices_with_balance</span><br><span class="line"><span class="keyword">SET</span> payment_total = invoice_total</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">3</span></span><br></pre></td></tr></table></figure><p>就会报错: </p><blockquote><p>Error Code: 1369. CHECK OPTION failed</p></blockquote><h1 id="5-视图的优点"><a href="#5-视图的优点" class="headerlink" title="5 视图的优点"></a>5 视图的优点</h1><blockquote><p>Simplify queries<br>Reduce the impact of changes<br>Restrict access to the data</p></blockquote>]]></content>
<summary type="html">
<h1 id="1-创建视图"><a href="#1-创建视图" class="headerlink" title="1 创建视图"></a>1 创建视图</h1><figure class="highlight sql"><table><tr><td class="gutte
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(6)--MySQL的基本函数</title>
<link href="http://yoursite.com/2021/05/19/20210519_SQL006/"/>
<id>http://yoursite.com/2021/05/19/20210519_SQL006/</id>
<published>2021-05-19T11:50:09.000Z</published>
<updated>2021-05-19T12:14:20.513Z</updated>
<content type="html"><![CDATA[<h1 id="1-数值函数"><a href="#1-数值函数" class="headerlink" title="1 数值函数"></a>1 数值函数</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">ROUND</span>(<span class="number">5.7355</span>, <span class="number">2</span>) <span class="comment">-- 保留小数点后2位, 四舍五入: 5.74</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">TRUNCATE</span>(<span class="number">5.7355</span>, <span class="number">2</span>) <span class="comment">-- 保留小数点后2位: 5.73</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">CEILING</span>(<span class="number">5.2</span>)<span class="comment">-- 6 (上取整)</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">FLOOR</span>(<span class="number">5.7</span>)<span class="comment">-- 5 (下取整)</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">ABS</span>(<span class="number">-5.2</span>)<span class="comment">-- 求绝对值</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">RAND</span>()<span class="comment">-- 得到0~1的随机值</span></span><br></pre></td></tr></table></figure><h1 id="2-字符串函数"><a href="#2-字符串函数" class="headerlink" title="2 字符串函数"></a>2 字符串函数</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">LENGTH</span>(<span class="string">'sky'</span>) <span class="comment">-- 3</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">UPPER</span>(<span class="string">'sky'</span>) <span class="comment">-- SKY</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">LOWER</span>(<span class="string">'Sky'</span>)<span class="comment">-- sky</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">LTRIM</span>(<span class="string">' Sky'</span>)<span class="comment">-- 去除左边的空格：Sky</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">RTRIM</span>(<span class="string">'Sky '</span>)<span class="comment">-- 去除右边的空格：Sky</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">TRIM</span>(<span class="string">' Sky '</span>)<span class="comment">-- 去除两边的空格：Sky</span></span><br></pre></td></tr></table></figure><p><strong>子串</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">LEFT</span>(<span class="string">'Kindergarten'</span>, <span class="number">4</span>)<span class="comment">-- Kind</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">RIGHT</span>(<span class="string">'Kindergarten'</span>, <span class="number">6</span>)<span class="comment">-- garten</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">SUBSTRING</span>(<span class="string">'Kindergarten'</span>, <span class="number">3</span>, <span class="number">5</span>)<span class="comment">-- nderg</span></span><br></pre></td></tr></table></figure><p><strong>查找位置</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">LOCATE</span>(<span class="string">'N'</span>, <span class="string">'Kindergarten'</span>) <span class="comment">-- 3 不区分大小写，若不在字符串中则返回0</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">LOCATE</span>(<span class="string">'garten'</span>, <span class="string">'Kindergarten'</span>)<span class="comment">-- 7</span></span><br></pre></td></tr></table></figure><p><strong>替换</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">REPLACE</span>(<span class="string">'Kindergarten'</span>, <span class="string">'garten'</span>, <span class="string">'garden'</span>)<span class="comment">-- Kindergarden</span></span><br></pre></td></tr></table></figure><p><strong>拼接</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">CONCAT</span>(<span class="string">'first'</span>, <span class="string">'last'</span>)<span class="comment">-- firstlast</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_store;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">CONCAT</span>(first_name, <span class="string">' '</span>, last_name) <span class="keyword">AS</span> full_name</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br></pre></td></tr></table></figure><h1 id="3-日期"><a href="#3-日期" class="headerlink" title="3 日期"></a>3 日期</h1><h2 id="3-1-日期函数"><a href="#3-1-日期函数" class="headerlink" title="3.1 日期函数"></a>3.1 日期函数</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">NOW</span>(), <span class="keyword">CURDATE</span>(), <span class="keyword">CURTIME</span>()</span><br></pre></td></tr></table></figure><div class="table-container"><table><thead><tr><th>NOW()</th><th>CURDATE()</th><th>CURTIME()</th></tr></thead><tbody><tr><td>2021-05-19 19:08:56</td><td>2021-05-19</td><td>19:08:56</td></tr></tbody></table></div><p><strong>获取指定的时间构成元素</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">YEAR</span>(<span class="keyword">NOW</span>())<span class="comment">-- 2021</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">MONTH</span>(<span class="keyword">NOW</span>())<span class="comment">-- 5</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DAY</span>(<span class="keyword">NOW</span>())<span class="comment">-- 19</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">HOUR</span>(<span class="keyword">NOW</span>())<span class="comment">-- 19</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">MINUTE</span>(<span class="keyword">NOW</span>())<span class="comment">-- 08</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">SECOND</span>(<span class="keyword">NOW</span>())<span class="comment">-- 56</span></span><br></pre></td></tr></table></figure><p><strong>获取时间的字符串</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DAYNAME</span>(<span class="keyword">NOW</span>()), MONTHNAME(<span class="keyword">NOW</span>())</span><br></pre></td></tr></table></figure><div class="table-container"><table><thead><tr><th>DAYNAME(NOW())</th><th>MONTHNAME(NOW())</th></tr></thead><tbody><tr><td>Wednesday</td><td>May</td></tr></tbody></table></div><p><strong>EXTRACT</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">EXTRACT</span>(<span class="keyword">DAY</span> <span class="keyword">FROM</span> <span class="keyword">NOW</span>()) <span class="comment">-- 19</span></span><br></pre></td></tr></table></figure><h2 id="3-2-格式化日期和时间"><a href="#3-2-格式化日期和时间" class="headerlink" title="3.2 格式化日期和时间"></a>3.2 格式化日期和时间</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DATE_FORMAT</span>(<span class="keyword">NOW</span>(), <span class="string">'%M %d %Y'</span>)</span><br></pre></td></tr></table></figure><blockquote><p>May 19 2021</p></blockquote><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DATE_FORMAT</span>(<span class="keyword">NOW</span>(), <span class="string">'%H:%i %p'</span>)</span><br></pre></td></tr></table></figure><blockquote><p>19:17 PM</p></blockquote><h2 id="3-3-计算日期和时间"><a href="#3-3-计算日期和时间" class="headerlink" title="3.3 计算日期和时间"></a>3.3 计算日期和时间</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DATE_ADD</span>(<span class="keyword">NOW</span>(), <span class="built_in">INTERVAL</span> <span class="number">1</span> <span class="keyword">DAY</span>)<span class="comment">-- 2021-05-20 19:21:32</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DATE_ADD</span>(<span class="keyword">NOW</span>(), <span class="built_in">INTERVAL</span> <span class="number">1</span> <span class="keyword">YEAR</span>)<span class="comment">-- 2022-05-19 19:21:32</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DATE_SUB</span>(<span class="keyword">NOW</span>(), <span class="built_in">INTERVAL</span> <span class="number">1</span> <span class="keyword">YEAR</span>)<span class="comment">-- 2020-05-19 19:21:32</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DATEDIFF</span>(<span class="string">'2021-01-05'</span>, <span class="string">'2021-01-01'</span>)<span class="comment">-- 4</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> TIME_TO_SEC(<span class="string">'09:00'</span>) - TIME_TO_SEC(<span class="string">'09:02'</span>) <span class="comment">-- -120 (单位:秒)</span></span><br></pre></td></tr></table></figure><h1 id="4-IFNULL-和-COALESCE"><a href="#4-IFNULL-和-COALESCE" class="headerlink" title="4 IFNULL 和 COALESCE"></a>4 IFNULL 和 COALESCE</h1><p><strong>IFNULL</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_store;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> order_id,</span><br><span class="line"> <span class="keyword">IFNULL</span>(shipper_id, <span class="string">'Not assigned'</span>) <span class="keyword">AS</span> shipper</span><br><span class="line"><span class="keyword">FROM</span> orders</span><br></pre></td></tr></table></figure><p>IFNULL(shipper_id, ‘Not assigned’) 即如果shipper_id为NULL, 就改为’Not assigned’</p><p>输出:</p><div class="table-container"><table><thead><tr><th>order_id</th><th>shipper</th></tr></thead><tbody><tr><td>1</td><td>Not assigned</td></tr><tr><td>3</td><td>Not assigned</td></tr><tr><td>8</td><td>Not assigned</td></tr><tr><td>9</td><td>1</td></tr><tr><td>2</td><td>4</td></tr></tbody></table></div><p><strong>COALESCE</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_store;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> order_id,</span><br><span class="line"> <span class="keyword">COALESCE</span>(shipper_id, comments, <span class="string">'Not assigned'</span>) <span class="keyword">AS</span> shipper</span><br><span class="line"><span class="keyword">FROM</span> orders</span><br></pre></td></tr></table></figure><p>如果shipper_id为空,就返回comments中的值,如果comments也为空,则返回’Not assigned’.</p><h1 id="5-IF-函数"><a href="#5-IF-函数" class="headerlink" title="5 IF 函数"></a>5 IF 函数</h1><p>格式:</p><blockquote><p>IF(expression, first, second)<br>如果expression为真,返回first, 为假则返回second</p></blockquote><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> order_id,</span><br><span class="line"> order_date,</span><br><span class="line"> <span class="keyword">IF</span>(</span><br><span class="line"> <span class="keyword">YEAR</span>(order_date) = <span class="keyword">YEAR</span>(<span class="keyword">DATE_SUB</span>(<span class="keyword">NOW</span>(), <span class="built_in">INTERVAL</span> <span class="number">2</span> <span class="keyword">YEAR</span>)),</span><br><span class="line"> <span class="string">'Active'</span>,</span><br><span class="line"> <span class="string">'Archived'</span>) <span class="keyword">AS</span> <span class="keyword">category</span></span><br><span class="line"><span class="keyword">FROM</span> orders</span><br></pre></td></tr></table></figure><p>当前年份为2021, 所以2019的为’Active’ (Mosh制作视频时为2019), 其他都是’Archived’:</p><div class="table-container"><table><thead><tr><th>order_id</th><th>order_date</th><th>category</th></tr></thead><tbody><tr><td>1</td><td>2019-01-30</td><td>Active</td></tr><tr><td>2</td><td>2018-08-02</td><td>Archived</td></tr><tr><td>3</td><td>2017-12-01</td><td>Archived</td></tr><tr><td>4</td><td>2017-01-22</td><td>Archived</td></tr><tr><td>5</td><td>2017-08-25</td><td>Archived</td></tr></tbody></table></div><h1 id="6-CASE-运算符"><a href="#6-CASE-运算符" class="headerlink" title="6 CASE 运算符"></a>6 CASE 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> order_id,</span><br><span class="line"> <span class="keyword">CASE</span></span><br><span class="line"> <span class="keyword">WHEN</span> <span class="keyword">YEAR</span>(order_date) = <span class="keyword">YEAR</span>(<span class="keyword">NOW</span>()) - <span class="number">2</span> <span class="keyword">THEN</span> <span class="string">'Active'</span></span><br><span class="line"> <span class="keyword">WHEN</span> <span class="keyword">YEAR</span>(order_date) = <span class="keyword">YEAR</span>(<span class="keyword">NOW</span>()) - <span class="number">3</span> <span class="keyword">THEN</span> <span class="string">'Last Year'</span></span><br><span class="line"> <span class="keyword">WHEN</span> <span class="keyword">YEAR</span>(order_date) < <span class="keyword">YEAR</span>(<span class="keyword">NOW</span>()) - <span class="number">3</span> <span class="keyword">THEN</span> <span class="string">'Archived'</span></span><br><span class="line"> <span class="keyword">ELSE</span> <span class="string">'Future'</span></span><br><span class="line"><span class="keyword">END</span> <span class="keyword">AS</span> <span class="keyword">category</span></span><br><span class="line"><span class="keyword">FROM</span> orders</span><br></pre></td></tr></table></figure><p>输出:</p><div class="table-container"><table><thead><tr><th>order_id</th><th>category</th></tr></thead><tbody><tr><td>1</td><td>Active</td></tr><tr><td>2</td><td>Last Year</td></tr><tr><td>3</td><td>Archived</td></tr><tr><td>4</td><td>Archived</td></tr><tr><td>5</td><td>Archived</td></tr><tr><td>6</td><td>Last Year</td></tr><tr><td>7</td><td>Last Year</td></tr><tr><td>8</td><td>Last Year</td></tr><tr><td>9</td><td>Archived</td></tr><tr><td>10</td><td>Last Year</td></tr></tbody></table></div>]]></content>
<summary type="html">
<h1 id="1-数值函数"><a href="#1-数值函数" class="headerlink" title="1 数值函数"></a>1 数值函数</h1><figure class="highlight sql"><table><tr><td class="gutte
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(5)--编写复杂查询</title>
<link href="http://yoursite.com/2021/05/18/20210518_SQL005/"/>
<id>http://yoursite.com/2021/05/18/20210518_SQL005/</id>
<published>2021-05-18T13:50:09.000Z</published>
<updated>2021-05-19T09:29:03.755Z</updated>
<content type="html"><![CDATA[<h1 id="1-子查询"><a href="#1-子查询" class="headerlink" title="1 子查询"></a>1 子查询</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Find products that are more </span></span><br><span class="line"><span class="comment">-- expensive than Lettuce (id = 3)</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> products</span><br><span class="line"><span class="keyword">WHERE</span> unit_price > (</span><br><span class="line"> <span class="keyword">SELECT</span> unit_price</span><br><span class="line"> <span class="keyword">FROM</span> products</span><br><span class="line"> <span class="keyword">WHERE</span> product_id = <span class="number">3</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure><h1 id="2-IN-运算符"><a href="#2-IN-运算符" class="headerlink" title="2 IN 运算符"></a>2 IN 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Find the products that have never been ordered</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">USE</span> sql_store;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> products</span><br><span class="line"><span class="keyword">WHERE</span> product_id <span class="keyword">NOT</span> <span class="keyword">IN</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">DISTINCT</span> product_id</span><br><span class="line"> <span class="keyword">FROM</span> order_items</span><br><span class="line">)</span><br></pre></td></tr></table></figure><h1 id="3-子查询-VS-连接"><a href="#3-子查询-VS-连接" class="headerlink" title="3 子查询 VS 连接"></a>3 子查询 VS 连接</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Find clients without invoices</span></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> clients</span><br><span class="line"><span class="keyword">WHERE</span> client_id <span class="keyword">NOT</span> <span class="keyword">IN</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">DISTINCT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>用连接的方法:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> clients</span><br><span class="line"><span class="keyword">LEFT</span> <span class="keyword">JOIN</span> invoices <span class="keyword">USING</span> (client_id)</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id <span class="keyword">IS</span> <span class="literal">NULL</span></span><br></pre></td></tr></table></figure><p>在时间一样的情况下, 选择更易读的那种方法</p><h1 id="4-ALL-关键字"><a href="#4-ALL-关键字" class="headerlink" title="4 ALL 关键字"></a>4 ALL 关键字</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Select invoices larger than all invoices of client 3</span></span><br><span class="line"><span class="keyword">USE</span> sql_invoicing;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_total > (</span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">MAX</span>(invoice_total)</span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line"> <span class="keyword">WHERE</span> client_id = <span class="number">3</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>改用all 关键字:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_total > <span class="keyword">ALL</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> invoice_total</span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line"> <span class="keyword">WHERE</span> client_id = <span class="number">3</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure><h1 id="5-ANY-关键字"><a href="#5-ANY-关键字" class="headerlink" title="5 ANY 关键字"></a>5 ANY 关键字</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Select clients with at least two invoices</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> clients</span><br><span class="line"><span class="keyword">WHERE</span> client_id <span class="keyword">IN</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line"> <span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id</span><br><span class="line"> <span class="keyword">HAVING</span> <span class="keyword">COUNT</span>(*) >= <span class="number">2</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>改用any 关键字</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Select clients with at least two invoices</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> clients</span><br><span class="line"><span class="keyword">WHERE</span> client_id = <span class="keyword">ANY</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line"> <span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id</span><br><span class="line"> <span class="keyword">HAVING</span> <span class="keyword">COUNT</span>(*) >= <span class="number">2</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>即 “IN” 相当于 “=ANY”</p><h1 id="6-相关子查询"><a href="#6-相关子查询" class="headerlink" title="6 相关子查询"></a>6 相关子查询</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Select employees whose salary is </span></span><br><span class="line"><span class="comment">-- above the average in their office</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">USE</span> sql_hr;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> employees e</span><br><span class="line"><span class="keyword">WHERE</span> salary > (</span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">AVG</span>(salary)</span><br><span class="line"> <span class="keyword">FROM</span> employees</span><br><span class="line"> <span class="keyword">WHERE</span> office_id = e.office_id</span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>注意到子查询用到了外查询的 e.office_id, 也就是说子查询和外查询是相关子查询.</p><p>对于非相关子查询, 执行一次就可以了.</p><p>但是对于相关子查询, 外查询每一条record都要执行一次子查询,所以执行时间相对较长.</p><h1 id="7-EXISTS-运算符"><a href="#7-EXISTS-运算符" class="headerlink" title="7 EXISTS 运算符"></a>7 EXISTS 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Select clients that have an invoice</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> clients</span><br><span class="line"><span class="keyword">WHERE</span> client_id <span class="keyword">IN</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> <span class="keyword">DISTINCT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>子查询返回一个列表, 然后外查询看在不在此列表中.如果子查询返回的列表非常大,就会很影响性能.</p><p><strong>使用EXISTS运算符</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Select clients that have an invoice</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> clients c</span><br><span class="line"><span class="keyword">WHERE</span> <span class="keyword">EXISTS</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line"> <span class="keyword">WHERE</span> client_id = c.client_id</span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>对于外查询的每一条record, EXISTS 子查询返回TRUE / FALSE, 若返回TRUE,就添加到结果集中.</p><p><strong>Exercise</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Find the products that have never been ordered</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> products p</span><br><span class="line"><span class="keyword">WHERE</span> <span class="keyword">NOT</span> <span class="keyword">EXISTS</span> (</span><br><span class="line"> <span class="keyword">SELECT</span> product_id</span><br><span class="line"> <span class="keyword">FROM</span> order_items</span><br><span class="line"> <span class="keyword">WHERE</span> product_id = p.product_id</span><br><span class="line">)</span><br></pre></td></tr></table></figure><h1 id="8-SELECT-子句中的子查询"><a href="#8-SELECT-子句中的子查询" class="headerlink" title="8 SELECT 子句中的子查询"></a>8 SELECT 子句中的子查询</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> invoice_id,</span><br><span class="line"> invoice_total,</span><br><span class="line"> (<span class="keyword">SELECT</span> <span class="keyword">AVG</span>(invoice_total)</span><br><span class="line"> <span class="keyword">FROM</span> invoices) <span class="keyword">AS</span> invoice_average,</span><br><span class="line">invoice_total - (<span class="keyword">SELECT</span> invoice_average) <span class="keyword">AS</span> <span class="keyword">difference</span> <span class="comment">-- 不能直接用别名</span></span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br></pre></td></tr></table></figure><p>得到这样一张表:</p><div class="table-container"><table><thead><tr><th>invoice_id</th><th>invoice_total</th><th>invoice_average</th><th>difference</th></tr></thead><tbody><tr><td>1</td><td>101.79</td><td>152.388235</td><td>-50.598235</td></tr><tr><td>2</td><td>175.32</td><td>152.388235</td><td>22.931765</td></tr><tr><td>3</td><td>147.99</td><td>152.388235</td><td>-4.398235</td></tr><tr><td>4</td><td>152.21</td><td>152.388235</td><td>-0.178235</td></tr><tr><td>5</td><td>169.36</td><td>152.388235</td><td>16.971765</td></tr><tr><td>6</td><td>157.78</td><td>152.388235</td><td>5.391765</td></tr><tr><td>7</td><td>133.87</td><td>152.388235</td><td>-18.518235</td></tr></tbody></table></div><p><strong>Exercise</strong></p><p>得到这样一张表:</p><div class="table-container"><table><thead><tr><th>client_id</th><th>name</th><th>total_sales</th><th>average</th><th>difference</th></tr></thead><tbody><tr><td>1</td><td>Vinte</td><td>802.89</td><td>152.388235</td><td>650.501765</td></tr><tr><td>2</td><td>Myworks</td><td>101.79</td><td>152.388235</td><td>-50.598235</td></tr><tr><td>3</td><td>Yadel</td><td>705.90</td><td>152.388235</td><td>553.511765</td></tr><tr><td>4</td><td>Kwideo</td><td></td><td>152.388235</td><td></td></tr><tr><td>5</td><td>Topiclounge</td><td>980.02</td><td>152.388235</td><td>827.631765</td></tr></tbody></table></div><p>Solution:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> </span><br><span class="line"> client_id,</span><br><span class="line"> <span class="keyword">name</span>,</span><br><span class="line"> (<span class="keyword">SELECT</span> <span class="keyword">SUM</span>(invoice_total) </span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line"> <span class="keyword">WHERE</span> client_id = c.client_id) <span class="keyword">AS</span> total_sales,</span><br><span class="line">(<span class="keyword">SELECT</span> <span class="keyword">AVG</span>(invoice_total) <span class="keyword">FROM</span> invoices) <span class="keyword">AS</span> average,</span><br><span class="line"> (<span class="keyword">SELECT</span> total_sales - average) <span class="keyword">AS</span> <span class="keyword">difference</span></span><br><span class="line"><span class="keyword">FROM</span> clients c</span><br></pre></td></tr></table></figure><h1 id="9-FROM-子句中的子查询"><a href="#9-FROM-子句中的子查询" class="headerlink" title="9 FROM 子句中的子查询"></a>9 FROM 子句中的子查询</h1><p>将上面的查询结果当成表来用,放在FROM 子句中:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> (</span><br><span class="line"> <span class="keyword">SELECT</span></span><br><span class="line"> client_id,</span><br><span class="line"> <span class="keyword">name</span>,</span><br><span class="line"> (<span class="keyword">SELECT</span> <span class="keyword">SUM</span>(invoice_total) </span><br><span class="line"> <span class="keyword">FROM</span> invoices</span><br><span class="line"> <span class="keyword">WHERE</span> client_id = c.client_id) <span class="keyword">AS</span> total_sales,</span><br><span class="line"> (<span class="keyword">SELECT</span> <span class="keyword">AVG</span>(invoice_total) <span class="keyword">FROM</span> invoices) <span class="keyword">AS</span> average,</span><br><span class="line"> (<span class="keyword">SELECT</span> total_sales - average) <span class="keyword">AS</span> <span class="keyword">difference</span></span><br><span class="line"> <span class="keyword">FROM</span> clients c</span><br><span class="line">) <span class="keyword">AS</span> sales_summary</span><br><span class="line"><span class="keyword">WHERE</span> total_sales <span class="keyword">IS</span> <span class="keyword">NOT</span> <span class="literal">NULL</span></span><br></pre></td></tr></table></figure><p>得到:</p><div class="table-container"><table><thead><tr><th>client_id</th><th>name</th><th>total_sales</th><th>average</th><th>difference</th></tr></thead><tbody><tr><td>1</td><td>Vinte</td><td>802.89</td><td>152.388235</td><td>650.501765</td></tr><tr><td>2</td><td>Myworks</td><td>101.79</td><td>152.388235</td><td>-50.598235</td></tr><tr><td>3</td><td>Yadel</td><td>705.90</td><td>152.388235</td><td>553.511765</td></tr><tr><td>5</td><td>Topiclounge</td><td>980.02</td><td>152.388235</td><td>827.631765</td></tr></tbody></table></div>]]></content>
<summary type="html">
<h1 id="1-子查询"><a href="#1-子查询" class="headerlink" title="1 子查询"></a>1 子查询</h1><figure class="highlight sql"><table><tr><td class="gutter"><
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(4)--数据汇总</title>
<link href="http://yoursite.com/2021/05/18/20210518_SQL004/"/>
<id>http://yoursite.com/2021/05/18/20210518_SQL004/</id>
<published>2021-05-18T09:50:09.000Z</published>
<updated>2021-05-18T07:38:23.494Z</updated>
<content type="html"><![CDATA[<h1 id="1-聚合函数（Aggregate-Functions）"><a href="#1-聚合函数（Aggregate-Functions）" class="headerlink" title="1 聚合函数（Aggregate Functions）"></a>1 聚合函数（Aggregate Functions）</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> </span><br><span class="line"> <span class="keyword">MAX</span>(invoice_total) <span class="keyword">AS</span> highest,</span><br><span class="line"> <span class="keyword">MIN</span>(invoice_total) <span class="keyword">AS</span> lowest,</span><br><span class="line"> <span class="keyword">AVG</span>(invoice_total) <span class="keyword">AS</span> average,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total * <span class="number">1.1</span>) <span class="keyword">AS</span> total2,</span><br><span class="line"> <span class="keyword">COUNT</span>(invoice_total) <span class="keyword">AS</span> number_of_invoices,</span><br><span class="line"> <span class="keyword">COUNT</span>(payment_date) <span class="keyword">AS</span> count_of_payments,<span class="comment">-- COUNT只计算非空值个数</span></span><br><span class="line"> <span class="keyword">COUNT</span>(*) <span class="keyword">AS</span> total_records,</span><br><span class="line"> <span class="keyword">COUNT</span>(<span class="keyword">DISTINCT</span> client_id) <span class="keyword">AS</span> total_clients <span class="comment">-- 去除重复值</span></span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_date > <span class="string">'2019-07-01'</span></span><br></pre></td></tr></table></figure><p><strong>Exercise</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> <span class="string">'First half of 2019'</span> <span class="keyword">AS</span> date_range,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales,</span><br><span class="line"> <span class="keyword">SUM</span>(payment_total) <span class="keyword">AS</span> total_payments,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total - payment_total) <span class="keyword">AS</span> what_we_expect</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_date <span class="keyword">BETWEEN</span> <span class="string">'2019-01-01'</span> <span class="keyword">AND</span> <span class="string">'2019-06-30'</span></span><br><span class="line"><span class="keyword">UNION</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> <span class="string">'Second half of 2019'</span> <span class="keyword">AS</span> date_range,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales,</span><br><span class="line"> <span class="keyword">SUM</span>(payment_total) <span class="keyword">AS</span> total_payments,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total - payment_total) <span class="keyword">AS</span> what_we_expect</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_date <span class="keyword">BETWEEN</span> <span class="string">'2019-07-01'</span> <span class="keyword">AND</span> <span class="string">'2019-12-31'</span></span><br><span class="line"><span class="keyword">UNION</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> <span class="string">'Total'</span> <span class="keyword">AS</span> date_range,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales,</span><br><span class="line"> <span class="keyword">SUM</span>(payment_total) <span class="keyword">AS</span> total_payments,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total - payment_total) <span class="keyword">AS</span> what_we_expect</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_date <span class="keyword">BETWEEN</span> <span class="string">'2019-01-01'</span> <span class="keyword">AND</span> <span class="string">'2019-12-31'</span></span><br></pre></td></tr></table></figure><div class="table-container"><table><thead><tr><th>date_range</th><th>total_sales</th><th>total_payments</th><th>what_we_expect</th></tr></thead><tbody><tr><td>First half of 2019</td><td>1539.07</td><td>212.97</td><td>1326.1</td></tr><tr><td>Second half of 2019</td><td>1051.53</td><td>148.41</td><td>903.12</td></tr><tr><td>Total</td><td>2590.6</td><td>361.38</td><td>2229.22</td></tr></tbody></table></div><h1 id="2-GROUP-BY-子句"><a href="#2-GROUP-BY-子句" class="headerlink" title="2 GROUP BY 子句"></a>2 GROUP BY 子句</h1><p><strong>Demo 1</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> client_id,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_date >= <span class="string">'2019-07-01'</span></span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> total_sales <span class="keyword">DESC</span></span><br></pre></td></tr></table></figure><p>⚠ 各个子句的顺序不能错，比如不能将WHERE移到ORDER BY 下面</p><p>得出每个client在2019年下半年的订单总金额</p><div class="table-container"><table><thead><tr><th>client_id</th><th>total_sales</th></tr></thead><tbody><tr><td>5</td><td>489.52</td></tr><tr><td>3</td><td>427.54</td></tr><tr><td>1</td><td>134.47</td></tr></tbody></table></div><p><strong>Demo 2</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> state,</span><br><span class="line"> city,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales</span><br><span class="line"><span class="keyword">FROM</span> invoices i</span><br><span class="line"><span class="keyword">JOIN</span> clients <span class="keyword">USING</span> (client_id)</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> state, city</span><br></pre></td></tr></table></figure><p>得到每个（state, city）组合的总金额：</p><div class="table-container"><table><thead><tr><th>state</th><th>city</th><th>total_sales</th></tr></thead><tbody><tr><td>NY</td><td>Syracuse</td><td>802.89</td></tr><tr><td>WV</td><td>Huntington</td><td>101.79</td></tr><tr><td>CA</td><td>San Francisco</td><td>705.90</td></tr><tr><td>OR</td><td>Portland</td><td>980.02</td></tr></tbody></table></div><p><strong>Exercise</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> <span class="built_in">date</span>,</span><br><span class="line"> pm.name <span class="keyword">AS</span> payment_method,</span><br><span class="line"> <span class="keyword">SUM</span>(amount) <span class="keyword">AS</span> total_payments</span><br><span class="line"><span class="keyword">FROM</span> payments p</span><br><span class="line"><span class="keyword">JOIN</span> payment_methods pm</span><br><span class="line"> <span class="keyword">ON</span> p.payment_method = pm.payment_method_id</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> <span class="built_in">date</span>, payment_method</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> <span class="built_in">date</span></span><br></pre></td></tr></table></figure><p>根据交易日期和交易方式求总金额。</p><div class="table-container"><table><thead><tr><th>date</th><th>payment_method</th><th>total_payments</th></tr></thead><tbody><tr><td>2019-01-03</td><td>Credit Card</td><td>74.55</td></tr><tr><td>2019-01-08</td><td>Credit Card</td><td>32.77</td></tr><tr><td>2019-01-08</td><td>Cash</td><td>10.00</td></tr><tr><td>2019-01-11</td><td>Credit Card</td><td>0.03</td></tr><tr><td>2019-01-15</td><td>Credit Card</td><td>148.41</td></tr><tr><td>2019-01-26</td><td>Credit Card</td><td>87.44</td></tr><tr><td>2019-02-12</td><td>Credit Card</td><td>8.18</td></tr></tbody></table></div><h1 id="3-HAVING-子句"><a href="#3-HAVING-子句" class="headerlink" title="3 HAVING 子句"></a>3 HAVING 子句</h1><blockquote><p>👉 使用WHERE子句可以在行分组之<strong>前</strong>过滤数据<br>👉 使用HAVING子句可以在行分组之<strong>后</strong>过滤数据</p></blockquote><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> client_id,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id</span><br><span class="line"><span class="keyword">HAVING</span> total_sales > <span class="number">500</span> <span class="comment">-- 必须是在select子句中的column</span></span><br></pre></td></tr></table></figure><div class="table-container"><table><thead><tr><th>client_id</th><th>total_sales</th></tr></thead><tbody><tr><td>1</td><td>802.89</td></tr><tr><td>3</td><td>705.90</td></tr><tr><td>5</td><td>980.02</td></tr></tbody></table></div><p><strong>复合搜索条件</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> client_id,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales,</span><br><span class="line"> <span class="keyword">COUNT</span>(*) <span class="keyword">AS</span> number_of_invoices</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id</span><br><span class="line"><span class="keyword">HAVING</span> total_sales > <span class="number">500</span> <span class="keyword">AND</span> number_of_invoices > <span class="number">5</span></span><br></pre></td></tr></table></figure><p>获取金额超过500并且多于5个订单的client的结果</p><div class="table-container"><table><thead><tr><th>client_id</th><th>total_sales</th><th>number_of_invoices</th></tr></thead><tbody><tr><td>5</td><td>980.02</td><td>6</td></tr></tbody></table></div><p><strong>Exercise</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- Get the customers located in VG</span></span><br><span class="line"><span class="comment">-- who have spent more than $100</span></span><br><span class="line"><span class="keyword">USE</span> sql_store;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> c.customer_id,</span><br><span class="line"> c.first_name,</span><br><span class="line"> c.last_name,</span><br><span class="line"> c.state,</span><br><span class="line"> <span class="keyword">SUM</span>(oi.quantity * oi.unit_price) <span class="keyword">AS</span> total_sales</span><br><span class="line"><span class="keyword">FROM</span> customers c</span><br><span class="line"><span class="keyword">JOIN</span> orders o <span class="keyword">USING</span> (customer_id)</span><br><span class="line"><span class="keyword">JOIN</span> order_items oi <span class="keyword">USING</span> (order_id)</span><br><span class="line"><span class="keyword">WHERE</span> state = <span class="string">'VA'</span></span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span></span><br><span class="line"> c.customer_id,</span><br><span class="line"> c.first_name,</span><br><span class="line"> c.last_name</span><br><span class="line"><span class="keyword">HAVING</span> total_sales > <span class="number">100</span></span><br></pre></td></tr></table></figure><div class="table-container"><table><thead><tr><th>customer_id</th><th>first_name</th><th>last_name</th><th>state</th><th>total_sales</th></tr></thead><tbody><tr><td>2</td><td>Ines</td><td>Brushfield</td><td>VA</td><td>157.92</td></tr></tbody></table></div><h1 id="4-ROLLUP-运算符"><a href="#4-ROLLUP-运算符" class="headerlink" title="4 ROLLUP 运算符"></a>4 ROLLUP 运算符</h1><p><strong>Demo 1</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> client_id,</span><br><span class="line"> <span class="keyword">SUM</span>(invoice_total) <span class="keyword">AS</span> total_sales</span><br><span class="line"><span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> client_id <span class="keyword">WITH</span> <span class="keyword">ROLLUP</span></span><br></pre></td></tr></table></figure><p>WITH ROLLUP 自动计算总数</p><div class="table-container"><table><thead><tr><th>client_id</th><th>total_sales</th></tr></thead><tbody><tr><td>1</td><td>802.89</td></tr><tr><td>2</td><td>101.79</td></tr><tr><td>3</td><td>705.90</td></tr><tr><td>5</td><td>980.02</td></tr><tr><td></td><td>2590.60</td></tr></tbody></table></div><p><strong>Demo 2</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> <span class="built_in">date</span>,</span><br><span class="line"> pm.name <span class="keyword">AS</span> payment_method,</span><br><span class="line"> <span class="keyword">SUM</span>(amount) <span class="keyword">AS</span> total_payments</span><br><span class="line"><span class="keyword">FROM</span> payments p</span><br><span class="line"><span class="keyword">JOIN</span> payment_methods pm</span><br><span class="line"><span class="keyword">ON</span> p.payment_method = pm.payment_method_id</span><br><span class="line"><span class="keyword">GROUP</span> <span class="keyword">BY</span> <span class="built_in">date</span>, pm.name <span class="keyword">WITH</span> <span class="keyword">ROLLUP</span></span><br></pre></td></tr></table></figure><p>注意: GROUP BY 子句中由于使用了 WITH ROLLUP, 就不能使用别名 payment_method</p><p>得到每个组及整个结果集的汇总集:</p><div class="table-container"><table><thead><tr><th>date</th><th>payment_method</th><th>total_payments</th></tr></thead><tbody><tr><td>2019-01-03</td><td>Credit Card</td><td>74.55</td></tr><tr><td>2019-01-03</td><td></td><td>74.55</td></tr><tr><td>2019-01-08</td><td>Cash</td><td>10.00</td></tr><tr><td>2019-01-08</td><td>Credit Card</td><td>32.77</td></tr><tr><td>2019-01-08</td><td></td><td>42.77</td></tr><tr><td>2019-01-11</td><td>Credit Card</td><td>0.03</td></tr><tr><td>2019-01-11</td><td></td><td>0.03</td></tr><tr><td>2019-01-15</td><td>Credit Card</td><td>148.41</td></tr><tr><td>2019-01-15</td><td></td><td>148.41</td></tr><tr><td>2019-01-26</td><td>Credit Card</td><td>87.44</td></tr><tr><td>2019-01-26</td><td></td><td>87.44</td></tr><tr><td>2019-02-12</td><td>Credit Card</td><td>8.18</td></tr><tr><td>2019-02-12</td><td></td><td>8.18</td></tr><tr><td></td><td></td><td>361.38</td></tr></tbody></table></div><p>注意到 2019-01-08 对 Cash 和 Credit Card 有一个汇总值.</p>]]></content>
<summary type="html">
<h1 id="1-聚合函数（Aggregate-Functions）"><a href="#1-聚合函数（Aggregate-Functions）" class="headerlink" title="1 聚合函数（Aggregate Functions）"></a>1 聚合函
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(3)--增删改</title>
<link href="http://yoursite.com/2021/05/17/20210517_SQL003/"/>
<id>http://yoursite.com/2021/05/17/20210517_SQL003/</id>
<published>2021-05-17T13:50:09.000Z</published>
<updated>2021-05-18T07:35:42.334Z</updated>
<content type="html"><![CDATA[<h1 id="1-列属性"><a href="#1-列属性" class="headerlink" title="1 列属性"></a>1 列属性</h1><blockquote><p><strong>Data type</strong></p><p>👉 VARCHAR: 可变长度字符串（节省空间）<br>👉 CHAR: 字符串，不满指定长度就会填充满</p><p><strong>Properties</strong></p><p>👉 PK: Primary Key<br>👉 NN: Not Null<br>👉 AI: Automatic Increment</p></blockquote><h1 id="2-插入"><a href="#2-插入" class="headerlink" title="2 插入"></a>2 插入</h1><h2 id="2-1-插入单行"><a href="#2-1-插入单行" class="headerlink" title="2.1 插入单行"></a>2.1 插入单行</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> customers</span><br><span class="line"><span class="keyword">VALUES</span> (</span><br><span class="line"> <span class="keyword">DEFAULT</span>, </span><br><span class="line"> <span class="string">'John'</span>, </span><br><span class="line"> <span class="string">'Smith'</span>, </span><br><span class="line"> <span class="string">'1990-01-01'</span>,</span><br><span class="line"> <span class="literal">NULL</span>,</span><br><span class="line"> <span class="string">'address'</span>,</span><br><span class="line"> <span class="string">'city'</span>,</span><br><span class="line"> <span class="string">'CA'</span>,</span><br><span class="line"> <span class="keyword">DEFAULT</span>)</span><br></pre></td></tr></table></figure><p>有些不是必填的, 可以skip掉:</p><p><strong>Another Way</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> customers (</span><br><span class="line"> first_name,</span><br><span class="line"> last_name,</span><br><span class="line"> birth_date,</span><br><span class="line"> address,</span><br><span class="line"> city,</span><br><span class="line"> state)</span><br><span class="line"><span class="keyword">VALUES</span> (</span><br><span class="line"> <span class="string">'John'</span>, </span><br><span class="line"> <span class="string">'Smith'</span>, </span><br><span class="line"> <span class="string">'1990-01-01'</span>,</span><br><span class="line"> <span class="string">'address'</span>,</span><br><span class="line"> <span class="string">'city'</span>,</span><br><span class="line"> <span class="string">'CA'</span>)</span><br></pre></td></tr></table></figure><h2 id="2-2-插入多行"><a href="#2-2-插入多行" class="headerlink" title="2.2 插入多行"></a>2.2 插入多行</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> shippers (<span class="keyword">name</span>)</span><br><span class="line"><span class="keyword">VALUES</span> (<span class="string">'Shipper1'</span>), </span><br><span class="line"> (<span class="string">'Shipper2'</span>), </span><br><span class="line"> (<span class="string">'Shipper3'</span>)</span><br></pre></td></tr></table></figure><h2 id="2-3-插入分层行"><a href="#2-3-插入分层行" class="headerlink" title="2.3 插入分层行"></a>2.3 插入分层行</h2><p>order_items是orders的子表, 列出更为详细的信息.在orders插入行, 相应的在order_items也要插入行:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> orders (customer_id, order_date, <span class="keyword">status</span>)</span><br><span class="line"><span class="keyword">VALUES</span> (<span class="number">1</span>, <span class="string">'2019-01-02'</span>, <span class="number">1</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> order_items</span><br><span class="line"><span class="keyword">VALUES</span> </span><br><span class="line"> (<span class="keyword">LAST_INSERT_ID</span>(), <span class="number">1</span>, <span class="number">1</span>, <span class="number">2.95</span>),</span><br><span class="line"> (<span class="keyword">LAST_INSERT_ID</span>(), <span class="number">2</span>, <span class="number">1</span>, <span class="number">3.95</span>)</span><br></pre></td></tr></table></figure><p>LAST_INSERT_ID()相当于一个method/function, 直接调用获取最近插入的record的id.</p><h1 id="3-创建复制表-a-copy-of-table"><a href="#3-创建复制表-a-copy-of-table" class="headerlink" title="3 创建复制表(a copy of table)"></a>3 创建复制表(a copy of table)</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TABLE</span> orders_archived <span class="keyword">AS</span></span><br><span class="line"><span class="keyword">SELECT</span> * <span class="keyword">FROM</span> orders</span><br></pre></td></tr></table></figure><p>通过这种方式创建的表复制会没有PK, AI这些属性</p><h2 id="3-1-子查询"><a href="#3-1-子查询" class="headerlink" title="3.1 子查询"></a>3.1 子查询</h2><p>上例中的SELECT * FROM orders是子查询.</p><p>INSERT INTO也可以用子查询:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">INSERT</span> <span class="keyword">INTO</span> orders_archived</span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> orders</span><br><span class="line"><span class="keyword">WHERE</span> order_date < <span class="string">'2019-01-01'</span></span><br></pre></td></tr></table></figure><p>orders_archived表中只有满足条件的records.</p><h1 id="4-更新"><a href="#4-更新" class="headerlink" title="4 更新"></a>4 更新</h1><h2 id="4-1-更新单行"><a href="#4-1-更新单行" class="headerlink" title="4.1 更新单行"></a>4.1 更新单行</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices</span><br><span class="line"><span class="keyword">SET</span> payment_total = <span class="number">10</span>, payment_date = <span class="string">'2019-03-01'</span></span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">1</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices</span><br><span class="line"><span class="keyword">SET</span> payment_total = <span class="keyword">DEFAULT</span>, payment_date = <span class="literal">NULL</span></span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">1</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices</span><br><span class="line"><span class="keyword">SET</span> </span><br><span class="line"> payment_total = invoice_total * <span class="number">0.5</span>, <span class="comment">-- 使用表达式更新</span></span><br><span class="line"> payment_date = due_date</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">3</span></span><br></pre></td></tr></table></figure><h2 id="4-2-更新多行"><a href="#4-2-更新多行" class="headerlink" title="4.2 更新多行"></a>4.2 更新多行</h2><p>在MySQL workbench需要取消safe update选项才能更新多行.</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices</span><br><span class="line"><span class="keyword">SET</span> </span><br><span class="line"> payment_total = invoice_total * <span class="number">0.5</span>, <span class="comment">-- 使用表达式更新</span></span><br><span class="line"> payment_date = due_date</span><br><span class="line"><span class="keyword">WHERE</span> client_id = <span class="number">3</span></span><br><span class="line"><span class="comment">-- WHERE client_id IN (3, 4)</span></span><br></pre></td></tr></table></figure><p>client_id=3对应有多项record, 都会被update</p><h2 id="4-3-在UPDATE中使用子查询"><a href="#4-3-在UPDATE中使用子查询" class="headerlink" title="4.3 在UPDATE中使用子查询"></a>4.3 在UPDATE中使用子查询</h2><p>将上例改为:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices</span><br><span class="line"><span class="keyword">SET</span> </span><br><span class="line"> payment_total = invoice_total * <span class="number">0.5</span>, <span class="comment">-- 使用表达式更新</span></span><br><span class="line"> payment_date = due_date</span><br><span class="line"><span class="keyword">WHERE</span> client_id = </span><br><span class="line"> (<span class="keyword">SELECT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> clients</span><br><span class="line"> <span class="keyword">WHERE</span> <span class="keyword">name</span> = <span class="string">'Myworks'</span>)</span><br></pre></td></tr></table></figure><p>如果子查询返回的是多个client_id:</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">UPDATE</span> invoices</span><br><span class="line"><span class="keyword">SET</span> </span><br><span class="line"> payment_total = invoice_total * <span class="number">0.5</span>, <span class="comment">-- 使用表达式更新</span></span><br><span class="line"> payment_date = due_date</span><br><span class="line"><span class="keyword">WHERE</span> client_id <span class="keyword">IN</span><span class="comment">-- 用IN</span></span><br><span class="line"> (<span class="keyword">SELECT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> clients</span><br><span class="line"> <span class="keyword">WHERE</span> state <span class="keyword">IN</span> (<span class="string">'CA'</span>, <span class="string">'NY'</span>))</span><br></pre></td></tr></table></figure><p>⚠ 在执行查询之前, 应该先执行子查询看看对不对.</p><h1 id="5-删除行"><a href="#5-删除行" class="headerlink" title="5 删除行"></a>5 删除行</h1><p><strong>根据条件删除某些行</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DELETE</span> <span class="keyword">FROM</span> invoices_archived</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = <span class="number">2</span></span><br></pre></td></tr></table></figure><p><strong>子查询</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DELETE</span> <span class="keyword">FROM</span> invoices</span><br><span class="line"><span class="keyword">WHERE</span> invoice_id = </span><br><span class="line"> (<span class="keyword">SELECT</span> client_id</span><br><span class="line"> <span class="keyword">FROM</span> clients</span><br><span class="line"> <span class="keyword">WHERE</span> <span class="keyword">name</span> = <span class="string">'Vinte'</span>)</span><br></pre></td></tr></table></figure><p><strong>删除整表所有records</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">DELETE</span> <span class="keyword">FROM</span> invoices_archived</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<h1 id="1-列属性"><a href="#1-列属性" class="headerlink" title="1 列属性"></a>1 列属性</h1><blockquote>
<p><strong>Data type</strong></p>
<p>👉 VARCHAR:
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(2)--连接</title>
<link href="http://yoursite.com/2021/05/17/20210517_SQL002/"/>
<id>http://yoursite.com/2021/05/17/20210517_SQL002/</id>
<published>2021-05-17T07:50:09.000Z</published>
<updated>2021-05-18T07:30:42.320Z</updated>
<content type="html"><![CDATA[<p><img src="/images/SQL1/002_sql-join.png" alt="002_sql-join"></p><h1 id="1-Inner-Joins-内连接"><a href="#1-Inner-Joins-内连接" class="headerlink" title="1 Inner Joins (内连接)"></a>1 Inner Joins (内连接)</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> order_id, o.customer_id, first_name, last_name</span><br><span class="line"><span class="keyword">FROM</span> orders o</span><br><span class="line"><span class="keyword">INNER</span> <span class="keyword">JOIN</span> customers c</span><br><span class="line"> <span class="keyword">ON</span> o.customer_id = c.customer_id</span><br></pre></td></tr></table></figure><p>返回表orders在表customers中有对应record的项</p><p>o和c分别是orders和customers的别名，以简化代码</p><p>关键字INNER可省略</p><h2 id="1-1-跨数据库连接"><a href="#1-1-跨数据库连接" class="headerlink" title="1.1 跨数据库连接"></a>1.1 跨数据库连接</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_store; <span class="comment">-- 选择了数据库sql_store为当前数据库</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> order_items oi</span><br><span class="line"><span class="keyword">JOIN</span> sql_inventory.products p <span class="comment">-- 另一个数据库sql_inventory</span></span><br><span class="line"> <span class="keyword">ON</span> oi.product_id = p.product_id</span><br></pre></td></tr></table></figure><h2 id="1-2-自连接"><a href="#1-2-自连接" class="headerlink" title="1.2 自连接"></a>1.2 自连接</h2><p>将表和自己连接，但是给不同的别名以区分。如下，manager也是一名员工，其信息也在表employees中，用自连接可以得到employee和TA的manager的具体员工信息。</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_hr; <span class="comment">-- 选择了数据库sql_hr为当前数据库</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> e.employee_id,</span><br><span class="line"> e.first_name,</span><br><span class="line"> m.first_name <span class="keyword">AS</span> manager</span><br><span class="line"><span class="keyword">FROM</span> employees e</span><br><span class="line"><span class="keyword">JOIN</span> employees m </span><br><span class="line"> <span class="keyword">ON</span> e.reports_to = m.employee_id</span><br></pre></td></tr></table></figure><h2 id="1-3-多表连接"><a href="#1-3-多表连接" class="headerlink" title="1.3 多表连接"></a>1.3 多表连接</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_store;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> o.order_id,</span><br><span class="line"> o.order_date,</span><br><span class="line"> c.first_name,</span><br><span class="line"> c.last_name,</span><br><span class="line"> os.name <span class="keyword">AS</span> <span class="keyword">status</span></span><br><span class="line"><span class="keyword">FROM</span> orders o</span><br><span class="line"><span class="keyword">JOIN</span> customers c</span><br><span class="line"> <span class="keyword">ON</span> o.customer_id = c.customer_id</span><br><span class="line"><span class="keyword">JOIN</span> order_statuses os</span><br><span class="line"> <span class="keyword">ON</span> o.status = os.order_status_id</span><br></pre></td></tr></table></figure><p>表orders先和表customers连接，然后与表order_statuses连接。</p><h2 id="1-4-复合连接条件"><a href="#1-4-复合连接条件" class="headerlink" title="1.4 复合连接条件"></a>1.4 复合连接条件</h2><p>对于有两个主键的表，要用两个主键作为条件与别的表连接。例如表order_items的主键是order_id和product_id</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> order_items oi</span><br><span class="line"><span class="keyword">JOIN</span> order_item_notes oin</span><br><span class="line"> <span class="keyword">ON</span> oi.order_id = oin.order_id</span><br><span class="line"> <span class="keyword">AND</span> oi.product_id = oin.product_id</span><br></pre></td></tr></table></figure><h2 id="1-5-隐式连接语法"><a href="#1-5-隐式连接语法" class="headerlink" title="1.5 隐式连接语法"></a>1.5 隐式连接语法</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> orders o</span><br><span class="line"><span class="keyword">JOIN</span> customers c</span><br><span class="line"> <span class="keyword">ON</span> o.customer_id = c.customer_id</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">-- Implicit Join Syntax</span></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> orders o, customers c</span><br><span class="line"><span class="keyword">WHERE</span> o.customer_id = c.customer_id</span><br></pre></td></tr></table></figure><p>不建议使用隐式连接语法，要是忘写WHERE就完了。</p><h1 id="2-外连接"><a href="#2-外连接" class="headerlink" title="2 外连接"></a>2 外连接</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> </span><br><span class="line"> c.customer_id,</span><br><span class="line"> c.first_name,</span><br><span class="line"> o.order_id</span><br><span class="line"><span class="keyword">FROM</span> customers c</span><br><span class="line"><span class="keyword">RIGHT</span> <span class="keyword">JOIN</span> orders o</span><br><span class="line"> <span class="keyword">ON</span> c.customer_id = o.customer_id</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> c.customer_id</span><br></pre></td></tr></table></figure><p>LEFT (OUTER) JOIN 和 RIGHT (OUTER) JOIN, OUTER 可省略</p><h2 id="2-1-多表外连接"><a href="#2-1-多表外连接" class="headerlink" title="2.1 多表外连接"></a>2.1 多表外连接</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> </span><br><span class="line"> c.customer_id,</span><br><span class="line"> c.first_name,</span><br><span class="line"> o.order_id,</span><br><span class="line"> sh.name <span class="keyword">AS</span> shipper</span><br><span class="line"><span class="keyword">FROM</span> customers c</span><br><span class="line"><span class="keyword">LEFT</span> <span class="keyword">JOIN</span> orders o</span><br><span class="line"> <span class="keyword">ON</span> c.customer_id = o.customer_id</span><br><span class="line"><span class="keyword">LEFT</span> <span class="keyword">JOIN</span> shippers sh</span><br><span class="line"> <span class="keyword">ON</span> o.shipper_id = sh.shipper_id</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> c.customer_id</span><br></pre></td></tr></table></figure><h2 id="2-2-自外连接"><a href="#2-2-自外连接" class="headerlink" title="2.2 自外连接"></a>2.2 自外连接</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">USE</span> sql_hr;</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> </span><br><span class="line"> e.employee_id,</span><br><span class="line"> e.first_name <span class="keyword">AS</span> employee,</span><br><span class="line"> m.employee_id <span class="keyword">AS</span> manager</span><br><span class="line"><span class="keyword">FROM</span> employees e</span><br><span class="line"><span class="keyword">LEFT</span> <span class="keyword">JOIN</span> employees m</span><br><span class="line"> <span class="keyword">ON</span> e.reports_to = m.employee_id</span><br></pre></td></tr></table></figure><h1 id="3-USING子句"><a href="#3-USING子句" class="headerlink" title="3 USING子句"></a>3 USING子句</h1><p>使用于两张表的键名完全一致的情况</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> </span><br><span class="line"> o.order_id, </span><br><span class="line"> c.first_name,</span><br><span class="line"> sh.name <span class="keyword">AS</span> shipper</span><br><span class="line"><span class="keyword">FROM</span> orders o</span><br><span class="line"><span class="keyword">JOIN</span> customers c</span><br><span class="line"> <span class="comment">-- ON o.customer_id = c.customer_id </span></span><br><span class="line"> <span class="keyword">USING</span> (customer_id)</span><br><span class="line"><span class="keyword">LEFT</span> <span class="keyword">JOIN</span> shippers sh</span><br><span class="line"> <span class="keyword">USING</span> (shipper_id)</span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> order_items oi</span><br><span class="line"><span class="keyword">JOIN</span> order_item_notes oin</span><br><span class="line"> <span class="comment">-- ON oi.order_id = oin.order_id</span></span><br><span class="line"> <span class="comment">-- AND oi.product_id = oin.product_id</span></span><br><span class="line"> <span class="keyword">USING</span> (order_id, product_id)</span><br></pre></td></tr></table></figure><h1 id="4-自然连接"><a href="#4-自然连接" class="headerlink" title="4 自然连接"></a>4 自然连接</h1><p>自动找到同名的column来连接。（容易出错，不建议使用）</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> o.order_id,</span><br><span class="line"> c.first_name</span><br><span class="line"><span class="keyword">FROM</span> orders o</span><br><span class="line"><span class="keyword">NATURAL</span> <span class="keyword">JOIN</span> customers c</span><br></pre></td></tr></table></figure><h1 id="5-交叉连接"><a href="#5-交叉连接" class="headerlink" title="5 交叉连接"></a>5 交叉连接</h1><p>连接两张表的所有records</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> c.first_name <span class="keyword">AS</span> customers,</span><br><span class="line"> p.name <span class="keyword">AS</span> product</span><br><span class="line"><span class="keyword">FROM</span> customers c</span><br><span class="line"><span class="keyword">CROSS</span> <span class="keyword">JOIN</span> products p</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> c.first_name</span><br></pre></td></tr></table></figure><p>应用场景：表一是大中小型号，表二是红黄蓝等各种颜色，将所有型号和所有颜色组合起来。</p><p><strong>隐式语法</strong></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> c.first_name <span class="keyword">AS</span> customers,</span><br><span class="line"> p.name <span class="keyword">AS</span> product</span><br><span class="line"><span class="keyword">FROM</span> customers c, products p</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> c.first_name</span><br></pre></td></tr></table></figure><h1 id="6-联合（Unions）"><a href="#6-联合（Unions）" class="headerlink" title="6 联合（Unions）"></a>6 联合（Unions）</h1><p>筛选某些records，用union将多次筛选的结果联合起来</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> order_id,</span><br><span class="line"> order_date,</span><br><span class="line"> <span class="string">'Active'</span> <span class="keyword">AS</span> <span class="keyword">status</span></span><br><span class="line"><span class="keyword">FROM</span> orders</span><br><span class="line"><span class="keyword">WHERE</span> order_date >= <span class="string">'2019-01-01'</span><span class="comment">-- 在此日期后的order, 状态设为active</span></span><br><span class="line"><span class="keyword">UNION</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> order_id,</span><br><span class="line"> order_date,</span><br><span class="line"> <span class="string">'Archived'</span> <span class="keyword">AS</span> <span class="keyword">status</span><span class="comment">-- 在此日期前的order, 状态设为archived</span></span><br><span class="line"><span class="keyword">FROM</span> orders</span><br><span class="line"><span class="keyword">WHERE</span> order_date < <span class="string">'2019-01-01'</span></span><br></pre></td></tr></table></figure><p>也可以联合多张表</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> first_name <span class="comment">-- 决定列名</span></span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">UNION</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">name</span></span><br><span class="line"><span class="keyword">FROM</span> shippers</span><br></pre></td></tr></table></figure><p><strong>练习</strong></p><p>根据积分将顾客划分为金银铜三档：</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> customer_id,</span><br><span class="line"> first_name,</span><br><span class="line"> points,</span><br><span class="line"> <span class="string">'Bronze'</span> <span class="keyword">AS</span> <span class="keyword">type</span></span><br><span class="line"><span class="keyword">FROM</span> customers c</span><br><span class="line"><span class="keyword">WHERE</span> points < <span class="number">2000</span></span><br><span class="line"><span class="keyword">UNION</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> customer_id,</span><br><span class="line"> first_name,</span><br><span class="line"> points,</span><br><span class="line"> <span class="string">'silver'</span> <span class="keyword">AS</span> <span class="keyword">type</span></span><br><span class="line"><span class="keyword">FROM</span> customers c</span><br><span class="line"><span class="keyword">WHERE</span> points <span class="keyword">BETWEEN</span> <span class="number">2000</span> <span class="keyword">AND</span> <span class="number">3000</span></span><br><span class="line"><span class="keyword">UNION</span></span><br><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> customer_id,</span><br><span class="line"> first_name,</span><br><span class="line"> points,</span><br><span class="line"> <span class="string">'Gold'</span> <span class="keyword">AS</span> <span class="keyword">type</span></span><br><span class="line"><span class="keyword">FROM</span> customers c</span><br><span class="line"><span class="keyword">WHERE</span> points > <span class="number">3000</span></span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> first_name</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p><img src="/images/SQL1/002_sql-join.png" alt="002_sql-join"></p>
<h1 id="1-Inner-Joins-内连接"><a href="#1-Inner-Joins-内连接" class="headerlin
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>MySQL笔记(1)--基础语句</title>
<link href="http://yoursite.com/2021/05/16/20210516_SQL001/"/>
<id>http://yoursite.com/2021/05/16/20210516_SQL001/</id>
<published>2021-05-16T11:50:09.000Z</published>
<updated>2021-05-18T07:31:16.468Z</updated>
<content type="html"><![CDATA[<h1 id="SELECT-子句"><a href="#SELECT-子句" class="headerlink" title="SELECT 子句"></a>SELECT 子句</h1><p>从一张表（customers）中选取某些列</p><p><img src="/images/SQL1/001.png" style="zoom: 150%;"></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span></span><br><span class="line"> first_name, </span><br><span class="line"> last_name, </span><br><span class="line"> points, </span><br><span class="line"> points * <span class="number">10</span> + <span class="number">100</span> <span class="keyword">AS</span> <span class="string">"discount factor"</span> <span class="comment">-- 可以进行数学运算,AS来rename</span></span><br><span class="line"><span class="keyword">FROM</span> customers</span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">DISTINCT</span> state <span class="comment">-- DISTINCT 只保留重复项中的一项</span></span><br><span class="line"><span class="keyword">FROM</span> customers</span><br></pre></td></tr></table></figure><h1 id="WHERE-子句"><a href="#WHERE-子句" class="headerlink" title="WHERE 子句"></a>WHERE 子句</h1><p>过滤作用，根据条件选择records</p><p>(各个WHERE子句只是提供不同的示例，运行时选择一个WHERE子句，其余注释掉，下文同)</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">WHERE</span> state = <span class="string">'va'</span> <span class="comment">-- va大小写一样的</span></span><br><span class="line"><span class="keyword">WHERE</span> state <> <span class="string">'va'</span> <span class="comment">-- not in VA</span></span><br><span class="line"><span class="keyword">WHERE</span> birth_date > <span class="string">'1990-01-01'</span> <span class="comment">-- born after 1990/01/01</span></span><br></pre></td></tr></table></figure><p>运算符有：</p><figure class="highlight"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">></span><br><span class="line">>=</span><br><span class="line"><</span><br><span class="line"><=</span><br><span class="line">=</span><br><span class="line">(not equal) != or <></span><br></pre></td></tr></table></figure><h1 id="AND-OR-NOT-运算符"><a href="#AND-OR-NOT-运算符" class="headerlink" title="AND, OR, NOT 运算符"></a>AND, OR, NOT 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">WHERE</span> <span class="keyword">NOT</span> (birth_date > <span class="string">'1990-01-01'</span> <span class="keyword">OR</span> points > <span class="number">1000</span>)</span><br></pre></td></tr></table></figure><p>AND优先级高于OR</p><h1 id="IN-运算符"><a href="#IN-运算符" class="headerlink" title="IN 运算符"></a>IN 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="comment">-- WHERE state = 'VA' OR state = 'GA' OR state = 'FL'</span></span><br><span class="line"><span class="comment">-- 用IN改写：</span></span><br><span class="line"><span class="keyword">WHERE</span> state <span class="keyword">IN</span> (<span class="string">'VA'</span>, <span class="string">'GA'</span>, <span class="string">'FL'</span>)</span><br><span class="line"><span class="comment">-- WHERE state NOT IN ('VA', 'GA', 'FL') -- 返回不在这些州的 records</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">-- 这样是错的：WHERE state = 'VA' OR 'GA' OR 'FL' </span></span><br><span class="line"><span class="comment">-- 因为我们不能用OR连接布尔表达式state = 'VA'和字符串'GA'</span></span><br></pre></td></tr></table></figure><h1 id="BETWEEN-运算符"><a href="#BETWEEN-运算符" class="headerlink" title="BETWEEN 运算符"></a>BETWEEN 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="comment">-- WHERE points >= 1000 AND points <=3000</span></span><br><span class="line"><span class="comment">-- 改写为：</span></span><br><span class="line"><span class="keyword">WHERE</span> points <span class="keyword">BETWEEN</span> <span class="number">1000</span> <span class="keyword">AND</span> <span class="number">3000</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- 也可用于非数值</span></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">WHERE</span> birth_date <span class="keyword">BETWEEN</span> <span class="string">"1990-01-01"</span> <span class="keyword">AND</span> <span class="string">"2000-01-01"</span></span><br></pre></td></tr></table></figure><h1 id="LIKE-运算符"><a href="#LIKE-运算符" class="headerlink" title="LIKE 运算符"></a>LIKE 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- 字符串模式匹配</span></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">WHERE</span> last_name <span class="keyword">LIKE</span> <span class="string">'b%'</span> </span><br><span class="line"><span class="comment">-- 得到last name以'B'或'b'开头的records</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name <span class="keyword">LIKE</span> <span class="string">'brush%'</span> </span><br><span class="line"><span class="comment">-- 得到last name以“brush”开头的records</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name <span class="keyword">LIKE</span> <span class="string">'%b%'</span> </span><br><span class="line"><span class="comment">-- 表示'b'前后可以有任意字符数('b'在开头、中间、结尾出现皆可)</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name <span class="keyword">LIKE</span> <span class="string">'%y'</span> </span><br><span class="line"><span class="comment">-- 得到last name以'y'结尾的records</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name <span class="keyword">LIKE</span> <span class="string">'_____y'</span> </span><br><span class="line"><span class="comment">-- 得到last name以'y'结尾且只有6个字符('_'填补表示任意字符)的records</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name <span class="keyword">LIKE</span> <span class="string">'b____y'</span> </span><br><span class="line"><span class="comment">-- 运行后得到last name以'b'开头以'y'结尾且只有6个字符的record(本例中为'Boagey')</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/***SUMMARY**/</span> </span><br><span class="line">% any number of characters</span><br><span class="line">_ single character</span><br></pre></td></tr></table></figure><h1 id="REGEXP-运算符"><a href="#REGEXP-运算符" class="headerlink" title="REGEXP 运算符"></a>REGEXP 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- regular expression (REGEXP) 正则表达式</span></span><br><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name <span class="keyword">LIKE</span> <span class="string">'%field%'</span></span><br><span class="line"><span class="comment">-- WHERE last_name REGEXP 'field'</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name REGEXP <span class="string">'^field'</span> </span><br><span class="line"><span class="comment">-- last name must starts with "field"</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name REGEXP <span class="string">'field$'</span> </span><br><span class="line"><span class="comment">-- last name must ends with "field"</span></span><br><span class="line"></span><br><span class="line"><span class="comment">/* use '|' for multiple search pattern */</span> </span><br><span class="line"><span class="keyword">WHERE</span> last_name REGEXP <span class="string">'field|mac|rose'</span> </span><br><span class="line"><span class="comment">-- last name must contains "field" or "mac" or "rose"</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name REGEXP <span class="string">'field$|mac|rose'</span> </span><br><span class="line"><span class="comment">-- last name must ends with "field" or contains "mac"/"rose"</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name REGEXP <span class="string">'[gim]e'</span> </span><br><span class="line"><span class="comment">-- last name must contains "ge"/"ie"/"me"</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name REGEXP <span class="string">'e[gim]'</span> </span><br><span class="line"><span class="comment">-- last name must contains "eg"/"ei"/"em"</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">WHERE</span> last_name REGEXP <span class="string">'[a-h]e'</span></span><br><span class="line"><span class="comment">-- the same as WHERE last_name REGEXP '[abcdefgh]e'</span></span><br></pre></td></tr></table></figure><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">^ beginning</span><br><span class="line">$ end</span><br><span class="line">| logical or</span><br><span class="line">[abcd]</span><br><span class="line">[a-f]</span><br></pre></td></tr></table></figure><h1 id="IS-NULL-运算符"><a href="#IS-NULL-运算符" class="headerlink" title="IS NULL 运算符"></a>IS NULL 运算符</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">WHERE</span> phone <span class="keyword">IS</span> <span class="literal">NULL</span> <span class="comment">-- 返回没有phone(该字段为NULL)的records</span></span><br><span class="line"><span class="keyword">WHERE</span> phone <span class="keyword">IS</span> <span class="keyword">NOT</span> <span class="literal">NULL</span> <span class="comment">-- phone不为空</span></span><br></pre></td></tr></table></figure><h1 id="ORDER-BY-子句"><a href="#ORDER-BY-子句" class="headerlink" title="ORDER BY 子句"></a>ORDER BY 子句</h1><p>默认是按照主键（primary key）排序，使用ORDER BY 子句可以指定排序的键</p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> first_name <span class="comment">-- 根据first_name字段值升序排序</span></span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> first_name <span class="keyword">DESC</span> <span class="comment">-- 根据first_name字段值降序排序</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> state <span class="keyword">DESC</span>, first_name </span><br><span class="line"><span class="comment">--先根据state降序排列，state相同的根据first_name升序排列</span></span><br></pre></td></tr></table></figure><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- birth_date不在选出来的列里，still work</span></span><br><span class="line"><span class="keyword">SELECT</span> first_name, last_name</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">ORDER</span> <span class="keyword">BY</span> birth_date</span><br></pre></td></tr></table></figure><h1 id="LIMIT-子句"><a href="#LIMIT-子句" class="headerlink" title="LIMIT 子句"></a>LIMIT 子句</h1><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> *</span><br><span class="line"><span class="keyword">FROM</span> customers</span><br><span class="line"><span class="keyword">LIMIT</span> <span class="number">3</span> <span class="comment">-- 返回前3个records</span></span><br><span class="line"><span class="keyword">LIMIT</span> <span class="number">300</span> <span class="comment">-- 300 > #record, 返回全部records</span></span><br><span class="line"><span class="keyword">LIMIT</span> <span class="number">6</span>, <span class="number">3</span> <span class="comment">-- 跳过前6个records，返回第7，8，9个records</span></span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<h1 id="SELECT-子句"><a href="#SELECT-子句" class="headerlink" title="SELECT 子句"></a>SELECT 子句</h1><p>从一张表（customers）中选取某些列</p>
<p><img src="/im
</summary>
<category term="Database" scheme="http://yoursite.com/categories/Database/"/>
<category term="job notes-database" scheme="http://yoursite.com/tags/job-notes-database/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(11)--Popular Python Package</title>
<link href="http://yoursite.com/2021/05/14/20210514_Python011/"/>
<id>http://yoursite.com/2021/05/14/20210514_Python011/</id>
<published>2021-05-14T08:50:09.000Z</published>
<updated>2021-05-17T04:24:23.058Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><h1 id="Web-Scraping"><a href="#Web-Scraping" class="headerlink" title="Web Scraping"></a>Web Scraping</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> requests</span><br><span class="line"><span class="keyword">from</span> bs4 <span class="keyword">import</span> BeautifulSoup</span><br><span class="line"></span><br><span class="line">response = requests.get(<span class="string">"https://stackoverflow.com/questions"</span>)</span><br><span class="line">soup = BeautifulSoup(response.text, <span class="string">"html.parser"</span>)</span><br><span class="line"></span><br><span class="line">questions = soup.select(<span class="string">".question-summary"</span>)</span><br><span class="line">print(questions[<span class="number">0</span>].attrs)</span><br><span class="line">print(questions[<span class="number">0</span>].get(<span class="string">"id"</span>, <span class="number">0</span>))</span><br><span class="line"></span><br><span class="line"><span class="keyword">for</span> question <span class="keyword">in</span> questions:</span><br><span class="line"> print(question.select_one(<span class="string">".question-hyperlink"</span>).getText())</span><br><span class="line"> print(question.select_one(<span class="string">".vote-count-post"</span>).getText())</span><br></pre></td></tr></table></figure><blockquote><p>{‘class’: [‘question-summary’], ‘id’: ‘question-summary-67532680’}<br>question-summary-67532680<br>GKE Nginx Ingress Controller Oauth2 Proxy redirect<br>0<br>Workaround on nested async completion blocks from network calls? Without using PromiseKit<br>0<br>ax.text not being printed when using transform<br>0<br>Using html & javascript Populate textbox<br>0<br>How to pass JSON data through the Django backend to frontend view using Angular<br>0<br>Gitlab CI cannot pull private registry with DOCKER_AUTH_CONFIG<br>0<br>Shopify SKU Lookup using GraphQL<br>0</p></blockquote><h1 id="Browser-Automation"><a href="#Browser-Automation" class="headerlink" title="Browser Automation"></a>Browser Automation</h1><p>自动打开chrome，跳转到github.com，登录。</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> selenium <span class="keyword">import</span> webdriver</span><br><span class="line"><span class="keyword">import</span> time</span><br><span class="line"></span><br><span class="line">browser = webdriver.Chrome()</span><br><span class="line">browser.set_window_size(<span class="number">1920</span>, <span class="number">1080</span>) <span class="comment"># 设置窗口大小</span></span><br><span class="line"></span><br><span class="line">browser.get(<span class="string">"https://github.com"</span>)</span><br><span class="line"></span><br><span class="line">signin_link = browser.find_element_by_link_text(<span class="string">"Sign in"</span>)</span><br><span class="line">signin_link.click()</span><br><span class="line"></span><br><span class="line">username_box = browser.find_element_by_id(<span class="string">"login_field"</span>)</span><br><span class="line">username_box.send_keys(<span class="string">"PurpleMStone"</span>)</span><br><span class="line">password_box = browser.find_element_by_id(<span class="string">"password"</span>)</span><br><span class="line">password_box.send_keys(<span class="string">"zq15071452268"</span>)</span><br><span class="line">password_box.submit()</span><br><span class="line"></span><br><span class="line"><span class="keyword">assert</span> <span class="string">"PurpleMStone"</span> <span class="keyword">in</span> browser.page_source</span><br><span class="line"></span><br><span class="line">time.sleep(<span class="number">0.5</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment"># profile_link = browser.find_element_by_class_name("user-profile-link")</span></span><br><span class="line"><span class="comment"># link_label = profile_link.get_attribute("innerHTML")</span></span><br><span class="line"><span class="comment"># assert "PurpleMStone" in link_label</span></span><br><span class="line"></span><br><span class="line">browser.quit()</span><br></pre></td></tr></table></figure><h1 id="Working-with-PDFs"><a href="#Working-with-PDFs" class="headerlink" title="Working with PDFs"></a>Working with PDFs</h1><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> PyPDF2</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> open(<span class="string">"first.pdf"</span>, <span class="string">"rb"</span>) <span class="keyword">as</span> file:</span><br><span class="line"> reader = PyPDF2.PdfFileReader(file)</span><br><span class="line"> print(reader.numPages)</span><br><span class="line"> page = reader.getPage(<span class="number">0</span>) <span class="comment"># 获取第一页</span></span><br><span class="line"> page.rotateClockwise(<span class="number">90</span>) <span class="comment"># 页面旋转90度</span></span><br><span class="line"> </span><br><span class="line"> writer = PyPDF2.PdfFileWriter()</span><br><span class="line"> writer.addPage(page)</span><br><span class="line"> <span class="keyword">with</span> open(<span class="string">"rotated.pdf"</span>, <span class="string">"wb"</span>) <span class="keyword">as</span> output:</span><br><span class="line"> writer.write(output)</span><br></pre></td></tr></table></figure><p><strong>Demo 2: 合并PDF</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> PyPDF2</span><br><span class="line"></span><br><span class="line">merger = PyPDF2.PdfFileMerger()</span><br><span class="line">file_names = [<span class="string">"first.pdf"</span>, <span class="string">"second.pdf"</span>]</span><br><span class="line"><span class="keyword">for</span> file_name <span class="keyword">in</span> file_names:</span><br><span class="line"> merger.append(file_name)</span><br><span class="line">merger.write(<span class="string">"combined.pdf"</span>)</span><br></pre></td></tr></table></figure><h1 id="Excel-Spreadsheets"><a href="#Excel-Spreadsheets" class="headerlink" title="Excel Spreadsheets"></a>Excel Spreadsheets</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> openpyxl</span><br><span class="line"></span><br><span class="line">wb = openpyxl.load_workbook(<span class="string">"transactions.xlsx"</span>)</span><br><span class="line">print(wb.sheetnames)</span><br><span class="line"></span><br><span class="line">sheet = wb[<span class="string">"Sheet1"</span>]</span><br><span class="line"></span><br><span class="line"><span class="comment"># wb.create_sheet(["Sheet2", 0])</span></span><br><span class="line"><span class="comment"># wb.remove_sheet(sheet)</span></span><br><span class="line"></span><br><span class="line">cell = sheet[<span class="string">"a1"</span>]</span><br><span class="line"><span class="comment"># print(cell.row)</span></span><br><span class="line"><span class="comment"># print(cell.column)</span></span><br><span class="line"><span class="comment"># print(cell.coordinate)</span></span><br><span class="line"><span class="comment"># cell = sheet.cell(row=1, column=1)</span></span><br><span class="line"></span><br><span class="line">print(sheet.max_row)</span><br><span class="line">print(sheet.max_column)</span><br><span class="line"></span><br><span class="line"><span class="keyword">for</span> row <span class="keyword">in</span> range(<span class="number">1</span>, sheet.max_row + <span class="number">1</span>):</span><br><span class="line"> <span class="keyword">for</span> column <span class="keyword">in</span> range(<span class="number">1</span>, sheet.max_column + <span class="number">1</span>):</span><br><span class="line"> cell = sheet.cell(row, column)</span><br><span class="line"> <span class="comment"># print(cell.value)</span></span><br><span class="line"></span><br><span class="line">column = sheet[<span class="string">"a"</span>]</span><br><span class="line">print(column)</span><br><span class="line">cells = sheet[<span class="string">"a:c"</span>] <span class="comment"># column</span></span><br><span class="line">print(cells)</span><br><span class="line"></span><br><span class="line">print(sheet[<span class="number">1</span>:<span class="number">3</span>]) <span class="comment"># row</span></span><br><span class="line"></span><br><span class="line">sheet.append([<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]) <span class="comment"># add a row at the end of the sheet</span></span><br><span class="line"><span class="comment"># sheet.insert_rows insert_columns, delete_rows, delete_columns</span></span><br><span class="line"></span><br><span class="line">wb.save(<span class="string">"transaction2.xlsx"</span>)</span><br></pre></td></tr></table></figure><h1 id="Numpy"><a href="#Numpy" class="headerlink" title="Numpy"></a>Numpy</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> numpy <span class="keyword">as</span> np</span><br><span class="line"></span><br><span class="line">array = np.array([[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>], [<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>]])</span><br><span class="line">print(array)</span><br><span class="line">print(type(np.array))</span><br><span class="line">print(array.shape)</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">array = np.zeros((<span class="number">3</span>, <span class="number">4</span>), dtype=int)</span><br><span class="line">print(array)</span><br><span class="line"></span><br><span class="line">array = np.ones((<span class="number">3</span>, <span class="number">4</span>), dtype=int)</span><br><span class="line">print(array)</span><br><span class="line"></span><br><span class="line">array = np.full((<span class="number">3</span>, <span class="number">4</span>), <span class="number">5</span>, dtype=int)</span><br><span class="line">print(array)</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">array = np.random.random((<span class="number">3</span>, <span class="number">4</span>))</span><br><span class="line">print(array)</span><br><span class="line">print(array[<span class="number">0</span>, <span class="number">0</span>])</span><br><span class="line"></span><br><span class="line">print(array > <span class="number">0.2</span>)</span><br><span class="line"></span><br><span class="line">print(array[array > <span class="number">0.2</span>])</span><br><span class="line"></span><br><span class="line">print(np.sum(array))</span><br><span class="line">print(np.floor(array))</span><br><span class="line">print(np.ceil(array))</span><br><span class="line">print(np.round(array))</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">first = np.array([<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>])</span><br><span class="line">second = np.array([<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>])</span><br><span class="line">print(first + second)</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<h1 id="Web-Scraping"><a href="#Web-Scraping" class=
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(10)--Python Package Index (Pypi)</title>
<link href="http://yoursite.com/2021/05/13/20210513_Python010/"/>
<id>http://yoursite.com/2021/05/13/20210513_Python010/</id>
<published>2021-05-13T15:50:09.000Z</published>
<updated>2021-05-17T04:24:23.053Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><p>Python标准库不一定够用。所以有：</p><p><a href="https://pypi.org/" target="_blank" rel="noopener">pypi官网</a></p><h1 id="Pip"><a href="#Pip" class="headerlink" title="Pip"></a>Pip</h1><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">pip install requests(安装包)</span><br><span class="line">pip list(查看已安装列表)</span><br><span class="line">pip install requests==2.9.0(安装早前版本的包)</span><br><span class="line">pip install requests==2.9.*(安装早前版本中符合条件的最近的包)</span><br><span class="line">pip install requests~=2.9.0(同上)</span><br><span class="line">pip uninstall requests(卸载)</span><br></pre></td></tr></table></figure><p><strong>使用</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> requests</span><br><span class="line"></span><br><span class="line">response = requests.get(<span class="string">"https://baidu.com"</span>)</span><br><span class="line">print(response)</span><br></pre></td></tr></table></figure><p>输出：</p><p><Response [200]></p><p>200表示成功。</p><h1 id="Virtual-Environments"><a href="#Virtual-Environments" class="headerlink" title="Virtual Environments"></a>Virtual Environments</h1><p>创建虚拟环境：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> python -m venv env</span><br></pre></td></tr></table></figure><p>可以看到当前目录下有一个env文件夹，里面有个pyvenv.cfg，内容如下:</p><blockquote><p>home = D:\Python39<br>include-system-site-packages = false<br>version = 3.9.4</p></blockquote><p>进入虚拟环境：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> env\Scripts\activate.bat</span><br></pre></td></tr></table></figure><p>退出虚拟环境：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> deactivate</span><br></pre></td></tr></table></figure><h2 id="Pipenv"><a href="#Pipenv" class="headerlink" title="Pipenv"></a>Pipenv</h2><p>先安装：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pip install pipenv</span><br></pre></td></tr></table></figure><p>查看虚拟环境在哪（一般不在当前工作目录下，为了节省空间）：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv --venv</span><br></pre></td></tr></table></figure><p>进入虚拟环境：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv shell</span><br></pre></td></tr></table></figure><p>退出虚拟环境:</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> <span class="built_in">exit</span></span><br></pre></td></tr></table></figure><h2 id="Virtual-Environments-in-VSCode"><a href="#Virtual-Environments-in-VSCode" class="headerlink" title="Virtual Environments in VSCode"></a>Virtual Environments in VSCode</h2><p>查看虚拟环境在哪：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv --venv</span><br></pre></td></tr></table></figure><p>得到路径: C:/Users/11405/.virtualenvs/HelloWorldBjQZ8Ory</p><p>在VSCode中，File→Preference→settings，打开settings.json，改这两项（没有就自己加）：</p><figure class="highlight"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">"python.pythonPath": "C:/Users/11405/.virtualenvs/HelloWorldBjQZ8Ory/Scripts/python.exe",</span><br><span class="line">"code-runner.executorMap": {</span><br><span class="line"> "python": "C:/Users/11405/.virtualenvs/HelloWorldBjQZ8Ory/Scripts/python.exe",</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>点击VSCode左下角，然后选择pipenv为解释器,即可。</p><h2 id="Pipfile"><a href="#Pipfile" class="headerlink" title="Pipfile"></a>Pipfile</h2><p>在用pipenv创建虚拟环境时，当前目录（不是虚拟环境所在目录）下会生成两个文件：</p><blockquote><p>Pipfile</p><p>Pipfile.lock</p></blockquote><p>Pipfile文件：</p><blockquote><p>[[source]]<br>url = “<a href="https://pypi.org/simple" target="_blank" rel="noopener">https://pypi.org/simple</a>“<br>verify_ssl = true<br>name = “pypi”</p><p>[packages]<br>requests = “*”</p><p>[dev-packages]<br>pylint = “*”</p><p>[requires]<br>python_version = “3.9”</p></blockquote><p>Pipfile.lock是一个json文件，存了所有包的版本，所以换一台机器跑这个代码，依赖的还是这些版本的包，就不会出问题。</p><p>在另一台机器，只要：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv install</span><br></pre></td></tr></table></figure><p>就会根据Pipfile创建一样的环境</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv install --ignore-pipfile</span><br></pre></td></tr></table></figure><p>就会根据Pipfile.lock安装依赖包。</p><h2 id="Managing-the-Dependencies"><a href="#Managing-the-Dependencies" class="headerlink" title="Managing the Dependencies"></a>Managing the Dependencies</h2><p>可以用</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv graph</span><br></pre></td></tr></table></figure><p>看已安装的包及其依赖项。</p><p>卸载某个包后，再用上述命令查看，会发现依赖项还在（因为这些依赖项可能在别的地方有用的）。</p><p>更新包和依赖项(想安装最新的，就改Pipfile中的版本号为*)：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv update requests</span><br></pre></td></tr></table></figure><p>更新所有包的依赖项：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pipenv update --outdated</span><br></pre></td></tr></table></figure><h1 id="Publishing-Packages"><a href="#Publishing-Packages" class="headerlink" title="Publishing Packages"></a>Publishing Packages</h1><p>在pypi官网上注册一个账号（记得激活邮箱）。</p><p>然后安装：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> pip install setuptools wheel twine</span><br></pre></td></tr></table></figure><p>新建一个项目，包含 stonepdf，test（单元测试），data（各种数据） 三个文件夹。</p><p>需要以下文件：</p><ol><li><p><strong>LICENSE</strong>：到 <a href="https://choosealicense.com/" target="_blank" rel="noopener">https://choosealicense.com/</a> copy一个</p></li><li><p><strong>README.md</strong>：介绍你的项目</p></li><li><p><strong>setup.py</strong></p></li></ol><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># setup.py</span></span><br><span class="line"><span class="keyword">import</span> setuptools</span><br><span class="line"><span class="keyword">import</span> setuptools</span><br><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">setuptools.setup(</span><br><span class="line"> name=<span class="string">"stonepdf"</span>,</span><br><span class="line"> version=<span class="number">1.0</span>,</span><br><span class="line"> long_description=Path(<span class="string">"README.md"</span>).read_text(),</span><br><span class="line"> packages=setuptools.find_packages(exclude=[<span class="string">"tests"</span>, <span class="string">"data"</span>]) <span class="comment"># 忽略这两个</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure><p>然后运行（sdist表示source distribution, bdist表示built distribution）：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> python setup.py sdist bdist_wheel</span><br></pre></td></tr></table></figure><p>然后上传：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> twine upload dist/*</span><br></pre></td></tr></table></figure><p>*表示上传dist目录下所有文件。</p><p>然后就可以像用其他包一样用我们的包啦。</p><h1 id="Docstrings"><a href="#Docstrings" class="headerlink" title="Docstrings"></a>Docstrings</h1><p>其他人提供的module在VSCode中有许多提示性信息显示出来，这就需要Docstrings。</p><p>比如（用””” “””包含的部分）：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="string">""" This module provides functions to convert a PDF to text. """</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Converter</span>:</span></span><br><span class="line"> <span class="string">""" A simple converter for converting PDFs to text. """</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">convert</span><span class="params">(self, path)</span>:</span></span><br><span class="line"> <span class="string">"""</span></span><br><span class="line"><span class="string"> Convert the given PDF to text. </span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string"> Parameters:</span></span><br><span class="line"><span class="string"> path (str): The path to a PDF file.</span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string"> Returns:</span></span><br><span class="line"><span class="string"> str: The content of the PDF file as text.</span></span><br><span class="line"><span class="string"> """</span></span><br><span class="line"></span><br><span class="line"> print(<span class="string">"pdf2text"</span>)</span><br></pre></td></tr></table></figure><p>这些信息在 VSCode 调用这些 module 的 method、class 时会显示出来，便于别人使用。</p><p><img src="/images/Python/003.png" alt="003"></p><h1 id="Pydoc"><a href="#Pydoc" class="headerlink" title="Pydoc"></a>Pydoc</h1><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">> pydoc math</span><br><span class="line">> pydoc stonepdf.pdf2text</span><br><span class="line">> pydoc -w stonepdf.pdf2text</span><br><span class="line">> pydoc -p 1234</span><br></pre></td></tr></table></figure><p>查看包的介绍文件。-w在html中显示。-p会提供网址。</p><p>在 Windows 用：</p><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">> python -m pydoc math</span><br></pre></td></tr></table></figure><p>其余同理。</p>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<p>Python标准库不一定够用。所以有：</p>
<p><a href="https://pypi.
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(9)--Python Standard Library</title>
<link href="http://yoursite.com/2021/05/12/20210512_Python009/"/>
<id>http://yoursite.com/2021/05/12/20210512_Python009/</id>
<published>2021-05-12T15:50:09.000Z</published>
<updated>2021-05-17T04:24:23.046Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><h1 id="Files"><a href="#Files" class="headerlink" title="Files"></a>Files</h1><h2 id="Paths"><a href="#Paths" class="headerlink" title="Paths"></a>Paths</h2><p><strong>Demo 1: Basic Methods</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">path = Path(<span class="string">r"ecommerce/__init__.py"</span>) <span class="comment"># create a path object</span></span><br><span class="line">print(path.exists()) <span class="comment"># file exists or not</span></span><br><span class="line">print(path.is_file()) <span class="comment"># is a file or not</span></span><br><span class="line">print(path.is_dir())<span class="comment"># is a directory or not</span></span><br><span class="line">print(path.name)</span><br><span class="line">print(path.stem) <span class="comment"># return the file name without extension</span></span><br><span class="line">print(path.suffix) <span class="comment"># return the extension</span></span><br><span class="line">print(path.parent) <span class="comment"># parent folder</span></span><br></pre></td></tr></table></figure><p>输出结果：</p><p>True<br>True<br>False<br>__init__.py<br>__init__<br>.py<br>ecommerce</p><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">path = Path(<span class="string">r"ecommerce/__init__.py"</span>) <span class="comment"># create a path object</span></span><br><span class="line"></span><br><span class="line">path = path.with_name(<span class="string">"file.txt"</span>) <span class="comment"># 产生一个新path object，(文件尚未产生)</span></span><br><span class="line">print(path)</span><br><span class="line"></span><br><span class="line">print(path.absolute())</span><br><span class="line"></span><br><span class="line">path = path.with_suffix(<span class="string">".txt"</span>) <span class="comment"># 产生一个新path object，(文件尚未产生)</span></span><br><span class="line">print(path)</span><br></pre></td></tr></table></figure><p>输出：</p><p>ecommerce\file.txt<br>e:\Stone\Python\HelloWorld\ecommerce\file.txt<br>ecommerce\file.txt</p><h2 id="Working-with-Directories"><a href="#Working-with-Directories" class="headerlink" title="Working with Directories"></a>Working with Directories</h2><p><strong>Demo 1: Basic Methods</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">path = Path(<span class="string">"ecommerce"</span>)</span><br><span class="line">path.exists()</span><br><span class="line">path.mkdir() <span class="comment"># creat this directory</span></span><br><span class="line">path.rmdir() <span class="comment"># remove this directory</span></span><br><span class="line">path.rename(<span class="string">"ecommerce2"</span>) <span class="comment"># rename</span></span><br></pre></td></tr></table></figure><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">path = Path(<span class="string">"ecommerce"</span>)</span><br><span class="line">print(path.iterdir())</span><br></pre></td></tr></table></figure><p>输出：</p><p><generator object Path.iterdir at 0x00000180577926D0></p><p>可见，得到的是一个generator object。因为目录下可能有大量文件，generator 更高效。</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">paths = [p <span class="keyword">for</span> p <span class="keyword">in</span> path.iterdir()]</span><br><span class="line">print(paths)</span><br></pre></td></tr></table></figure><p>输出：</p><p>[WindowsPath(‘ecommerce/customer’), WindowsPath(‘ecommerce/shopping’), WindowsPath(‘ecommerce/__init__.py’), WindowsPath(‘ecommerce/__pycache__‘)]</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># filter: 只获取目录</span></span><br><span class="line">paths = [p <span class="keyword">for</span> p <span class="keyword">in</span> path.iterdir() <span class="keyword">if</span> p.is_dir()]</span><br><span class="line">print(paths)</span><br></pre></td></tr></table></figure><p>输出：</p><p>[WindowsPath(‘ecommerce/customer’), WindowsPath(‘ecommerce/shopping’), WindowsPath(‘ecommerce/__pycache__‘)]</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># 当前目录下符合条件的文件</span></span><br><span class="line">py_files = [p <span class="keyword">for</span> p <span class="keyword">in</span> path.glob(<span class="string">"*.py"</span>)]</span><br><span class="line">print(py_files)</span><br></pre></td></tr></table></figure><p>输出：</p><p>[WindowsPath(‘ecommerce/__init__.py’)]</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># recursively (包括子目录下符合条件的文件)</span></span><br><span class="line">py_files = [p <span class="keyword">for</span> p <span class="keyword">in</span> path.rglob(<span class="string">"*.py"</span>)]</span><br><span class="line">print(py_files)</span><br></pre></td></tr></table></figure><p>输出：</p><p>[WindowsPath(‘ecommerce/__init__.py’), WindowsPath(‘ecommerce/customer/contact.py’), WindowsPath(‘ecommerce/customer/__init__.py’), WindowsPath(‘ecommerce/shopping/sales.py’), WindowsPath(‘ecommerce/shopping/__init__.py’)]</p><h2 id="Working-with-Files"><a href="#Working-with-Files" class="headerlink" title="Working with Files"></a>Working with Files</h2><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">path = Path(<span class="string">"ecommerce/__init__.py"</span>)</span><br><span class="line">path.exists()</span><br><span class="line">path.rename(<span class="string">"init.txt"</span>)</span><br><span class="line">path.unlink() <span class="comment"># delete the file</span></span><br></pre></td></tr></table></figure><p><strong>Demo 2: 查看文件状态</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">print(path.stat())</span><br></pre></td></tr></table></figure><p>输出：</p><p>os.stat_result(st_mode=33206, st_ino=1688849860483389, st_dev=971387, st_nlink=1, st_uid=0, st_gid=0, st_size=32, st_atime=1620876102, st_mtime=1620816590, st_ctime=1620814908)</p><p>将时间转为容易识别的形式：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> time <span class="keyword">import</span> ctime</span><br><span class="line"></span><br><span class="line">print(ctime(path.stat().st_ctime))<span class="comment"># 打印文件的新建时间</span></span><br></pre></td></tr></table></figure><p>输出：</p><p>Wed May 12 18:21:48 2021</p><p><strong>Demo 3</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">print(path.read_bytes())</span><br><span class="line">print(path.read_text())</span><br><span class="line">path.write_bytes(<span class="string">"..."</span>)</span><br><span class="line">path.write_text(<span class="string">"..."</span>)</span><br></pre></td></tr></table></figure><p>输出：</p><p>b’print(“Ecommerce initialized”)\r\n’<br>print(“Ecommerce initialized”)</p><p><strong>Demo 4：拷贝文件</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"><span class="keyword">import</span> shutil</span><br><span class="line"></span><br><span class="line">source = Path(<span class="string">"ecommerce/__init__.py"</span>)</span><br><span class="line">target = Path() / <span class="string">"__init__.py"</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># target.write_text(source.read_text())# 简单粗暴：读->写</span></span><br><span class="line">shutil.copy(source, target)</span><br></pre></td></tr></table></figure><h2 id="Working-with-Zip-Files"><a href="#Working-with-Zip-Files" class="headerlink" title="Working with Zip Files"></a>Working with Zip Files</h2><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"><span class="keyword">from</span> zipfile <span class="keyword">import</span> ZipFile</span><br><span class="line"></span><br><span class="line">zip = ZipFile(<span class="string">"files.zip"</span>, <span class="string">"w"</span>)<span class="comment"># 生成一个zip文件</span></span><br><span class="line"><span class="keyword">for</span> path <span class="keyword">in</span> Path(<span class="string">"ecommerce"</span>).rglob(<span class="string">"*.*"</span>):</span><br><span class="line"> zip.write(path)<span class="comment"># 将这些文件存到zip文件中</span></span><br><span class="line">zip.close()<span class="comment"># 关闭</span></span><br></pre></td></tr></table></figure><p><strong>Simpler way</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"><span class="keyword">from</span> zipfile <span class="keyword">import</span> ZipFile</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> ZipFile(<span class="string">"files.zip"</span>, <span class="string">"w"</span>) <span class="keyword">as</span> zip:</span><br><span class="line"> <span class="keyword">for</span> path <span class="keyword">in</span> Path(<span class="string">"ecommerce"</span>).rglob(<span class="string">"*.*"</span>):</span><br><span class="line"> zip.write(path)</span><br></pre></td></tr></table></figure><p>不需要手动close释放。</p><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"><span class="keyword">from</span> zipfile <span class="keyword">import</span> ZipFile</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> ZipFile(<span class="string">"files.zip"</span>) <span class="keyword">as</span> zip:</span><br><span class="line"> <span class="comment"># print(zip.namelist())# zip中的文件列表</span></span><br><span class="line"> info = zip.getinfo(<span class="string">"ecommerce/__init__.py"</span>)<span class="comment"># 获取文件信息</span></span><br><span class="line"> print(info.file_size)<span class="comment"># 文件大小</span></span><br><span class="line"> print(info.compress_size)<span class="comment"># 压缩大小</span></span><br><span class="line"> zip.extractall(<span class="string">"extract"</span>)<span class="comment"># 解压到extract文件夹</span></span><br></pre></td></tr></table></figure><h2 id="Working-with-CSV-Files"><a href="#Working-with-CSV-Files" class="headerlink" title="Working with CSV Files"></a>Working with CSV Files</h2><p><strong>Demo 1: 写csv</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> csv</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> open(<span class="string">"data.csv"</span>, <span class="string">"w"</span>, newline=<span class="string">''</span>) <span class="keyword">as</span> file:</span><br><span class="line"> writer = csv.writer(file)</span><br><span class="line"> writer.writerow([<span class="string">"transaction_id"</span>, <span class="string">"product_id"</span>, <span class="string">"price"</span>])</span><br><span class="line"> writer.writerow([<span class="number">1000</span>, <span class="number">1</span>, <span class="number">5</span>])</span><br><span class="line"> writer.writerow([<span class="number">1001</span>, <span class="number">2</span>, <span class="number">15</span>])</span><br></pre></td></tr></table></figure><p>如果不加newline=’’，文件会多空行</p><p><strong>Demo 2: 读csv</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> csv</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> open(<span class="string">"data.csv"</span>) <span class="keyword">as</span> file:</span><br><span class="line"> reader = csv.reader(file)</span><br><span class="line"> print(list(reader))</span><br></pre></td></tr></table></figure><p>输出：</p><p>[[‘transaction_id’, ‘product_id’, ‘price’], [‘1000’, ‘1’, ‘5’], [‘1001’, ‘2’, ‘15’]]</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> csv</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> open(<span class="string">"data.csv"</span>) <span class="keyword">as</span> file:</span><br><span class="line"> reader = csv.reader(file)</span><br><span class="line"> <span class="keyword">for</span> row <span class="keyword">in</span> reader:<span class="comment"># reader is iterable</span></span><br><span class="line"> print(row)</span><br></pre></td></tr></table></figure><p>输出：</p><p>[‘transaction_id’, ‘product_id’, ‘price’]<br>[‘1000’, ‘1’, ‘5’]<br>[‘1001’, ‘2’, ‘15’]</p><h2 id="Working-with-JSON-Files"><a href="#Working-with-JSON-Files" class="headerlink" title="Working with JSON Files"></a>Working with JSON Files</h2><p><strong>Demo 1: write JSON files</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> json</span><br><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">movies = [</span><br><span class="line"> {<span class="string">"id"</span>: <span class="number">1</span>, <span class="string">"title"</span>: <span class="string">"Terminator"</span>, <span class="string">"year"</span>: <span class="number">1989</span>},</span><br><span class="line"> {<span class="string">"id"</span>: <span class="number">2</span>, <span class="string">"title"</span>: <span class="string">"Kindergarten Cop"</span>, <span class="string">"year"</span>: <span class="number">1993</span>},</span><br><span class="line">]</span><br><span class="line"></span><br><span class="line">data = json.dumps(movies)</span><br><span class="line">Path(<span class="string">"movies.json"</span>).write_text(data)</span><br></pre></td></tr></table></figure><p><strong>Demo 2: read JSON Files</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> json</span><br><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">data = Path(<span class="string">"movies.json"</span>).read_text()</span><br><span class="line">movies = json.loads(data)</span><br><span class="line">print(movies[<span class="number">0</span>])</span><br></pre></td></tr></table></figure><p>输出：</p><p>{‘id’: 1, ‘title’: ‘Terminator’, ‘year’: 1989}</p><h2 id="Working-with-a-SQLite-Database"><a href="#Working-with-a-SQLite-Database" class="headerlink" title="Working with a SQLite Database"></a>Working with a SQLite Database</h2><p><strong>Demo 1: write</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sqlite3</span><br><span class="line"><span class="keyword">import</span> json</span><br><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"></span><br><span class="line">movies = json.loads(Path(<span class="string">"movies.json"</span>).read_text())</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> sqlite3.connect(<span class="string">"db.sqlite3"</span>) <span class="keyword">as</span> conn:</span><br><span class="line"> command = <span class="string">"INSERT INTO Movies VALUES(?, ?, ?)"</span></span><br><span class="line"> <span class="keyword">for</span> movie <span class="keyword">in</span> movies:</span><br><span class="line"> conn.execute(command, tuple(movie.values()))</span><br><span class="line"> conn.commit()</span><br></pre></td></tr></table></figure><p><strong>Demo 2: read</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sqlite3</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> sqlite3.connect(<span class="string">"db.sqlite3"</span>) <span class="keyword">as</span> conn:</span><br><span class="line"> command = <span class="string">"SELECT * FROM Movies"</span></span><br><span class="line"> cursor = conn.execute(command)</span><br><span class="line"> <span class="keyword">for</span> row <span class="keyword">in</span> cursor:</span><br><span class="line"> print(row)</span><br></pre></td></tr></table></figure><p>或：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sqlite3</span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> sqlite3.connect(<span class="string">"db.sqlite3"</span>) <span class="keyword">as</span> conn:</span><br><span class="line"> command = <span class="string">"SELECT * FROM Movies"</span></span><br><span class="line"> cursor = conn.execute(command)</span><br><span class="line"> movies = cursor.fetchall()</span><br><span class="line"> print(movies)</span><br></pre></td></tr></table></figure><h1 id="Time"><a href="#Time" class="headerlink" title="Time"></a>Time</h1><h2 id="Working-with-Timestamps"><a href="#Working-with-Timestamps" class="headerlink" title="Working with Timestamps"></a>Working with Timestamps</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> time</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">send_emails</span><span class="params">()</span>:</span></span><br><span class="line"> <span class="keyword">for</span> i <span class="keyword">in</span> range(<span class="number">1000000</span>):</span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">start = time.time()</span><br><span class="line">send_emails()</span><br><span class="line">end = time.time()</span><br><span class="line">duration = end - start</span><br><span class="line">print(duration)</span><br></pre></td></tr></table></figure><p>输出：</p><p>0.016989707946777344</p><h2 id="Working-with-DateTimes"><a href="#Working-with-DateTimes" class="headerlink" title="Working with DateTimes"></a>Working with DateTimes</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> datetime <span class="keyword">import</span> datetime</span><br><span class="line"><span class="keyword">import</span> time</span><br><span class="line"></span><br><span class="line">dt1 = datetime(<span class="number">2018</span>, <span class="number">1</span>, <span class="number">1</span>)</span><br><span class="line">print(dt1)<span class="comment"># 2018-01-01 00:00:00</span></span><br><span class="line">dt2 = datetime.now()</span><br><span class="line">print(dt2)<span class="comment"># 2021-05-13 16:51:52.269074</span></span><br><span class="line">print(dt2 > dt1)<span class="comment"># True</span></span><br><span class="line"></span><br><span class="line">dt = datetime.strptime(<span class="string">"2018/01/01"</span>, <span class="string">"%Y/%m/%d"</span>) <span class="comment"># 指定时间</span></span><br><span class="line">print(dt)<span class="comment"># 2018-01-01 00:00:00</span></span><br><span class="line"></span><br><span class="line">dt = datetime.fromtimestamp(time.time())</span><br><span class="line">print(<span class="string">f"<span class="subst">{dt.year}</span>/<span class="subst">{dt.month}</span>"</span>)<span class="comment"># 2021/5</span></span><br><span class="line">print(dt.strftime(<span class="string">"%Y/%m"</span>)) <span class="comment"># 2021/05 (当地时间)</span></span><br></pre></td></tr></table></figure><h2 id="Working-with-Time-Deltas"><a href="#Working-with-Time-Deltas" class="headerlink" title="Working with Time Deltas"></a>Working with Time Deltas</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> datetime <span class="keyword">import</span> datetime, timedelta</span><br><span class="line"></span><br><span class="line">dt1 = datetime(<span class="number">2020</span>, <span class="number">1</span>, <span class="number">1</span>) + timedelta(days=<span class="number">1</span>, seconds=<span class="number">1000</span>)</span><br><span class="line">print(dt1)</span><br><span class="line">dt2 = datetime.now()</span><br><span class="line"></span><br><span class="line">duration = dt2 - dt1</span><br><span class="line">print(duration)</span><br><span class="line">print(<span class="string">"days"</span>, duration.days)</span><br><span class="line">print(<span class="string">"seconds"</span>, duration.seconds)<span class="comment"># 不计天数</span></span><br><span class="line">print(<span class="string">"total_seconds"</span>, duration.total_seconds())<span class="comment"># 计天数</span></span><br></pre></td></tr></table></figure><p>输出：</p><p>2020-01-02 00:16:40<br>497 days, 16:44:06.578196<br>days 497<br>seconds 60246<br>total_seconds 43001046.578196</p><h1 id="Random-Values"><a href="#Random-Values" class="headerlink" title="Random Values"></a>Random Values</h1><h2 id="Generating-Random-Values"><a href="#Generating-Random-Values" class="headerlink" title="Generating Random Values"></a>Generating Random Values</h2><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> random</span><br><span class="line"><span class="keyword">import</span> string</span><br><span class="line"></span><br><span class="line">print(random.random())</span><br><span class="line">print(random.randint(<span class="number">1</span>, <span class="number">10</span>))</span><br><span class="line">print(random.choice([<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]))<span class="comment"># 从给定的值中随机选一个</span></span><br><span class="line">print(random.choices([<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>], k=<span class="number">4</span>))<span class="comment"># 从给定的值中随机选出序列</span></span><br><span class="line">print(<span class="string">""</span>.join(random.choices(<span class="string">"abcdefg"</span>, k=<span class="number">4</span>)))</span><br></pre></td></tr></table></figure><p>输出：</p><p>0.9791978038762406<br>2<br>3<br>[4, 1, 3, 3]<br>eefe</p><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> random</span><br><span class="line"><span class="keyword">import</span> string</span><br><span class="line"></span><br><span class="line">print(string.ascii_letters)<span class="comment"># 所有字母</span></span><br><span class="line">print(string.digits)<span class="comment"># 所有数字</span></span><br><span class="line">print(<span class="string">""</span>.join(random.choices(string.ascii_letters + string.digits, k=<span class="number">4</span>)))</span><br></pre></td></tr></table></figure><p>输出：</p><p>abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ<br>0123456789<br>g02V</p><p><strong>Demo 3：打乱顺序</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> random</span><br><span class="line"></span><br><span class="line">numbers = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]</span><br><span class="line">random.shuffle(numbers)</span><br><span class="line">print(numbers)</span><br></pre></td></tr></table></figure><p>输出：</p><p>[4, 2, 3, 1]</p><h1 id="Browser"><a href="#Browser" class="headerlink" title="Browser"></a>Browser</h1><h2 id="Opening-the-Browser"><a href="#Opening-the-Browser" class="headerlink" title="Opening the Browser"></a>Opening the Browser</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> webbrowser</span><br><span class="line"></span><br><span class="line">print(<span class="string">"Deployment completed"</span>)</span><br><span class="line">webbrowser.open(<span class="string">"https://baidu.com"</span>)</span><br></pre></td></tr></table></figure><h2 id="Sending-Emails"><a href="#Sending-Emails" class="headerlink" title="Sending Emails"></a>Sending Emails</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> email.mime.multipart <span class="keyword">import</span> MIMEMultipart</span><br><span class="line"><span class="keyword">from</span> email.mime.text <span class="keyword">import</span> MIMEText</span><br><span class="line"><span class="keyword">from</span> email.mime.image <span class="keyword">import</span> MIMEImage</span><br><span class="line"><span class="keyword">from</span> pathlib <span class="keyword">import</span> Path</span><br><span class="line"><span class="keyword">import</span> smtplib</span><br><span class="line"></span><br><span class="line">message = MIMEMultipart()</span><br><span class="line">message[<span class="string">"from"</span>] = <span class="string">"name_of_sender"</span></span><br><span class="line">message[<span class="string">"to"</span>] = <span class="string">"[email protected]"</span></span><br><span class="line">message[<span class="string">"subject"</span>] = <span class="string">"This is a test"</span></span><br><span class="line">message.attach(MIMEText(<span class="string">"Body"</span>))<span class="comment"># 正文</span></span><br><span class="line">message.attach(MIMEImage(Path(<span class="string">"ww.jpg"</span>).read_bytes()))<span class="comment"># 附件图片</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">with</span> smtplib.SMTP(host=<span class="string">"smtp.whu.edu.cn"</span>, port=<span class="number">25</span>) <span class="keyword">as</span> smtp:</span><br><span class="line"> smtp.ehlo() <span class="comment"># say hello to smtp server</span></span><br><span class="line"> smtp.starttls() <span class="comment"># TLS</span></span><br><span class="line"> smtp.login(<span class="string">"[email protected]"</span>, <span class="string">"password"</span>)</span><br><span class="line"> smtp.send_message(message)</span><br><span class="line"> print(<span class="string">"Sent..."</span>)</span><br></pre></td></tr></table></figure><h2 id="Working-with-Templates"><a href="#Working-with-Templates" class="headerlink" title="Working with Templates"></a>Working with Templates</h2><p>新建一个html文件 template.html，输入！回车会自动生成模板，删除head块中的内容，body块写入内容。最后如下：</p><figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta"><!DOCTYPE html></span></span><br><span class="line"><span class="tag"><<span class="name">html</span> <span class="attr">lang</span>=<span class="string">"en"</span>></span></span><br><span class="line"></span><br><span class="line"><span class="tag"><<span class="name">head</span>></span></span><br><span class="line"><span class="tag"></<span class="name">head</span>></span></span><br><span class="line"></span><br><span class="line"><span class="tag"><<span class="name">body</span>></span></span><br><span class="line"> Hi <span class="tag"><<span class="name">strong</span>></span>$name<span class="tag"></<span class="name">strong</span>></span>, this is our test email.</span><br><span class="line"><span class="tag"></<span class="name">body</span>></span></span><br><span class="line"></span><br><span class="line"><span class="tag"></<span class="name">html</span>></span></span><br></pre></td></tr></table></figure><p>strong表示字体加粗。</p><p>在上述email例子中：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> string <span class="keyword">import</span> Template</span><br><span class="line"></span><br><span class="line">template = Template(Path(<span class="string">"template.html"</span>).read_text())<span class="comment"># 读入模板</span></span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># 相应语句改为</span></span><br><span class="line">body = template.substitute({<span class="string">"name"</span>: <span class="string">"John"</span>}) <span class="comment"># 或者name="John"</span></span><br><span class="line">message.attach(MIMEText(body, <span class="string">"html"</span>))<span class="comment"># "html"</span></span><br></pre></td></tr></table></figure><p>即可。</p><h1 id="Command-line-Arguments"><a href="#Command-line-Arguments" class="headerlink" title="Command-line Arguments"></a>Command-line Arguments</h1><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sys</span><br><span class="line"></span><br><span class="line">print(sys.argv)</span><br></pre></td></tr></table></figure><p>>python app.py -a -b -c</p><p>[‘app.py’, ‘-a’, ‘-b’, ‘-c’]</p><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sys</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> len(sys.argv) == <span class="number">1</span>:</span><br><span class="line"> print(<span class="string">"USAGE: python3 app.py <password>"</span>)</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line"> password = sys.argv[<span class="number">1</span>]</span><br><span class="line"> print(<span class="string">"password"</span>, password)</span><br></pre></td></tr></table></figure><p>> python app.py</p><p>USAGE: python3 app.py <password></p><p>> python app.py 1234</p><p>password 1234</p><h1 id="Running-External-Programs"><a href="#Running-External-Programs" class="headerlink" title="Running External Programs"></a>Running External Programs</h1><h2 id="Python执行命令"><a href="#Python执行命令" class="headerlink" title="Python执行命令"></a>Python执行命令</h2><p>在 window 10 上跑的。</p><p>用python 执行windows命令dir</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> subprocess</span><br><span class="line"></span><br><span class="line">completed = subprocess.run([<span class="string">"dir"</span>, <span class="string">"."</span>], </span><br><span class="line"> shell=<span class="literal">True</span>,</span><br><span class="line"> capture_output=<span class="literal">True</span>, </span><br><span class="line"> text=<span class="literal">True</span>)</span><br><span class="line">print(type(completed))</span><br></pre></td></tr></table></figure><p>capture_output=True使得输出在completed.stdout。</p><p>输出：</p><p><class ‘subprocess.CompletedProcess’></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">print(<span class="string">"args"</span>, completed.args)</span><br><span class="line">print(<span class="string">"returncode"</span>, completed.returncode) <span class="comment"># 0: success</span></span><br><span class="line">print(<span class="string">"stderr"</span>, completed.stderr) <span class="comment"># standard error</span></span><br><span class="line">print(<span class="string">"stdout"</span>, completed.stdout) <span class="comment"># standard output</span></span><br></pre></td></tr></table></figure><p>输出：</p><p>> python app.py</p><p>args [‘dir’, ‘.’]<br>returncode 0<br>stderr<br>stdout 驱动器 E 中的卷没有标签。<br> 卷的序列号是 000E-D27B</p><p> E:\Stone\Python\HelloWorld 的目录</p><p>2021/05/13 19:24 <DIR> .<br>2021/05/13 19:24 <DIR> ..<br>2021/05/06 16:44 <DIR> .vscode<br>2021/05/13 19:24 341 app.py<br>2021/05/06 16:15 8 content.txt<br>2021/05/13 15:36 54 data.csv<br>2021/05/13 15:55 0 db.sqlite3<br>2021/05/12 18:49 <DIR> ecommerce<br>2021/05/13 15:27 <DIR> extract<br>2021/05/13 14:42 3,647 files.zip<br>2021/05/13 15:48 102 movies.json<br>2021/05/13 18:03 138 template.html<br>2021/05/13 17:42 78,556 ww.jpg<br>2021/05/12 18:12 <DIR> __pycache__<br> 8 个文件 82,846 字节<br> 6 个目录 46,196,027,392 可用字节</p><h2 id="Python执行另一个python脚本"><a href="#Python执行另一个python脚本" class="headerlink" title="Python执行另一个python脚本"></a>Python执行另一个python脚本</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># other.py</span></span><br><span class="line">print(<span class="string">"Here is a complicated script."</span>)</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># app.py</span></span><br><span class="line"><span class="keyword">import</span> subprocess</span><br><span class="line"></span><br><span class="line">completed = subprocess.run([<span class="string">"python"</span>, <span class="string">"other.py"</span>], </span><br><span class="line"> shell=<span class="literal">True</span>,</span><br><span class="line"> capture_output=<span class="literal">True</span>, </span><br><span class="line"> text=<span class="literal">True</span>)</span><br><span class="line">print(<span class="string">"args"</span>, completed.args)</span><br><span class="line">print(<span class="string">"returncode"</span>, completed.returncode) <span class="comment"># 0: success</span></span><br><span class="line">print(<span class="string">"stderr"</span>, completed.stderr) <span class="comment"># standard error</span></span><br><span class="line">print(<span class="string">"stdout"</span>, completed.stdout) <span class="comment"># standard output</span></span><br></pre></td></tr></table></figure><p>> python app.py</p><p>args [‘python’, ‘other.py’]<br>returncode 0<br>stderr<br>stdout Here is a complicated script.</p><p>看到 stdout 就是 other.py 的输出。</p><h2 id="错误命令"><a href="#错误命令" class="headerlink" title="错误命令"></a>错误命令</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> subprocess</span><br><span class="line"></span><br><span class="line">completed = subprocess.run([<span class="string">"False"</span>], </span><br><span class="line"> shell=<span class="literal">True</span>,</span><br><span class="line"> capture_output=<span class="literal">True</span>, </span><br><span class="line"> text=<span class="literal">True</span>)</span><br><span class="line">print(<span class="string">"args"</span>, completed.args)</span><br><span class="line">print(<span class="string">"returncode"</span>, completed.returncode) <span class="comment"># 0: success</span></span><br><span class="line">print(<span class="string">"stderr"</span>, completed.stderr) <span class="comment"># standard error</span></span><br><span class="line">print(<span class="string">"stdout"</span>, completed.stdout) <span class="comment"># standard output</span></span><br></pre></td></tr></table></figure><p>> python app.py</p><p>args [‘False’]<br>returncode 1<br>stderr ‘False’ 不是内部或外部命令，也不是可运行的程序<br>或批处理文件。</p><p>stdout</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> completed.returncode != <span class="number">0</span>:<span class="comment"># 可以这样判断处理</span></span><br><span class="line"> print(completed.stderr)</span><br></pre></td></tr></table></figure><p>也可以这样处理：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> subprocess</span><br><span class="line"></span><br><span class="line"><span class="keyword">try</span>:</span><br><span class="line"> completed = subprocess.run([<span class="string">"False"</span>],</span><br><span class="line"> shell=<span class="literal">True</span>,</span><br><span class="line"> capture_output=<span class="literal">True</span>,</span><br><span class="line"> text=<span class="literal">True</span>,</span><br><span class="line"> check=<span class="literal">True</span>)<span class="comment"># 自动检查</span></span><br><span class="line"> print(<span class="string">"args"</span>, completed.args)</span><br><span class="line"> print(<span class="string">"returncode"</span>, completed.returncode) <span class="comment"># 0: success</span></span><br><span class="line"> print(<span class="string">"stderr"</span>, completed.stderr) <span class="comment"># standard error</span></span><br><span class="line"> print(<span class="string">"stdout"</span>, completed.stdout) <span class="comment"># standard output</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">except</span> subprocess.CalledProcessError <span class="keyword">as</span> ex:</span><br><span class="line"> print(ex)</span><br></pre></td></tr></table></figure><p>输出：</p><p>Command ‘[‘False’]’ returned non-zero exit status 1.</p>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<h1 id="Files"><a href="#Files" class="headerlink" t
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(8)--Modules</title>
<link href="http://yoursite.com/2021/05/12/20210512_Python008/"/>
<id>http://yoursite.com/2021/05/12/20210512_Python008/</id>
<published>2021-05-12T09:50:09.000Z</published>
<updated>2021-05-17T04:24:23.039Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># sales.py</span></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">calc_tax</span><span class="params">()</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">calc_shipping</span><span class="params">()</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># app.py</span></span><br><span class="line"><span class="keyword">from</span> sales <span class="keyword">import</span> calc_shipping, calc_tax</span><br><span class="line"></span><br><span class="line">calc_shipping()</span><br><span class="line">calc_tax()</span><br></pre></td></tr></table></figure><p>或：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sales</span><br><span class="line"></span><br><span class="line">sales.calc_shipping()</span><br><span class="line">sales.calc_tax()</span><br></pre></td></tr></table></figure><h1 id="Compiled-Python-Files"><a href="#Compiled-Python-Files" class="headerlink" title="Compiled Python Files"></a>Compiled Python Files</h1><p>编译后在__pycache__生成 sales.cpython-39.pyc 文件，如果sales.py未作更改，则此文件直接用（相当于缓存），可以加快程序执行。如果sales.py改了，则要重新生成此文件。</p><h1 id="Module-Search-Path"><a href="#Module-Search-Path" class="headerlink" title="Module Search Path"></a>Module Search Path</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sys</span><br><span class="line"></span><br><span class="line">print(sys.path)</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[<span class="string">'e:\\Stone\\Python\\HelloWorld'</span>, <span class="string">'D:\\Python39\\python39.zip'</span>, <span class="string">'D:\\Python39\\DLLs'</span>, <span class="string">'D:\\Python39\\lib'</span>, <span class="string">'D:\\Python39'</span>, <span class="string">'C:\\Users\\11405\\AppData\\Roaming\\Python\\Python39\\site-packages'</span>, <span class="string">'D:\\Python39\\lib\\site-packages'</span>]</span><br></pre></td></tr></table></figure><h1 id="Packages"><a href="#Packages" class="headerlink" title="Packages"></a>Packages</h1><p>package 对应于文件夹；module 对应于文件</p><p>比如在 app.py 所在目录下有个 ecommerce 文件夹，ecommerce 文件夹内有多个文件，包括 sales.py，假如改文件夹内有文件__init__.py，则 python 会将此文件夹视为 package.</p><p>那么可以这样用：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># app.py</span></span><br><span class="line"><span class="keyword">from</span> ecommerce <span class="keyword">import</span> sales</span><br><span class="line"></span><br><span class="line">sales.calc_tax()</span><br><span class="line">sales.calc_shipping()</span><br></pre></td></tr></table></figure><h2 id="Sub-packages"><a href="#Sub-packages" class="headerlink" title="Sub-packages"></a>Sub-packages</h2><p>ecommerce 文件夹内有 shopping 文件夹, shopping 文件夹内有文件__init__.py 和 sales.py，则：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># app.py</span></span><br><span class="line"><span class="keyword">from</span> ecommerce.shopping <span class="keyword">import</span> sales</span><br></pre></td></tr></table></figure><h2 id="Intra-package-References"><a href="#Intra-package-References" class="headerlink" title="Intra-package References"></a>Intra-package References</h2><p><img src="/images/Python/002.png" alt="002"></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># sales.py</span></span><br><span class="line"><span class="keyword">from</span> ecommerce.customer <span class="keyword">import</span> contact</span><br><span class="line"><span class="keyword">from</span> ..customer <span class="keyword">import</span> contact<span class="comment"># ..表示父目录</span></span><br><span class="line"></span><br><span class="line">contact.contact_customer()</span><br></pre></td></tr></table></figure><h2 id="The-dir-Function"><a href="#The-dir-Function" class="headerlink" title="The dir() Function"></a>The dir() Function</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># app.py</span></span><br><span class="line"><span class="keyword">from</span> ecommerce.shopping <span class="keyword">import</span> sales</span><br><span class="line"></span><br><span class="line">print(dir(sales))</span><br></pre></td></tr></table></figure><p>输出sales的各种方法：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[<span class="string">'__builtins__'</span>, <span class="string">'__cached__'</span>, <span class="string">'__doc__'</span>, <span class="string">'__file__'</span>, <span class="string">'__loader__'</span>, <span class="string">'__name__'</span>, <span class="string">'__package__'</span>, <span class="string">'__spec__'</span>, <span class="string">'calc_shipping'</span>, <span class="string">'calc_tax'</span>, <span class="string">'contact'</span>]</span><br></pre></td></tr></table></figure><p>比如：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># app.py</span></span><br><span class="line"><span class="keyword">from</span> ecommerce.shopping <span class="keyword">import</span> sales</span><br><span class="line"></span><br><span class="line">print(sales.__name__)</span><br><span class="line">print(sales.__package__)</span><br><span class="line">print(sales.__file__)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">ecommerce.shopping.sales</span><br><span class="line">ecommerce.shopping</span><br><span class="line">e:\Stone\Python\HelloWorld\ecommerce\shopping\sales.py</span><br></pre></td></tr></table></figure><h1 id="Executing-Modules-as-Scripts"><a href="#Executing-Modules-as-Scripts" class="headerlink" title="Executing Modules as Scripts"></a>Executing Modules as Scripts</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># ecommerce/shopping/sales.py</span></span><br><span class="line">print(<span class="string">"Sales initialized"</span>, __name__)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">calc_tax</span><span class="params">()</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">calc_shipping</span><span class="params">()</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># ecommerce/__init__.py</span></span><br><span class="line">print(<span class="string">"Ecommerce initialized"</span>)</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># app.py</span></span><br><span class="line"><span class="keyword">from</span> ecommerce.shopping <span class="keyword">import</span> sales</span><br></pre></td></tr></table></figure><p>执行 app.py，输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Ecommerce initialized</span><br><span class="line">Sales initialized ecommerce.shopping.sales</span><br></pre></td></tr></table></figure><p>如果执行sales.py, 输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Sales initialized __main__</span><br></pre></td></tr></table></figure><p>可见__name__对于不同py文件是不同的。</p><p>如果加上：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> __name__ == <span class="string">"__main__"</span>:</span><br><span class="line"> <span class="comment"># bla bla bla</span></span><br></pre></td></tr></table></figure><p>则只在当前py文件运行时执行这些 # blablabla 的code。</p>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<figure class="highlight python"><table><tr><td clas
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(7)--Classes</title>
<link href="http://yoursite.com/2021/05/11/20210511_Python007/"/>
<id>http://yoursite.com/2021/05/11/20210511_Python007/</id>
<published>2021-05-11T15:50:09.000Z</published>
<updated>2021-05-17T04:24:23.019Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><p>👉 Class: blueprint for creating new objects — Human</p><p>👉 Object: instance of a class — John, Mary, Jack</p><h1 id="Creating-Classes"><a href="#Creating-Classes" class="headerlink" title="Creating Classes"></a>Creating Classes</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"draw"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point()</span><br><span class="line">print(type(point))</span><br><span class="line"></span><br><span class="line">print(isinstance(point, Point)) <span class="comment"># 判断point是不是Point的实例</span></span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><<span class="class"><span class="keyword">class</span> '<span class="title">__main__</span>.<span class="title">Point</span>'></span></span><br><span class="line"><span class="class"><span class="title">True</span></span></span><br></pre></td></tr></table></figure><h2 id="Constructors"><a href="#Constructors" class="headerlink" title="Constructors"></a>Constructors</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span><span class="comment"># Constructor</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">f"Point (<span class="subst">{self.x}</span>, <span class="subst">{self.y}</span>)"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">print(point.x)</span><br><span class="line">point.draw()</span><br></pre></td></tr></table></figure><p>self 指向当前 working 的 object, 比如 draw 不需要显式写 point.draw(point), 编译器会自动将当前的 object point 作为 draw() 的参数。</p><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">1</span></span><br><span class="line">Point (<span class="number">1</span>, <span class="number">2</span>)</span><br></pre></td></tr></table></figure><h1 id="Class-vs-Instance-Attributes"><a href="#Class-vs-Instance-Attributes" class="headerlink" title="Class vs Instance Attributes"></a>Class vs Instance Attributes</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> default_color = <span class="string">"red"</span><span class="comment"># class attribute</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x<span class="comment"># object/instance attribute</span></span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">f"Point (<span class="subst">{self.x}</span>, <span class="subst">{self.y}</span>)"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">point.draw()<span class="comment"># x和y是object attribute，每个object不同</span></span><br><span class="line"></span><br><span class="line">another = Point(<span class="number">3</span>, <span class="number">4</span>)</span><br><span class="line">another.draw()</span><br><span class="line"></span><br><span class="line">Point.default_color = <span class="string">"yellow"</span><span class="comment"># 直接用类调用更改。类中所有实例共享</span></span><br><span class="line">print(point.default_color)</span><br><span class="line">print(Point.default_color)</span><br><span class="line">print(another.default_color)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">Point (<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">Point (<span class="number">3</span>, <span class="number">4</span>)</span><br><span class="line">yellow</span><br><span class="line">yellow</span><br><span class="line">yellow</span><br></pre></td></tr></table></figure><h1 id="Class-vs-Instance-Methods"><a href="#Class-vs-Instance-Methods" class="headerlink" title="Class vs Instance Methods"></a>Class vs Instance Methods</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"><span class="meta"> @classmethod</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">zero</span><span class="params">(cls)</span>:</span> <span class="comment"># cls指向这个class本身</span></span><br><span class="line"> <span class="keyword">return</span> cls(<span class="number">0</span>, <span class="number">0</span>) <span class="comment"># Point(0, 0)</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">f"Point (<span class="subst">{self.x}</span>, <span class="subst">{self.y}</span>)"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point.zero()<span class="comment"># 工厂方法:将复杂的对象构造(比如多种初始值)交给类方法去完成</span></span><br><span class="line">point.draw()<span class="comment"># 输出 Point (0, 0)</span></span><br></pre></td></tr></table></figure><h1 id="Magic-Methods"><a href="#Magic-Methods" class="headerlink" title="Magic Methods"></a>Magic Methods</h1><p>编译器会自动调用Magic Methods。</p><p><a href="https://rszalski.github.io/magicmethods/" target="_blank" rel="noopener">guide book</a></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">f"Point (<span class="subst">{self.x}</span>, <span class="subst">{self.y}</span>)"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">print(point)</span><br><span class="line">print(str(point))</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><__main__.Point object at 0x0000020E4C1A6FD0></span><br><span class="line"><__main__.Point object at 0x0000020E4C1A6FD0></span><br></pre></td></tr></table></figure><p>自定义magic function __str__：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__str__</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> <span class="string">f"(<span class="subst">{self.x}</span>, <span class="subst">{self.y}</span>)"</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">f"Point (<span class="subst">{self.x}</span>, <span class="subst">{self.y}</span>)"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">print(point)</span><br><span class="line">print(str(point))</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">(<span class="number">1</span>, <span class="number">2</span>)</span><br></pre></td></tr></table></figure><h2 id="Comparing-Objects"><a href="#Comparing-Objects" class="headerlink" title="Comparing Objects"></a>Comparing Objects</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">other = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">print(point == other)</span><br></pre></td></tr></table></figure><p>输出：False。因为这样比较的是两个reference，是不同的。</p><p>方法：重定义magic functions</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__eq__</span><span class="params">(self, other)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> self.x == other.x <span class="keyword">and</span> self.y == other.y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__gt__</span><span class="params">(self, other)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> self.x > other.x <span class="keyword">and</span> self.y > other.y</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point(<span class="number">10</span>, <span class="number">20</span>)</span><br><span class="line">other = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">print(point == other)</span><br><span class="line">print(point > other)</span><br></pre></td></tr></table></figure><p>这时就是比较两个 object 的 x 和 y 的值了</p><h2 id="Supporting-Arithmetic-Operations"><a href="#Supporting-Arithmetic-Operations" class="headerlink" title="Supporting Arithmetic Operations"></a>Supporting Arithmetic Operations</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__add__</span><span class="params">(self, other)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> Point(self.x + other.x, self.y + other.y)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = Point(<span class="number">10</span>, <span class="number">20</span>)</span><br><span class="line">other = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">combined = point + other</span><br><span class="line">print(<span class="string">f"Point (<span class="subst">{combined.x}</span>, <span class="subst">{combined.y}</span>)"</span>)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Point (<span class="number">11</span>, <span class="number">22</span>)</span><br></pre></td></tr></table></figure><h1 id="Creating-Custom-Containers"><a href="#Creating-Custom-Containers" class="headerlink" title="Creating Custom Containers"></a>Creating Custom Containers</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TagCloud</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> self.tags = {}</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">add</span><span class="params">(self, tag)</span>:</span></span><br><span class="line"> self.tags[tag.lower()] = self.tags.get(tag.lower(), <span class="number">0</span>) + <span class="number">1</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__getitem__</span><span class="params">(self, tag)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> self.tags.get(tag.lower(), <span class="number">0</span>)</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__setitem__</span><span class="params">(self, tag, count)</span>:</span></span><br><span class="line"> self.tags[tag.lower()] = count</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__len__</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> len(self.tags)</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__iter__</span><span class="params">(self)</span>:</span> <span class="comment"># make it iterable</span></span><br><span class="line"> <span class="keyword">return</span> iter(self.tags)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">cloud = TagCloud()</span><br><span class="line">cloud.add(<span class="string">"Python"</span>)</span><br><span class="line">cloud.add(<span class="string">"python"</span>)</span><br><span class="line">cloud.add(<span class="string">"python"</span>)</span><br><span class="line">print(cloud.tags)</span><br><span class="line"></span><br><span class="line">print(cloud[<span class="string">"python"</span>]) <span class="comment"># __getitem__实现</span></span><br><span class="line">cloud[<span class="string">"java"</span>] = <span class="number">10</span> <span class="comment"># __setitem__实现</span></span><br><span class="line">print(cloud.tags)</span><br><span class="line"></span><br><span class="line">print(len(cloud)) <span class="comment"># __len__实现</span></span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">{<span class="string">'python'</span>: <span class="number">3</span>}</span><br><span class="line"><span class="number">3</span></span><br><span class="line">{<span class="string">'python'</span>: <span class="number">3</span>, <span class="string">'java'</span>: <span class="number">10</span>}</span><br><span class="line"><span class="number">2</span></span><br></pre></td></tr></table></figure><h1 id="Private-Members"><a href="#Private-Members" class="headerlink" title="Private Members"></a>Private Members</h1><p>将光标放到 tags 那，F2，改成__tags，回车，所有地方都改成__tags了，这就是 private member</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TagCloud</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> self.__tags = {}</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">cloud = TagCloud()</span><br><span class="line">print(cloud.__dict__)</span><br><span class="line">print(cloud._TagCloud__tags)</span><br><span class="line">print(cloud.__tags)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">{<span class="string">'_TagCloud__tags'</span>: {}}</span><br><span class="line">{}</span><br><span class="line">Traceback (most recent call last):</span><br><span class="line"> File <span class="string">"e:\HelloWorld\app.py"</span>, line <span class="number">24</span>, <span class="keyword">in</span> <module></span><br><span class="line"> print(cloud.__tags)</span><br><span class="line">AttributeError: <span class="string">'TagCloud'</span> object has no attribute <span class="string">'__tags'</span></span><br></pre></td></tr></table></figure><p>不能直接用名字调用，但是实际上python的 private member还是外部可见的，用__dict__就能查看到，用那个名字’_TagCloud__tags’也还能访问到。</p><h1 id="Properties"><a href="#Properties" class="headerlink" title="Properties"></a>Properties</h1><p>Java 风格的 getter 和 setter，不 pythonic:</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Product</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, price)</span>:</span></span><br><span class="line"> self.set_price(price)</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">get_price</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> self.__price</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">set_price</span><span class="params">(self, value)</span>:</span></span><br><span class="line"> <span class="keyword">if</span> value < <span class="number">0</span>:</span><br><span class="line"> <span class="keyword">raise</span> ValueError(<span class="string">"Price cannot be negative."</span>)</span><br><span class="line"> self.__price = value</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">product = Product(<span class="number">-50</span>)</span><br></pre></td></tr></table></figure><p>pythonic coding：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Product</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, price)</span>:</span></span><br><span class="line"> self.price = price</span><br><span class="line"></span><br><span class="line"><span class="meta"> @property</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">price</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> self.__price</span><br><span class="line"></span><br><span class="line"><span class="meta"> @price.setter</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">price</span><span class="params">(self, value)</span>:</span></span><br><span class="line"> <span class="keyword">if</span> value < <span class="number">0</span>:</span><br><span class="line"> <span class="keyword">raise</span> ValueError(<span class="string">"Price cannot be negative."</span>)</span><br><span class="line"> self.__price = value</span><br><span class="line"></span><br><span class="line"> <span class="comment"># price = property(get_price, set_price)</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">product = Product(<span class="number">-10</span>)</span><br><span class="line">print(product.price)</span><br></pre></td></tr></table></figure><p>如果将setter删掉，price 就是只读的。</p><h1 id="Inheritance"><a href="#Inheritance" class="headerlink" title="Inheritance"></a>Inheritance</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Animal</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> self.age = <span class="number">1</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">eat</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"eat"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment"># Animal: Parent, Base</span></span><br><span class="line"><span class="comment"># Mammal: Child, Sub</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Mammal</span><span class="params">(Animal)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">walk</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"walk"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Fish</span><span class="params">(Animal)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">swim</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"swim"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">m = Mammal()</span><br><span class="line">m.eat()</span><br><span class="line">print(m.age)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">eat</span><br><span class="line"><span class="number">1</span></span><br></pre></td></tr></table></figure><h2 id="The-Object-Class"><a href="#The-Object-Class" class="headerlink" title="The Object Class"></a>The Object Class</h2><p>所有 class 都继承于 object 类</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Animal</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> self.age = <span class="number">1</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">eat</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"eat"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment"># Animal: Parent, Base</span></span><br><span class="line"><span class="comment"># Mammal: Child, Sub</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Mammal</span><span class="params">(Animal)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">walk</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"walk"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">m = Mammal()</span><br><span class="line">print(isinstance(m, Animal))<span class="comment"># True</span></span><br><span class="line">print(isinstance(m, Mammal))<span class="comment"># True</span></span><br><span class="line">print(isinstance(m, object))<span class="comment"># True</span></span><br><span class="line">print(issubclass(Mammal, Animal))<span class="comment"># True</span></span><br><span class="line">print(issubclass(Mammal, object))<span class="comment"># True</span></span><br></pre></td></tr></table></figure><h2 id="Method-Overriding"><a href="#Method-Overriding" class="headerlink" title="Method Overriding"></a>Method Overriding</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Animal</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Animal Constructor"</span>)</span><br><span class="line"> self.age = <span class="number">1</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">eat</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"eat"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment"># Animal: Parent, Base</span></span><br><span class="line"><span class="comment"># Mammal: Child, Sub</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Mammal</span><span class="params">(Animal)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> super().__init__() <span class="comment"># 调用父类的__init__(),否则会被子类的覆盖掉而不执行</span></span><br><span class="line"> print(<span class="string">"Mammal Constructor"</span>)</span><br><span class="line"> self.weight = <span class="number">2</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">walk</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"walk"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">m = Mammal()</span><br><span class="line">print(m.age)</span><br><span class="line">print(m.weight)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">Animal Constructor</span><br><span class="line">Mammal Constructor</span><br><span class="line"><span class="number">1</span></span><br><span class="line"><span class="number">2</span></span><br></pre></td></tr></table></figure><h2 id="Multi-level-Inheritance"><a href="#Multi-level-Inheritance" class="headerlink" title="Multi-level Inheritance"></a>Multi-level Inheritance</h2><p>Abuse inheritance:</p><p> Employee - Person - LivingCreature - Thing</p><p>make software complex</p><h2 id="Multiple-Inheritance"><a href="#Multiple-Inheritance" class="headerlink" title="Multiple Inheritance"></a>Multiple Inheritance</h2><p>Abuse case:</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Employee</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">greet</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Employee Greet"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Person</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">greet</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Person Greet"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Manager</span><span class="params">(Employee, Person)</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">manager = Manager()</span><br><span class="line">manager.greet()</span><br></pre></td></tr></table></figure><p>将 class Manager(Employee, Person) 中的父类换个顺序就有完全不同的输出：</p><p>(Employee, Person) 输出： Employee Greet</p><p>(Person, Employee) 输出： Person Greet</p><p><strong>Good Example</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Flyer</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">fly</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Swimmer</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">swim</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">FlyingFish</span><span class="params">(Flyer, Swimmer)</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br></pre></td></tr></table></figure><p>子类的两个父类应该是没有什么共同点的，而子类兼具两个父类的method。</p><h2 id="A-Good-Example-of-Inheritance"><a href="#A-Good-Example-of-Inheritance" class="headerlink" title="A Good Example of Inheritance"></a>A Good Example of Inheritance</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">InvalidOperationError</span><span class="params">(Exception)</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Stream</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> self.opened = <span class="literal">False</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">open</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">if</span> self.opened:</span><br><span class="line"> <span class="keyword">raise</span> InvalidOperationError(<span class="string">"Stream is already opened."</span>)</span><br><span class="line"> self.opened = <span class="literal">True</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">close</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">if</span> <span class="keyword">not</span> self.opened:</span><br><span class="line"> <span class="keyword">raise</span> InvalidOperationError(<span class="string">"Stream is already closed."</span>)</span><br><span class="line"> self.opened = <span class="literal">False</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">FileStream</span><span class="params">(Stream)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">read</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Reading data from a file."</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">NetworkStream</span><span class="params">(Stream)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">read</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Reading data from a network."</span>)</span><br></pre></td></tr></table></figure><h1 id="Abstract-Base-Classes"><a href="#Abstract-Base-Classes" class="headerlink" title="Abstract Base Classes"></a>Abstract Base Classes</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> abc <span class="keyword">import</span> ABC, abstractmethod</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">InvalidOperationError</span><span class="params">(Exception)</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Stream</span><span class="params">(ABC)</span>:</span><span class="comment"># 继承自ABC(抽象类简写)</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self)</span>:</span></span><br><span class="line"> self.opened = <span class="literal">False</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">open</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">if</span> self.opened:</span><br><span class="line"> <span class="keyword">raise</span> InvalidOperationError(<span class="string">"Stream is already opened."</span>)</span><br><span class="line"> self.opened = <span class="literal">True</span></span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">close</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">if</span> <span class="keyword">not</span> self.opened:</span><br><span class="line"> <span class="keyword">raise</span> InvalidOperationError(<span class="string">"Stream is already closed."</span>)</span><br><span class="line"> self.opened = <span class="literal">False</span></span><br><span class="line"></span><br><span class="line"><span class="meta"> @abstractmethod</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">read</span><span class="params">(self)</span>:</span><span class="comment"># 抽象方法</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">FileStream</span><span class="params">(Stream)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">read</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Reading data from a file."</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">NetworkStream</span><span class="params">(Stream)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">read</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Reading data from a network."</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MemoryStream</span><span class="params">(Stream)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">read</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"Reading data from the memory."</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">stream = MemoryStream()</span><br><span class="line">stream.read()</span><br></pre></td></tr></table></figure><p>👉 Stream 不能创建实例</p><p>👉 Stream 的子类必须实现 read() ,或者也是抽象类</p><h1 id="Polymorphism"><a href="#Polymorphism" class="headerlink" title="Polymorphism"></a>Polymorphism</h1><p>Many Forms</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> abc <span class="keyword">import</span> ABC, abstractmethod</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">UIControl</span><span class="params">(ABC)</span>:</span></span><br><span class="line"><span class="meta"> @abstractmethod</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">pass</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TextBox</span><span class="params">(UIControl)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"TextBox"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">DropDownList</span><span class="params">(UIControl)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"DropDownList"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(controls)</span>:</span><span class="comment"># 在run-time才知道control具体是什么</span></span><br><span class="line"> <span class="keyword">for</span> control <span class="keyword">in</span> controls:</span><br><span class="line"> control.draw()</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">ddl = DropDownList()</span><br><span class="line">textbox = TextBox()</span><br><span class="line">draw([ddl, textbox])<span class="comment"># 多种对象实现一种方法</span></span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">DropDownList</span><br><span class="line">TextBox</span><br></pre></td></tr></table></figure><h2 id="Duck-Typing"><a href="#Duck-Typing" class="headerlink" title="Duck Typing"></a>Duck Typing</h2><p><strong>If it walks and quacks like a duck, it is a duck.</strong></p><p>上例中并不需要抽象类UIControl。只要类中实现了draw()方法，python就认为是个UIControl，就能执行draw(controls)方法。</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> abc <span class="keyword">import</span> ABC, abstractmethod</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TextBox</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"TextBox"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">DropDownList</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(self)</span>:</span></span><br><span class="line"> print(<span class="string">"DropDownList"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">draw</span><span class="params">(controls)</span>:</span></span><br><span class="line"> <span class="keyword">for</span> control <span class="keyword">in</span> controls:</span><br><span class="line"> control.draw()</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">ddl = DropDownList()</span><br><span class="line">textbox = TextBox()</span><br><span class="line">draw([ddl, textbox])</span><br></pre></td></tr></table></figure><h1 id="Extending-Built-in-Types"><a href="#Extending-Built-in-Types" class="headerlink" title="Extending Built-in Types"></a>Extending Built-in Types</h1><p><strong>Demo 1: 扩展str类</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Text</span><span class="params">(str)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">duplicate</span><span class="params">(self)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> self + self</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">text = Text(<span class="string">"Python"</span>)</span><br><span class="line">print(text.duplicate())<span class="comment"># 本类方法</span></span><br><span class="line">print(text.lower()) <span class="comment"># 父类方法</span></span><br></pre></td></tr></table></figure><p>输出:</p><p>PythonPython</p><p>python</p><p><strong>Demo 2: 扩展list类</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TrackableList</span><span class="params">(list)</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">append</span><span class="params">(self, object)</span>:</span></span><br><span class="line"> print(<span class="string">"Append called"</span>)</span><br><span class="line"> super().append(object)<span class="comment"># 调用父类方法</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">list = TrackableList()</span><br><span class="line">list.append(<span class="string">"1"</span>)</span><br></pre></td></tr></table></figure><p>输出:</p><p>Append called</p><h1 id="Data-Classes"><a href="#Data-Classes" class="headerlink" title="Data Classes"></a>Data Classes</h1><p>对于一个只有data而没有方法的类：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Point</span>:</span></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(self, x, y)</span>:</span></span><br><span class="line"> self.x = x</span><br><span class="line"> self.y = y</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">def</span> <span class="title">__eq__</span><span class="params">(self, other)</span> -> bool:</span></span><br><span class="line"> <span class="keyword">return</span> self.x == other.x <span class="keyword">and</span> self.y == other.y</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">p1 = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">p2 = Point(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">print(p1 == p2)</span><br></pre></td></tr></table></figure><p>可以用 namedtuple make code cleaner:</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> collections <span class="keyword">import</span> namedtuple</span><br><span class="line"></span><br><span class="line">Point = namedtuple(<span class="string">"Point"</span>, [<span class="string">"x"</span>, <span class="string">"y"</span>])</span><br><span class="line">p1 = Point(x=<span class="number">1</span>, y=<span class="number">2</span>) <span class="comment"># must be keyword arguments</span></span><br><span class="line">p2 = Point(x=<span class="number">1</span>, y=<span class="number">2</span>)</span><br><span class="line"></span><br><span class="line">print(p1 == p2)<span class="comment"># True</span></span><br><span class="line">print(p1.x)<span class="comment"># 像类一样调用：1</span></span><br><span class="line"></span><br><span class="line">p1.x = <span class="number">10</span><span class="comment"># AttributeError: can't set attribute</span></span><br></pre></td></tr></table></figure><p>只能新建一个namedtuple，用p1指向：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">p1 = Point(x=<span class="number">10</span>, y=<span class="number">2</span>)</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<p>👉 Class: blueprint for creating new objects — Hu
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(6)--Exceptions</title>
<link href="http://yoursite.com/2021/05/11/20210511_Python006/"/>
<id>http://yoursite.com/2021/05/11/20210511_Python006/</id>
<published>2021-05-11T09:50:09.000Z</published>
<updated>2021-05-17T04:24:22.995Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><h1 id="Handling-Exceptions"><a href="#Handling-Exceptions" class="headerlink" title="Handling Exceptions"></a>Handling Exceptions</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span>:</span><br><span class="line"> age = int(input(<span class="string">"Age: "</span>))</span><br><span class="line"><span class="keyword">except</span> ValueError <span class="keyword">as</span> ex:</span><br><span class="line"> print(<span class="string">"You didn't enter a valid age."</span>)</span><br><span class="line"> print(ex)<span class="comment"># 输出错误信息</span></span><br><span class="line"> print(type(ex))<span class="comment"># 显示错误类型</span></span><br><span class="line"><span class="keyword">else</span>:<span class="comment"># 没有exception就执行这一块</span></span><br><span class="line"> print(<span class="string">"No exceptions were thrown."</span>)</span><br><span class="line"></span><br><span class="line">print(<span class="string">"Execution continues."</span>)</span><br></pre></td></tr></table></figure><p>运行结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">>python app.py</span><br><span class="line">Age: <span class="number">10</span></span><br><span class="line">No exceptions were thrown.</span><br><span class="line">Execution continues.</span><br><span class="line"></span><br><span class="line">>python app.py</span><br><span class="line">Age: a</span><br><span class="line">You didn<span class="string">'t enter a valid age.</span></span><br><span class="line"><span class="string">invalid literal for int() with base 10: '</span>a<span class="string">' </span></span><br><span class="line"><span class="string"><class '</span>ValueErro<span class="string">r'></span></span><br><span class="line"><span class="string">Execution continues.</span></span><br></pre></td></tr></table></figure><h2 id="Handling-Different-Exceptions"><a href="#Handling-Different-Exceptions" class="headerlink" title="Handling Different Exceptions"></a>Handling Different Exceptions</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span>:</span><br><span class="line"> age = int(input(<span class="string">"Age: "</span>))</span><br><span class="line"> xfactor = <span class="number">10</span> / age</span><br><span class="line"><span class="keyword">except</span> (ValueError, ZeroDivisionError):<span class="comment"># 多种exception之一即执行</span></span><br><span class="line"> print(<span class="string">"You didn't enter a valid age."</span>)</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line"> print(<span class="string">"No exceptions were thrown."</span>)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">> python app.py</span><br><span class="line">Age: <span class="number">0</span></span><br><span class="line">You didn<span class="string">'t enter a valid age.</span></span><br></pre></td></tr></table></figure><h1 id="Cleaning-Up"><a href="#Cleaning-Up" class="headerlink" title="Cleaning Up"></a>Cleaning Up</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span>:</span><br><span class="line"> file = open(<span class="string">"app.py"</span>)</span><br><span class="line"> age = int(input(<span class="string">"Age: "</span>))</span><br><span class="line"> xfactor = <span class="number">10</span> / age</span><br><span class="line"><span class="keyword">except</span> (ValueError, ZeroDivisionError):</span><br><span class="line"> print(<span class="string">"You didn't enter a valid age."</span>)</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line"> print(<span class="string">"No exceptions were thrown."</span>)</span><br><span class="line"><span class="keyword">finally</span>:</span><br><span class="line"> file.close()</span><br></pre></td></tr></table></figure><p>无论有没有出现exception，都要释放资源，那就放在finally块里，无论有没有exception都会执行的。</p><h2 id="The-With-Statement"><a href="#The-With-Statement" class="headerlink" title="The With Statement"></a>The With Statement</h2><p>使用with语句打开文件，随后会自动释放掉，就不用file.close()了：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">with</span> open(<span class="string">"app.py"</span>) <span class="keyword">as</span> file:</span><br><span class="line"> print(<span class="string">"File opened."</span>)</span><br></pre></td></tr></table></figure><p>file有__enter__和__exit__两个magic function，最后会自动调用__exit__，所以不用手动释放。</p><h1 id="Raising-Exceptions"><a href="#Raising-Exceptions" class="headerlink" title="Raising Exceptions"></a>Raising Exceptions</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">calculate_xfactor</span><span class="params">(age)</span>:</span></span><br><span class="line"> <span class="keyword">if</span> age <= <span class="number">0</span>:</span><br><span class="line"> <span class="keyword">raise</span> ValueError(<span class="string">"Age cannot be 0 or less."</span>)</span><br><span class="line"> <span class="keyword">return</span> <span class="number">10</span> / age</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">try</span>:</span><br><span class="line"> calculate_xfactor(<span class="number">-1</span>)</span><br><span class="line"><span class="keyword">except</span> ValueError <span class="keyword">as</span> error:</span><br><span class="line"> print(error)</span><br></pre></td></tr></table></figure><h1 id="Cost-of-Raising-Exceptions"><a href="#Cost-of-Raising-Exceptions" class="headerlink" title="Cost of Raising Exceptions"></a>Cost of Raising Exceptions</h1><p>Mosh 不建议raise exceptions，因为代价有点大。</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> timeit <span class="keyword">import</span> timeit</span><br><span class="line"></span><br><span class="line">code1 = <span class="string">"""</span></span><br><span class="line"><span class="string">def calculate_xfactor(age):</span></span><br><span class="line"><span class="string"> if age <= 0:</span></span><br><span class="line"><span class="string"> raise ValueError("Age cannot be 0 or less.")</span></span><br><span class="line"><span class="string"> return 10 / age</span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string">try:</span></span><br><span class="line"><span class="string"> calculate_xfactor(-1)</span></span><br><span class="line"><span class="string">except ValueError as error:</span></span><br><span class="line"><span class="string"> pass</span></span><br><span class="line"><span class="string">"""</span></span><br><span class="line"></span><br><span class="line">code2 = <span class="string">"""</span></span><br><span class="line"><span class="string">def calculate_xfactor(age):</span></span><br><span class="line"><span class="string"> if age <= 0:</span></span><br><span class="line"><span class="string"> return None</span></span><br><span class="line"><span class="string"> return 10 / age</span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string">xfactor = calculate_xfactor(-1)</span></span><br><span class="line"><span class="string">if xfactor == None:</span></span><br><span class="line"><span class="string"> pass</span></span><br><span class="line"><span class="string">"""</span></span><br><span class="line"></span><br><span class="line">print(<span class="string">"first code="</span>, timeit(code1, number=<span class="number">10000</span>))</span><br><span class="line">print(<span class="string">"second code="</span>, timeit(code2, number=<span class="number">10000</span>))</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">first code= <span class="number">0.0040202000000000016</span></span><br><span class="line">second code= <span class="number">0.0015166999999999958</span></span><br></pre></td></tr></table></figure><p>可以看到raise exception的用时是普通方法的两倍多。</p>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<h1 id="Handling-Exceptions"><a href="#Handling-Exce
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(5)--Data Structures</title>
<link href="http://yoursite.com/2021/05/09/20210509_Python005/"/>
<id>http://yoursite.com/2021/05/09/20210509_Python005/</id>
<published>2021-05-09T09:50:09.000Z</published>
<updated>2021-05-17T04:24:22.987Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><h1 id="List"><a href="#List" class="headerlink" title="List"></a>List</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">letters = [<span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>]</span><br><span class="line">matrix = [[<span class="number">0</span>, <span class="number">1</span>], [<span class="number">2</span>, <span class="number">3</span>]]</span><br><span class="line"></span><br><span class="line">zeros = [<span class="number">0</span>] * <span class="number">5</span> <span class="comment"># [0,0,0,0,0]</span></span><br><span class="line">combined = zeros + letters <span class="comment"># [0, 0, 0, 0, 0, 'a', 'b', 'c']</span></span><br><span class="line">numbers = list(range(<span class="number">20</span>))</span><br><span class="line">chars = list(<span class="string">"Hello World"</span>)<span class="comment"># ['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd']</span></span><br><span class="line">print(len(chars)) <span class="comment"># 11</span></span><br></pre></td></tr></table></figure><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">letters = [<span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>, <span class="string">"d"</span>]</span><br><span class="line">print(letters[<span class="number">0</span>])</span><br><span class="line">print(letters[<span class="number">-1</span>])</span><br><span class="line">letters[<span class="number">0</span>] = <span class="string">"A"</span></span><br><span class="line">print(letters)</span><br><span class="line">print(letters[:<span class="number">3</span>])</span><br><span class="line">print(letters[::<span class="number">2</span>])</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">a</span><br><span class="line">d</span><br><span class="line">[<span class="string">'A'</span>, <span class="string">'b'</span>, <span class="string">'c'</span>, <span class="string">'d'</span>]</span><br><span class="line">[<span class="string">'A'</span>, <span class="string">'b'</span>, <span class="string">'c'</span>]</span><br><span class="line">[<span class="string">'A'</span>, <span class="string">'c'</span>]</span><br></pre></td></tr></table></figure><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">numbers = list(range(<span class="number">10</span>))</span><br><span class="line">print(numbers)</span><br><span class="line">print(numbers[::<span class="number">2</span>])</span><br><span class="line">print(numbers[::<span class="number">-1</span>])<span class="comment"># 倒序输出</span></span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">[<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>, <span class="number">9</span>]</span><br><span class="line">[<span class="number">0</span>, <span class="number">2</span>, <span class="number">4</span>, <span class="number">6</span>, <span class="number">8</span>]</span><br><span class="line">[<span class="number">9</span>, <span class="number">8</span>, <span class="number">7</span>, <span class="number">6</span>, <span class="number">5</span>, <span class="number">4</span>, <span class="number">3</span>, <span class="number">2</span>, <span class="number">1</span>, <span class="number">0</span>]</span><br></pre></td></tr></table></figure><h2 id="Unpacking-Lists"><a href="#Unpacking-Lists" class="headerlink" title="Unpacking Lists"></a>Unpacking Lists</h2><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">numbers = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]</span><br><span class="line">first, second, third = numbers<span class="comment"># first=1, second=2, third=3</span></span><br><span class="line">first, second, _ = numbers</span><br></pre></td></tr></table></figure><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">numbers2 = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">4</span>]</span><br><span class="line">first, second, *other = numbers2</span><br><span class="line">print(first)<span class="comment"># 1</span></span><br><span class="line">print(other)<span class="comment"># [3, 4, 4, 4, 4, 4, 4]</span></span><br></pre></td></tr></table></figure><p>与此同理：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">multiply</span><span class="params">(*numbers)</span>:</span></span><br><span class="line"> total = <span class="number">1</span></span><br><span class="line"> <span class="keyword">for</span> number <span class="keyword">in</span> numbers:</span><br><span class="line"> total *= number</span><br><span class="line"> <span class="keyword">return</span> total</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">print(multiply(<span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>))</span><br><span class="line">print(multiply(<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">2</span>, <span class="number">5</span>))</span><br></pre></td></tr></table></figure><p>Another demo:</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">numbers2 = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">4</span>, <span class="number">9</span>]</span><br><span class="line">first, *other, last = numbers2</span><br><span class="line">print(first, last)<span class="comment"># 1 9</span></span><br><span class="line">print(other)<span class="comment"># [2, 3, 4, 4, 4, 4, 4]</span></span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">{'id': 1, 'name': 'John', 'age': 22}</span><br><span class="line">John</span><br></pre></td></tr></table></figure><h2 id="Looping-over-Lists"><a href="#Looping-over-Lists" class="headerlink" title="Looping over Lists"></a>Looping over Lists</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">letters = [<span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>]</span><br><span class="line"><span class="keyword">for</span> letter <span class="keyword">in</span> enumerate(letters):</span><br><span class="line"> print(letter)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">(<span class="number">0</span>, <span class="string">'a'</span>)</span><br><span class="line">(<span class="number">1</span>, <span class="string">'b'</span>)</span><br><span class="line">(<span class="number">2</span>, <span class="string">'c'</span>)</span><br></pre></td></tr></table></figure><p><strong>unpacking tuple:</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">letters = [<span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>]</span><br><span class="line"><span class="keyword">for</span> index, letter <span class="keyword">in</span> enumerate(letters):</span><br><span class="line"> print(index, letter)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">0</span> a</span><br><span class="line"><span class="number">1</span> b</span><br><span class="line"><span class="number">2</span> c</span><br></pre></td></tr></table></figure><h2 id="Adding-Removing-Items"><a href="#Adding-Removing-Items" class="headerlink" title="Adding/Removing Items"></a>Adding/Removing Items</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">letters = [<span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>, <span class="string">"d"</span>, <span class="string">"e"</span>]</span><br><span class="line"></span><br><span class="line"><span class="comment"># Add</span></span><br><span class="line">letters.append(<span class="string">"f"</span>)<span class="comment"># 在末尾加入</span></span><br><span class="line">letters.insert(<span class="number">0</span>, <span class="string">"-"</span>)<span class="comment"># 在首部加入</span></span><br><span class="line">print(letters)</span><br><span class="line"></span><br><span class="line"><span class="comment"># Remove</span></span><br><span class="line">letters.pop()<span class="comment"># 删除最后一个元素</span></span><br><span class="line">print(letters)</span><br><span class="line"></span><br><span class="line">letters.remove(<span class="string">"b"</span>)<span class="comment"># 删除特定元素</span></span><br><span class="line">print(letters)</span><br><span class="line"></span><br><span class="line"><span class="keyword">del</span> letters[<span class="number">0</span>:<span class="number">3</span>]<span class="comment"># 删除多个元素</span></span><br><span class="line">print(letters)</span><br><span class="line"></span><br><span class="line">letters.clear()<span class="comment"># 清除所有元素</span></span><br><span class="line">print(letters)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">['-', 'a', 'b', 'c', 'd', 'e', 'f']</span><br><span class="line">['-', 'a', 'b', 'c', 'd', 'e']</span><br><span class="line">['-', 'a', 'c', 'd', 'e']</span><br><span class="line">['d', 'e']</span><br><span class="line">[]</span><br></pre></td></tr></table></figure><h2 id="Finding-Items"><a href="#Finding-Items" class="headerlink" title="Finding Items"></a>Finding Items</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">letters = [<span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>]</span><br><span class="line">print(letters.count(<span class="string">"d"</span>))<span class="comment"># 0</span></span><br><span class="line"><span class="keyword">if</span> <span class="string">"c"</span> <span class="keyword">in</span> letters:</span><br><span class="line"> print(letters.index(<span class="string">"c"</span>))<span class="comment"># 2</span></span><br></pre></td></tr></table></figure><h2 id="Sorting-Lists"><a href="#Sorting-Lists" class="headerlink" title="Sorting Lists"></a>Sorting Lists</h2><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">numbers = [<span class="number">3</span>, <span class="number">51</span>, <span class="number">2</span>, <span class="number">8</span>, <span class="number">6</span>]</span><br><span class="line">numbers.sort(reverse=<span class="literal">True</span>)<span class="comment"># 倒序</span></span><br><span class="line">print(numbers)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[51, 8, 6, 3, 2]</span><br></pre></td></tr></table></figure><p><strong>Demo 2</strong></p><p>built-in function: 不改变原数组</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">numbers = [<span class="number">3</span>, <span class="number">51</span>, <span class="number">2</span>, <span class="number">8</span>, <span class="number">6</span>]</span><br><span class="line">print(sorted(numbers, reverse=<span class="literal">True</span>))</span><br><span class="line">print(numbers)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">[<span class="number">51</span>, <span class="number">8</span>, <span class="number">6</span>, <span class="number">3</span>, <span class="number">2</span>]</span><br><span class="line">[<span class="number">3</span>, <span class="number">51</span>, <span class="number">2</span>, <span class="number">8</span>, <span class="number">6</span>]</span><br></pre></td></tr></table></figure><p><strong>Demo 3</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">items = [</span><br><span class="line"> (<span class="string">"product1"</span>, <span class="number">10</span>),</span><br><span class="line"> (<span class="string">"product2"</span>, <span class="number">9</span>),</span><br><span class="line"> (<span class="string">"product3"</span>, <span class="number">12</span>),</span><br><span class="line">]</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">sort_item</span><span class="params">(item)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> item[<span class="number">1</span>]</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">items.sort(key=sort_item)</span><br><span class="line">print(items)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[(<span class="string">'product2'</span>, <span class="number">9</span>), (<span class="string">'product1'</span>, <span class="number">10</span>), (<span class="string">'product3'</span>, <span class="number">12</span>)]</span><br></pre></td></tr></table></figure><h2 id="Lambdas"><a href="#Lambdas" class="headerlink" title="Lambdas"></a>Lambdas</h2><p>改造上述的例子(Demo 3)：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">items = [</span><br><span class="line"> (<span class="string">"product1"</span>, <span class="number">10</span>),</span><br><span class="line"> (<span class="string">"product2"</span>, <span class="number">9</span>),</span><br><span class="line"> (<span class="string">"product3"</span>, <span class="number">12</span>),</span><br><span class="line">]</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">items.sort(key=<span class="keyword">lambda</span> item: item[<span class="number">1</span>])</span><br><span class="line">print(items)</span><br></pre></td></tr></table></figure><p>语法：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">lambda</span> parameters:expression</span><br></pre></td></tr></table></figure><h3 id="Map-Function"><a href="#Map-Function" class="headerlink" title="Map Function"></a>Map Function</h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line">items = [</span><br><span class="line"> (<span class="string">"product1"</span>, <span class="number">10</span>),</span><br><span class="line"> (<span class="string">"product2"</span>, <span class="number">9</span>),</span><br><span class="line"> (<span class="string">"product3"</span>, <span class="number">12</span>),</span><br><span class="line">]</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment"># prices = []</span></span><br><span class="line"><span class="comment"># for item in items:</span></span><br><span class="line"><span class="comment"># prices.append(item[1])</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># print(prices)</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># A better way</span></span><br><span class="line">prices = list(map(<span class="keyword">lambda</span> item: item[<span class="number">1</span>], items))</span><br><span class="line">print(prices)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[10, 9, 12]</span><br></pre></td></tr></table></figure><h3 id="Filter-Function"><a href="#Filter-Function" class="headerlink" title="Filter Function"></a>Filter Function</h3><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">items = [</span><br><span class="line"> (<span class="string">"product1"</span>, <span class="number">10</span>),</span><br><span class="line"> (<span class="string">"product2"</span>, <span class="number">9</span>),</span><br><span class="line"> (<span class="string">"product3"</span>, <span class="number">12</span>),</span><br><span class="line">]</span><br><span class="line"></span><br><span class="line">filtered = list(filter(<span class="keyword">lambda</span> item: item[<span class="number">1</span>] >= <span class="number">10</span>, items))</span><br><span class="line">print(filtered)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[('product1', 10), ('product3', 12)]</span><br></pre></td></tr></table></figure><h2 id="List-Comprehension"><a href="#List-Comprehension" class="headerlink" title="List Comprehension"></a>List Comprehension</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[expression <span class="keyword">for</span> item <span class="keyword">in</span> items]</span><br></pre></td></tr></table></figure><p>改造上面的 map function 和 filter function:</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">prices = [item[<span class="number">1</span>] <span class="keyword">for</span> item <span class="keyword">in</span> items]</span><br><span class="line"></span><br><span class="line">filtered = [item <span class="keyword">for</span> item <span class="keyword">in</span> items <span class="keyword">if</span> item[<span class="number">1</span>] >= <span class="number">10</span>]</span><br></pre></td></tr></table></figure><h2 id="Zip-Function"><a href="#Zip-Function" class="headerlink" title="Zip Function"></a>Zip Function</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">list1 = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]</span><br><span class="line">list2 = [<span class="number">10</span>, <span class="number">20</span>, <span class="number">30</span>]</span><br><span class="line"></span><br><span class="line">print(list(zip(<span class="string">"abc"</span>, list1, list2)))</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[('a', 1, 10), ('b', 2, 20), ('c', 3, 30)]</span><br></pre></td></tr></table></figure><h1 id="Stacks"><a href="#Stacks" class="headerlink" title="Stacks"></a>Stacks</h1><p><strong>LIFO: Last In - First Out</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">browsing_session = []</span><br><span class="line">browsing_session.append(<span class="number">1</span>)<span class="comment"># 压栈</span></span><br><span class="line">browsing_session.append(<span class="number">2</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> <span class="keyword">not</span> <span class="keyword">not</span> browsing_session:</span><br><span class="line"> print(browsing_session.pop())<span class="comment"># 出栈</span></span><br></pre></td></tr></table></figure><h1 id="Queues"><a href="#Queues" class="headerlink" title="Queues"></a>Queues</h1><p><strong>FIFO: First In - First Out</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> collections <span class="keyword">import</span> deque</span><br><span class="line"></span><br><span class="line">queue = deque([])</span><br><span class="line">queue.append(<span class="number">1</span>)<span class="comment"># 入队</span></span><br><span class="line">queue.append(<span class="number">2</span>)</span><br><span class="line">queue.append(<span class="number">3</span>)</span><br><span class="line">queue.popleft()<span class="comment"># 出队</span></span><br><span class="line">print(queue)</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> <span class="keyword">not</span> queue:<span class="comment"># 检查队列是否为空</span></span><br><span class="line"> print(<span class="string">"Empty"</span>)</span><br></pre></td></tr></table></figure><h1 id="Tuples"><a href="#Tuples" class="headerlink" title="Tuples"></a>Tuples</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># 都是tuple：</span></span><br><span class="line">point = <span class="number">1</span>, <span class="number">2</span></span><br><span class="line"><span class="comment"># point = 1,</span></span><br><span class="line"><span class="comment"># point = ()</span></span><br><span class="line">print(type(point))</span><br></pre></td></tr></table></figure><p><strong>Demo</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">point = (<span class="number">1</span>, <span class="number">2</span>) + (<span class="number">3</span>, <span class="number">4</span>)</span><br><span class="line">print(point)<span class="comment"># (1, 2, 3, 4)</span></span><br><span class="line"></span><br><span class="line">point = (<span class="number">1</span>, <span class="number">2</span>) * <span class="number">3</span></span><br><span class="line">print(point)<span class="comment"># (1, 2, 1, 2, 1, 2)</span></span><br><span class="line"></span><br><span class="line">point = tuple([<span class="number">1</span>, <span class="number">2</span>])</span><br><span class="line">print(point)<span class="comment"># (1, 2)</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">point = tuple(<span class="string">"hello"</span>)</span><br><span class="line">print(point)<span class="comment"># ('h', 'e', 'l', 'l', 'o')</span></span><br><span class="line"></span><br><span class="line">point = (<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>)</span><br><span class="line">print(point[<span class="number">0</span>:<span class="number">2</span>])<span class="comment"># (1, 2)</span></span><br><span class="line"></span><br><span class="line">x, y, z = point</span><br><span class="line"><span class="keyword">if</span> <span class="number">10</span> <span class="keyword">in</span> point:</span><br><span class="line"> print(<span class="string">"exists"</span>)</span><br></pre></td></tr></table></figure><h2 id="Swapping-Variables"><a href="#Swapping-Variables" class="headerlink" title="Swapping Variables"></a>Swapping Variables</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">x = <span class="number">10</span></span><br><span class="line">y = <span class="number">11</span></span><br><span class="line"></span><br><span class="line">z = x</span><br><span class="line">x = y</span><br><span class="line">y = z</span><br><span class="line">print(x, y)<span class="comment"># 11 10</span></span><br></pre></td></tr></table></figure><p>simpler way:</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">x, y = y, x</span><br><span class="line">print(x, y)</span><br></pre></td></tr></table></figure><p>实际上是先定义了一个tuple，然后给x, y赋值，即 x, y = (11, 10)</p><h1 id="Arrays"><a href="#Arrays" class="headerlink" title="Arrays"></a>Arrays</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> array <span class="keyword">import</span> array</span><br><span class="line"></span><br><span class="line">numbers = array(<span class="string">"i"</span>, [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>])</span><br><span class="line"><span class="comment"># numbers[0] = 1.0# TypeError: integer argument expected, got float</span></span><br><span class="line"></span><br><span class="line">list1 = [<span class="number">1.0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]</span><br><span class="line">print(type(list1[<span class="number">0</span>]))<span class="comment"># <class 'float'></span></span><br><span class="line">print(type(list1[<span class="number">1</span>]))<span class="comment"># <class 'int'></span></span><br></pre></td></tr></table></figure><p>arrays 可以调用 append, pop, index 等与 lists 相同的 built-in function，但是只能有一种数据类型（由参数 typecode 如 ”i“ 指定）</p><p>而一个 list 里面可以有多种数据类型：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">list1 = [<span class="number">1.0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]</span><br><span class="line">list1.append(<span class="string">"hello"</span>)</span><br><span class="line">print(list1)<span class="comment"># [1.0, 1, 2, 3, 'hello']</span></span><br></pre></td></tr></table></figure><h1 id="Sets"><a href="#Sets" class="headerlink" title="Sets"></a>Sets</h1><p>一个 set 内不能有重复项</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">numbers = [<span class="number">1</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]</span><br><span class="line">first = set(numbers)</span><br><span class="line">print(first)<span class="comment"># {1, 2, 3, 4}</span></span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">second = {<span class="number">1</span>, <span class="number">4</span>}</span><br><span class="line">second.add(<span class="number">5</span>)</span><br><span class="line">print(second) <span class="comment"># {1, 4, 5}</span></span><br><span class="line"></span><br><span class="line">second.remove(<span class="number">5</span>)</span><br><span class="line">len(second)</span><br><span class="line">print(second)<span class="comment"># {1, 4}</span></span><br></pre></td></tr></table></figure><p><strong>Demo</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">numbers = [<span class="number">1</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]</span><br><span class="line">first = set(numbers)</span><br><span class="line"></span><br><span class="line">second = {<span class="number">1</span>, <span class="number">5</span>}</span><br><span class="line"></span><br><span class="line">print(first | second) <span class="comment"># 在first或在second</span></span><br><span class="line">print(first & second) <span class="comment"># 在first也在second</span></span><br><span class="line">print(first - second) <span class="comment"># 在first不在second</span></span><br><span class="line">print(first ^ second) <span class="comment"># 在first或在second但不同时在两个set</span></span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">{1, 2, 3, 4, 5}</span><br><span class="line">{1}</span><br><span class="line">{2, 3, 4}</span><br><span class="line">{2, 3, 4, 5}</span><br></pre></td></tr></table></figure><p>set是无序的，不能用index，如first[0]是不合法的</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> <span class="number">1</span> <span class="keyword">in</span> first:<span class="comment"># 判断set里是否有某元素</span></span><br><span class="line"> print(<span class="string">"yes"</span>)</span><br></pre></td></tr></table></figure><h1 id="Dictionary"><a href="#Dictionary" class="headerlink" title="Dictionary"></a>Dictionary</h1><p><strong>创建</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># Two ways to create a dictionary</span></span><br><span class="line">point = {<span class="string">"x"</span>: <span class="number">1</span>, <span class="string">"y"</span>: <span class="number">2</span>}</span><br><span class="line">point = dict(x=<span class="number">1</span>, y=<span class="number">2</span>)</span><br></pre></td></tr></table></figure><p><strong>增改</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">point[<span class="string">"x"</span>] = <span class="number">10</span><span class="comment"># modify element</span></span><br><span class="line">print(point)<span class="comment"># {'x': 10, 'y': 2}</span></span><br><span class="line">point[<span class="string">"z"</span>] = <span class="number">20</span><span class="comment"># create new element</span></span><br><span class="line">print(point)<span class="comment"># {'x': 10, 'y': 2, 'z': 20}</span></span><br></pre></td></tr></table></figure><p><strong>获取</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">print(point.get(<span class="string">"a"</span>))<span class="comment"># None</span></span><br><span class="line">print(point.get(<span class="string">"a"</span>, <span class="number">0</span>))<span class="comment"># 0 (若没有指定的key,则返回指定值0)</span></span><br></pre></td></tr></table></figure><p><strong>删除</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">del</span> point[<span class="string">"x"</span>]<span class="comment"># 删除元素</span></span><br><span class="line">print(point)<span class="comment"># {'y': 2, 'z': 20}</span></span><br></pre></td></tr></table></figure><p><strong>for loop</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> key <span class="keyword">in</span> point:</span><br><span class="line"> print(key, point[key])</span><br><span class="line"></span><br><span class="line"><span class="keyword">for</span> item <span class="keyword">in</span> point.items():<span class="comment"># 返回tuple</span></span><br><span class="line"> print(item)</span><br><span class="line"></span><br><span class="line"><span class="keyword">for</span> key, value <span class="keyword">in</span> point.items():</span><br><span class="line"> print(key, value)</span><br></pre></td></tr></table></figure><p>返回：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">y <span class="number">2</span></span><br><span class="line">z <span class="number">20</span></span><br><span class="line">(<span class="string">'y'</span>, <span class="number">2</span>)</span><br><span class="line">(<span class="string">'z'</span>, <span class="number">20</span>)</span><br><span class="line">y <span class="number">2</span></span><br><span class="line">z <span class="number">20</span></span><br></pre></td></tr></table></figure><h2 id="Dictionary-Comprehensions"><a href="#Dictionary-Comprehensions" class="headerlink" title="Dictionary Comprehensions"></a>Dictionary Comprehensions</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># values = []</span></span><br><span class="line"><span class="comment"># for x in range(5):</span></span><br><span class="line"><span class="comment"># values.append(x * 2)</span></span><br><span class="line"></span><br><span class="line">values = [x * <span class="number">2</span> <span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">5</span>)]<span class="comment"># 作用同上</span></span><br></pre></td></tr></table></figure><p>set、dictionary 的：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">values = {x * <span class="number">2</span> <span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">5</span>)}</span><br><span class="line">print(values)<span class="comment"># {0, 2, 4, 6, 8}</span></span><br><span class="line"></span><br><span class="line">values = {x: x * <span class="number">2</span> <span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">5</span>)}</span><br><span class="line">print(values)<span class="comment"># {0: 0, 1: 2, 2: 4, 3: 6, 4: 8}</span></span><br></pre></td></tr></table></figure><h1 id="Generators"><a href="#Generators" class="headerlink" title="Generators"></a>Generators</h1><p>用于有大量（无限）元素，要节省内存空间的情形</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> sys <span class="keyword">import</span> getsizeof</span><br><span class="line"></span><br><span class="line">values = (x * <span class="number">2</span> <span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">100000</span>))<span class="comment"># generator</span></span><br><span class="line">print(<span class="string">"gen:"</span>, getsizeof(values))</span><br><span class="line"></span><br><span class="line">values = [x * <span class="number">2</span> <span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">100000</span>)]<span class="comment"># list</span></span><br><span class="line">print(<span class="string">"list:"</span>, getsizeof(values))</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">gen: <span class="number">112</span></span><br><span class="line">list: <span class="number">800984</span></span><br></pre></td></tr></table></figure><p>对于 generator object, 无论有多少元素，都只占112的位置（就算只有一个元素也是）。generator object 在迭代时才生成元素，而不是将所有的元素都存在内存中。</p><h1 id="Unpacking-Operator"><a href="#Unpacking-Operator" class="headerlink" title="Unpacking Operator"></a>Unpacking Operator</h1><p><strong>Demo 1</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">numbers = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]</span><br><span class="line">print(numbers)<span class="comment"># [1, 2, 3]</span></span><br><span class="line">print(*numbers)<span class="comment"># 1 2 3</span></span><br></pre></td></tr></table></figure><p>numbers 是一个list, 而 *numbers 是 unpack 之后的3个独立的数</p><p><strong>Demo 2</strong></p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">values = list(range(<span class="number">5</span>))</span><br><span class="line">values = [*range(<span class="number">5</span>), *<span class="string">"Hello"</span>] </span><br><span class="line">print(values)<span class="comment"># [0, 1, 2, 3, 4, 'H', 'e', 'l', 'l', 'o']</span></span><br></pre></td></tr></table></figure><p><strong>Demo 3</strong></p><p>组合两个lists：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">first = [<span class="number">1</span>, <span class="number">2</span>]</span><br><span class="line">second = [<span class="number">3</span>]</span><br><span class="line">values = [*first, <span class="string">"a"</span>, *second, *<span class="string">"Hello"</span>]</span><br><span class="line">print(values)<span class="comment"># [1, 2, 'a', 3, 'H', 'e', 'l', 'l', 'o']</span></span><br></pre></td></tr></table></figure><p><strong>Demo 4</strong></p><p>组合两个字典：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">first = {<span class="string">"x"</span>: <span class="number">1</span>}</span><br><span class="line">second = {<span class="string">"x"</span>: <span class="number">10</span>, <span class="string">"y"</span>: <span class="number">2</span>}</span><br><span class="line">combined = {**first, **second, <span class="string">"z"</span>: <span class="number">1</span>}</span><br><span class="line">print(combined) <span class="comment"># {'x': 10, 'y': 2, 'z': 1}</span></span><br></pre></td></tr></table></figure><p>相同 key 值的元素 (“x”) 取最后一个元素的 value (10)</p><h1 id="Exercise"><a href="#Exercise" class="headerlink" title="Exercise"></a>Exercise</h1><p>得出一个句子中出现频率最高的字母：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pprint <span class="keyword">import</span> pprint</span><br><span class="line"></span><br><span class="line">sentence = <span class="string">"This is a common interview question"</span></span><br><span class="line"></span><br><span class="line">char_frequency = {}</span><br><span class="line"><span class="keyword">for</span> char <span class="keyword">in</span> sentence:</span><br><span class="line"> <span class="keyword">if</span> char <span class="keyword">in</span> char_frequency:</span><br><span class="line"> char_frequency[char] += <span class="number">1</span></span><br><span class="line"> <span class="keyword">else</span>:</span><br><span class="line"> char_frequency[char] = <span class="number">1</span></span><br><span class="line">pprint(char_frequency, width=<span class="number">1</span>) <span class="comment"># 一行只打印一个</span></span><br><span class="line"></span><br><span class="line">char_frequency_sorted = sorted(</span><br><span class="line"> char_frequency.items(),</span><br><span class="line"> key=<span class="keyword">lambda</span> kv: kv[<span class="number">1</span>],</span><br><span class="line"> reverse=<span class="literal">True</span>)</span><br><span class="line"></span><br><span class="line">print(char_frequency_sorted[<span class="number">0</span>])</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">{<span class="string">' '</span>: <span class="number">5</span>,</span><br><span class="line"> <span class="string">'T'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'a'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'c'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'e'</span>: <span class="number">3</span>,</span><br><span class="line"> <span class="string">'h'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'i'</span>: <span class="number">5</span>,</span><br><span class="line"> <span class="string">'m'</span>: <span class="number">2</span>,</span><br><span class="line"> <span class="string">'n'</span>: <span class="number">3</span>,</span><br><span class="line"> <span class="string">'o'</span>: <span class="number">3</span>,</span><br><span class="line"> <span class="string">'q'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'r'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'s'</span>: <span class="number">3</span>,</span><br><span class="line"> <span class="string">'t'</span>: <span class="number">2</span>,</span><br><span class="line"> <span class="string">'u'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'v'</span>: <span class="number">1</span>,</span><br><span class="line"> <span class="string">'w'</span>: <span class="number">1</span>}</span><br><span class="line">(<span class="string">'i'</span>, <span class="number">5</span>)</span><br></pre></td></tr></table></figure><p>我的改进：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">#char_frequency = {}</span></span><br><span class="line"><span class="comment">#for char in sentence:</span></span><br><span class="line"><span class="comment"># if char in char_frequency:</span></span><br><span class="line"><span class="comment"># char_frequency[char] += 1</span></span><br><span class="line"><span class="comment"># else:</span></span><br><span class="line"><span class="comment"># char_frequency[char] = 1</span></span><br><span class="line"><span class="comment"># pprint(char_frequency, width=1) # 一行只打印一个</span></span><br><span class="line"></span><br><span class="line">char_frequency = {}</span><br><span class="line"><span class="keyword">for</span> char <span class="keyword">in</span> sentence:</span><br><span class="line"> char_frequency[char] = char_frequency.get(char, <span class="number">0</span>) + <span class="number">1</span></span><br><span class="line">pprint(char_frequency, width=<span class="number">1</span>)</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<h1 id="List"><a href="#List" class="headerlink" tit
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(4)--Functions</title>
<link href="http://yoursite.com/2021/05/06/20210506_Python004/"/>
<id>http://yoursite.com/2021/05/06/20210506_Python004/</id>
<published>2021-05-06T09:50:09.000Z</published>
<updated>2021-05-17T04:24:22.981Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><h1 id="Arguments"><a href="#Arguments" class="headerlink" title="Arguments"></a>Arguments</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">greet</span><span class="params">(first_name, last_name)</span>:</span></span><br><span class="line"> print(<span class="string">f"Hi <span class="subst">{first_name}</span> <span class="subst">{last_name}</span>"</span>)</span><br><span class="line"> print(<span class="string">"Welcome aboard"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">greet(<span class="string">"Stone"</span>, <span class="string">"Purple"</span>)</span><br><span class="line">greet(<span class="string">"Stephen"</span>, <span class="string">"Curry"</span>)</span><br></pre></td></tr></table></figure><h1 id="Types-of-Functions"><a href="#Types-of-Functions" class="headerlink" title="Types of Functions"></a>Types of Functions</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">👉 <span class="number">1</span> - Perform a task</span><br><span class="line">👉 <span class="number">2</span> - Return a value</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">greet</span><span class="params">(name)</span>:</span></span><br><span class="line"> <span class="string">""" Type 1 function """</span></span><br><span class="line"> print(<span class="string">f"Hi <span class="subst">{name}</span>"</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">get_greeting</span><span class="params">(name)</span>:</span></span><br><span class="line"> <span class="string">""" Type 2 function """</span></span><br><span class="line"> <span class="keyword">return</span> <span class="string">f"Hi <span class="subst">{name}</span>"</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">message = get_greeting(<span class="string">"Stone"</span>)</span><br><span class="line">file = open(<span class="string">"content.txt"</span>, <span class="string">"w"</span>)</span><br><span class="line">file.write(message)</span><br></pre></td></tr></table></figure><p>python return <strong>None</strong> by default.</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">print(greet(<span class="string">"Stone"</span>))</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">None</span><br></pre></td></tr></table></figure><h1 id="Types-of-Arguments"><a href="#Types-of-Arguments" class="headerlink" title="Types of Arguments"></a>Types of Arguments</h1><h2 id="Keyword-Arguments"><a href="#Keyword-Arguments" class="headerlink" title="Keyword Arguments"></a>Keyword Arguments</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">increment</span><span class="params">(number, by)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> number + by</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment"># more readable: by=...</span></span><br><span class="line">print(increment(<span class="number">2</span>, by=<span class="number">1</span>))</span><br></pre></td></tr></table></figure><h2 id="Default-Arguments"><a href="#Default-Arguments" class="headerlink" title="Default Arguments"></a>Default Arguments</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">increment</span><span class="params">(number, by=<span class="number">1</span>)</span>:</span></span><br><span class="line"> <span class="keyword">return</span> number + by</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">print(increment(<span class="number">2</span>))</span><br></pre></td></tr></table></figure><p>所有 optional arguments 应该在无 default value 的 argument 后面</p><h2 id="args"><a href="#args" class="headerlink" title="*args"></a>*args</h2><p>输入参数数量可变(存成一个tuple，且iterable)</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">multiply</span><span class="params">(*numbers)</span>:</span></span><br><span class="line"> total = <span class="number">1</span></span><br><span class="line"> <span class="keyword">for</span> number <span class="keyword">in</span> numbers:</span><br><span class="line"> total *= number</span><br><span class="line"> <span class="keyword">return</span> total</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">print(multiply(<span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>))</span><br><span class="line">print(multiply(<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">2</span>, <span class="number">5</span>))</span><br></pre></td></tr></table></figure><p>输出</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">120</span><br><span class="line">1200</span><br></pre></td></tr></table></figure><h2 id="args-1"><a href="#args-1" class="headerlink" title="**args"></a>**args</h2><p>打包 keyword arguments 成 dict 给函数体调用</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">save_user</span><span class="params">(**user)</span>:</span></span><br><span class="line"> print(user)</span><br><span class="line"> print(user[<span class="string">"name"</span>])</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">save_user(id=<span class="number">1</span>, name=<span class="string">"John"</span>, age=<span class="number">22</span>)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">{'id': 1, 'name': 'John', 'age': 22}</span><br><span class="line">John</span><br></pre></td></tr></table></figure><h1 id="Scope"><a href="#Scope" class="headerlink" title="Scope"></a>Scope</h1><p>局部变量局部可见</p><p>全局变量全局可见</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">message = <span class="string">"a"</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">greet</span><span class="params">(name)</span>:</span></span><br><span class="line"> message = <span class="string">"b"</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">greet(<span class="string">"Stone"</span>)</span><br><span class="line">print(message)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">a</span><br></pre></td></tr></table></figure><p>因为 Python 将 greet 函数内的 message 视为 local variable，尽管它与全局变量同名。</p><h1 id="Debugging"><a href="#Debugging" class="headerlink" title="Debugging"></a>Debugging</h1><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">👉 F9 -- 设置断点</span><br><span class="line">👉 F5 -- 开始调试</span><br><span class="line">👉 F10 -- 下一步</span><br><span class="line">👉 F11 -- 进入 function 内单步 debug</span><br><span class="line">👉 Shift+F11 -- 退出 function 内单步 debug</span><br></pre></td></tr></table></figure><h1 id="VSCode-Tricks-Windows"><a href="#VSCode-Tricks-Windows" class="headerlink" title="VSCode Tricks (Windows)"></a>VSCode Tricks (Windows)</h1><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">👉 Home 光标移到行首</span><br><span class="line">👉 End 光标移到行尾</span><br><span class="line">👉 Ctrl+Home 光标移到文件头</span><br><span class="line">👉 Ctrl+End 光标移到文件尾</span><br><span class="line">👉 Ctrl+L 选择当前行</span><br><span class="line">👉 Alt+↑or↓ 代码上移或下移</span><br><span class="line">👉 Shift+Alt+↓选中某（几）行后用此组合键复制粘贴</span><br><span class="line">👉 Ctrl+/多行注释</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<h1 id="Arguments"><a href="#Arguments" class="heade
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(3)--Control Flow</title>
<link href="http://yoursite.com/2021/05/06/20210506_Python003/"/>
<id>http://yoursite.com/2021/05/06/20210506_Python003/</id>
<published>2021-05-06T08:50:09.000Z</published>
<updated>2021-05-17T04:24:22.973Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><h1 id="Comparison-Operators"><a href="#Comparison-Operators" class="headerlink" title="Comparison Operators"></a>Comparison Operators</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span><span class="number">10</span> > <span class="number">3</span></span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">>>> </span><span class="number">10</span> >= <span class="number">3</span></span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">>>> </span><span class="number">10</span> < <span class="number">20</span></span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">>>> </span><span class="number">10</span> == <span class="number">10</span></span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">>>> </span><span class="number">10</span> == <span class="string">"10"</span> </span><br><span class="line"><span class="literal">False</span></span><br><span class="line"><span class="meta">>>> </span><span class="number">10</span> != <span class="string">"10"</span> </span><br><span class="line"><span class="literal">True</span></span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">>>> </span><span class="string">"bag"</span> > <span class="string">"apple"</span> </span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="meta">>>> </span><span class="string">"bag"</span> == <span class="string">"BAG"</span> </span><br><span class="line"><span class="literal">False</span></span><br><span class="line"><span class="meta">>>> </span>ord(<span class="string">"b"</span>) </span><br><span class="line"><span class="number">98</span> </span><br><span class="line"><span class="meta">>>> </span>ord(<span class="string">"B"</span>) </span><br><span class="line"><span class="number">66</span></span><br></pre></td></tr></table></figure><h2 id="Chaining-Comparison-Operators"><a href="#Chaining-Comparison-Operators" class="headerlink" title="Chaining Comparison Operators"></a>Chaining Comparison Operators</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># age should be between 18 and 65</span></span><br><span class="line">age = <span class="number">22</span></span><br><span class="line"><span class="keyword">if</span> <span class="number">18</span> <= age < <span class="number">65</span>:</span><br><span class="line"> print(<span class="string">"Eligible"</span>)</span><br></pre></td></tr></table></figure><h1 id="Conditional-Statements"><a href="#Conditional-Statements" class="headerlink" title="Conditional Statements"></a>Conditional Statements</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">temperature = <span class="number">15</span></span><br><span class="line"><span class="keyword">if</span> temperature > <span class="number">30</span>:</span><br><span class="line"> print(<span class="string">"It is warm"</span>)</span><br><span class="line"> print(<span class="string">"Drink water"</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">elif</span> temperature > <span class="number">20</span>:</span><br><span class="line"> print(<span class="string">"It's nice"</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line"> print(<span class="string">"It's cold"</span>)</span><br><span class="line"> </span><br><span class="line">print(<span class="string">"Done"</span>)</span><br></pre></td></tr></table></figure><h1 id="Ternary-Operator"><a href="#Ternary-Operator" class="headerlink" title="Ternary Operator"></a>Ternary Operator</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">age = <span class="number">2</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># if age >= 18:</span></span><br><span class="line"><span class="comment"># message = "Eligible"</span></span><br><span class="line"><span class="comment"># else:</span></span><br><span class="line"><span class="comment"># message = "Not eligible"</span></span><br><span class="line"></span><br><span class="line">message = <span class="string">"Eligible"</span> <span class="keyword">if</span> age >= <span class="number">18</span> <span class="keyword">else</span> <span class="string">"Not eligible"</span></span><br><span class="line"></span><br><span class="line">print(message)</span><br></pre></td></tr></table></figure><h1 id="Logical-Operators"><a href="#Logical-Operators" class="headerlink" title="Logical Operators"></a>Logical Operators</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># logical operators: and, or, not</span></span><br><span class="line"></span><br><span class="line">high_income = <span class="literal">True</span></span><br><span class="line">good_credit = <span class="literal">False</span></span><br><span class="line">student = <span class="literal">False</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> (high_income <span class="keyword">or</span> good_credit) <span class="keyword">and</span> <span class="keyword">not</span> student:</span><br><span class="line"> print(<span class="string">"Eligible"</span>)</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line"> print(<span class="string">"Not eligible"</span>)</span><br></pre></td></tr></table></figure><h2 id="Short-circuit-Evaluation"><a href="#Short-circuit-Evaluation" class="headerlink" title="Short-circuit Evaluation"></a>Short-circuit Evaluation</h2><p>多条件判断时：</p><p>and 前面的 expression 为 False, 则不再(执行)判断后面的expression</p><p>or 前面的 expression 为 True, 则不再(执行)判断后面的expression</p><h1 id="For-Loops"><a href="#For-Loops" class="headerlink" title="For Loops"></a>For Loops</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> number <span class="keyword">in</span> range(<span class="number">1</span>, <span class="number">10</span>, <span class="number">2</span>):<span class="comment"># step: 2</span></span><br><span class="line"> print(<span class="string">"Attempt"</span>, number, number * <span class="string">"."</span>)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">Attempt 1 .</span><br><span class="line">Attempt 3 ...</span><br><span class="line">Attempt 5 .....</span><br><span class="line">Attempt 7 .......</span><br><span class="line">Attempt 9 .........</span><br></pre></td></tr></table></figure><h2 id="For…Else"><a href="#For…Else" class="headerlink" title="For…Else"></a>For…Else</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">successful = <span class="literal">False</span></span><br><span class="line"><span class="keyword">for</span> number <span class="keyword">in</span> range(<span class="number">3</span>):</span><br><span class="line"> print(<span class="string">"Attempt"</span>)</span><br><span class="line"> <span class="keyword">if</span> successful:</span><br><span class="line"> print(<span class="string">"Successful"</span>)</span><br><span class="line"> <span class="keyword">break</span></span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line"> print(<span class="string">"Attempted 3 times and failed."</span>)</span><br></pre></td></tr></table></figure><p>如果一直没有执行break，就执行下面的else</p><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">Attempt</span><br><span class="line">Attempt</span><br><span class="line">Attempt</span><br><span class="line">Attempted <span class="number">3</span> times <span class="keyword">and</span> failed</span><br></pre></td></tr></table></figure><h2 id="Nested-Loops"><a href="#Nested-Loops" class="headerlink" title="Nested Loops"></a>Nested Loops</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> x <span class="keyword">in</span> range(<span class="number">5</span>):</span><br><span class="line"> <span class="keyword">for</span> y <span class="keyword">in</span> range(<span class="number">3</span>):</span><br><span class="line"> print(<span class="string">f"(<span class="subst">{x}</span>, <span class="subst">{y}</span>)"</span>)</span><br></pre></td></tr></table></figure><p>输出结果</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line">(<span class="number">0</span>, <span class="number">0</span>)</span><br><span class="line">(<span class="number">0</span>, <span class="number">1</span>)</span><br><span class="line">(<span class="number">0</span>, <span class="number">2</span>)</span><br><span class="line">(<span class="number">1</span>, <span class="number">0</span>)</span><br><span class="line">(<span class="number">1</span>, <span class="number">1</span>)</span><br><span class="line">(<span class="number">1</span>, <span class="number">2</span>)</span><br><span class="line">(<span class="number">2</span>, <span class="number">0</span>)</span><br><span class="line">(<span class="number">2</span>, <span class="number">1</span>)</span><br><span class="line">(<span class="number">2</span>, <span class="number">2</span>)</span><br><span class="line">(<span class="number">3</span>, <span class="number">0</span>)</span><br><span class="line">(<span class="number">3</span>, <span class="number">1</span>)</span><br><span class="line">(<span class="number">3</span>, <span class="number">2</span>)</span><br><span class="line">(<span class="number">4</span>, <span class="number">0</span>)</span><br><span class="line">(<span class="number">4</span>, <span class="number">1</span>)</span><br><span class="line">(<span class="number">4</span>, <span class="number">2</span>)</span><br></pre></td></tr></table></figure><h2 id="Iterables"><a href="#Iterables" class="headerlink" title="Iterables"></a>Iterables</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># Iterable : range(), string, list...</span></span><br><span class="line"><span class="keyword">for</span> x <span class="keyword">in</span> <span class="string">"Python"</span>:</span><br><span class="line"> print(x)</span><br><span class="line"></span><br><span class="line"><span class="keyword">for</span> x <span class="keyword">in</span> [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]:</span><br><span class="line"> print(x)</span><br></pre></td></tr></table></figure><p>输出：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">P</span><br><span class="line">y</span><br><span class="line">t</span><br><span class="line">h</span><br><span class="line">o</span><br><span class="line">n</span><br><span class="line"><span class="number">1</span></span><br><span class="line"><span class="number">2</span></span><br><span class="line"><span class="number">3</span></span><br><span class="line"><span class="number">4</span></span><br></pre></td></tr></table></figure><h1 id="While-Loops"><a href="#While-Loops" class="headerlink" title="While Loops"></a>While Loops</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">command = <span class="string">""</span></span><br><span class="line"><span class="keyword">while</span> command.lower() != <span class="string">"quit"</span>:</span><br><span class="line"> command = input(<span class="string">">"</span>)</span><br><span class="line"> print(<span class="string">"ECHO"</span>, command)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">><span class="number">2</span>+<span class="number">2</span></span><br><span class="line">ECHO <span class="number">2</span>+<span class="number">2</span></span><br><span class="line">><span class="number">3</span>*<span class="number">4</span> </span><br><span class="line">ECHO <span class="number">3</span>*<span class="number">4</span></span><br><span class="line">>quit </span><br><span class="line">ECHO quit</span><br></pre></td></tr></table></figure><h1 id="Infinite-Loops"><a href="#Infinite-Loops" class="headerlink" title="Infinite Loops"></a>Infinite Loops</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">while</span> <span class="literal">True</span>:</span><br><span class="line"> command = input(<span class="string">">"</span>)</span><br><span class="line"> print(<span class="string">"ECHO"</span>, command)</span><br><span class="line"> <span class="keyword">if</span> command.lower() == <span class="string">"quit"</span>:</span><br><span class="line"> <span class="keyword">break</span></span><br></pre></td></tr></table></figure><h1 id="Exercise"><a href="#Exercise" class="headerlink" title="Exercise"></a>Exercise</h1><p>编写一段code，输出1~10之间（不含10）的偶数，如下：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">2</span></span><br><span class="line"><span class="number">4</span></span><br><span class="line"><span class="number">6</span></span><br><span class="line"><span class="number">8</span></span><br><span class="line">We have <span class="number">4</span> even numbers</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<h1 id="Comparison-Operators"><a href="#Comparison-O
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
<entry>
<title>Mosh的Python课程笔记(2)--基本类型</title>
<link href="http://yoursite.com/2021/05/06/20210506_Python002/"/>
<id>http://yoursite.com/2021/05/06/20210506_Python002/</id>
<published>2021-05-06T06:25:09.000Z</published>
<updated>2021-05-17T04:24:22.966Z</updated>
<content type="html"><![CDATA[<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p><h1 id="Primitive-Types"><a href="#Primitive-Types" class="headerlink" title="Primitive Types"></a>Primitive Types</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">students_count = <span class="number">1000</span></span><br><span class="line">rating = <span class="number">4.99</span></span><br><span class="line">is_published = <span class="literal">False</span> <span class="comment"># case sensitive (区分大小写)</span></span><br><span class="line">course_name = <span class="string">"Python Programming"</span></span><br></pre></td></tr></table></figure><h1 id="String"><a href="#String" class="headerlink" title="String"></a>String</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">course_name = <span class="string">"Python Programming"</span></span><br><span class="line">message = <span class="string">"""</span></span><br><span class="line"><span class="string">Hi John</span></span><br><span class="line"><span class="string">This is Stone from China</span></span><br><span class="line"><span class="string">blablabla</span></span><br><span class="line"><span class="string">"""</span></span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">course_name = <span class="string">"Python Programming"</span></span><br><span class="line">print(len(course_name))<span class="comment"># 18</span></span><br><span class="line">print(course_name[<span class="number">0</span>])<span class="comment"># P</span></span><br><span class="line">print(course_name[<span class="number">-1</span>])<span class="comment"># g</span></span><br><span class="line">print(course_name[<span class="number">0</span>:<span class="number">3</span>])<span class="comment"># Pyt</span></span><br><span class="line">print(course_name[<span class="number">0</span>:])<span class="comment"># Python Programming</span></span><br><span class="line">print(course_name[:<span class="number">3</span>])<span class="comment"># Pyt</span></span><br><span class="line">print(course_name[:])<span class="comment"># Python Programming</span></span><br></pre></td></tr></table></figure><h2 id="Escape-Sequences"><a href="#Escape-Sequences" class="headerlink" title="Escape Sequences"></a>Escape Sequences</h2><p>转义字符</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">course_name = <span class="string">"Python \"Programming"</span></span><br><span class="line">print(course_name) <span class="comment"># Python "Programming</span></span><br><span class="line"><span class="comment"># \" \' \\ \n</span></span><br></pre></td></tr></table></figure><h2 id="Formatted-Strings"><a href="#Formatted-Strings" class="headerlink" title="Formatted Strings"></a>Formatted Strings</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">first = <span class="string">"Stone"</span></span><br><span class="line">last = <span class="string">"Purple"</span></span><br><span class="line">full = <span class="string">f"<span class="subst">{first}</span> <span class="subst">{last}</span>"</span></span><br><span class="line">print(full)<span class="comment"># Stone Purple</span></span><br><span class="line">print(<span class="string">f"<span class="subst">{len(first)}</span> <span class="subst">{<span class="number">2</span> + <span class="number">2</span>}</span>"</span>)<span class="comment"># 5 4</span></span><br></pre></td></tr></table></figure><h2 id="String-Methods"><a href="#String-Methods" class="headerlink" title="String Methods"></a>String Methods</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">course = <span class="string">" Python Programming"</span></span><br><span class="line">print(course.upper())</span><br><span class="line">print(course.lower())</span><br><span class="line">print(course.title())</span><br><span class="line">print(course.strip()) <span class="comment"># lstrip, rstrip</span></span><br><span class="line">print(course.find(<span class="string">"Pro"</span>))</span><br><span class="line">print(course.replace(<span class="string">"P"</span>, <span class="string">"J"</span>))</span><br><span class="line">print(<span class="string">"Pro"</span> <span class="keyword">in</span> course)</span><br><span class="line">print(<span class="string">"swift"</span> <span class="keyword">not</span> <span class="keyword">in</span> course)</span><br></pre></td></tr></table></figure><p>输出结果：</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"> PYTHON PROGRAMMING</span><br><span class="line"> python programming</span><br><span class="line"> Python Programming</span><br><span class="line">Python Programming</span><br><span class="line"><span class="number">9</span></span><br><span class="line"> Jython Jrogramming</span><br><span class="line"><span class="literal">True</span></span><br><span class="line"><span class="literal">True</span></span><br></pre></td></tr></table></figure><h1 id="Numbers"><a href="#Numbers" class="headerlink" title="Numbers"></a>Numbers</h1><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">x = <span class="number">1</span></span><br><span class="line">x = <span class="number">1.1</span></span><br><span class="line">x = <span class="number">1</span> + <span class="number">2j</span> <span class="comment"># complex number</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># + - * / % // **</span></span><br><span class="line">print(<span class="number">10</span> / <span class="number">3</span>) <span class="comment"># 3.3333333333333335</span></span><br><span class="line">print(<span class="number">10</span> // <span class="number">3</span>)<span class="comment"># 3</span></span><br><span class="line">print(<span class="number">10</span> ** <span class="number">3</span>)<span class="comment"># 1000</span></span><br><span class="line"></span><br><span class="line">x = <span class="number">10</span></span><br><span class="line">x = x + <span class="number">3</span></span><br><span class="line">x += <span class="number">3</span></span><br></pre></td></tr></table></figure><h2 id="Working-with-Numbers"><a href="#Working-with-Numbers" class="headerlink" title="Working with Numbers"></a>Working with Numbers</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> math</span><br><span class="line"></span><br><span class="line">print(round(<span class="number">2.9</span>))<span class="comment"># 3</span></span><br><span class="line">print(abs(<span class="number">-2.9</span>))<span class="comment"># 2.9</span></span><br><span class="line"></span><br><span class="line">print(math.ceil(<span class="number">2.2</span>))<span class="comment"># 3</span></span><br></pre></td></tr></table></figure><h2 id="Type-Conversion"><a href="#Type-Conversion" class="headerlink" title="Type Conversion"></a>Type Conversion</h2><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">x = input(<span class="string">"x: "</span>)</span><br><span class="line">print(type(x))</span><br><span class="line">y = int(x) + <span class="number">1</span><span class="comment"># type conversion</span></span><br><span class="line">print(<span class="string">f"x: <span class="subst">{x}</span>, y: <span class="subst">{y}</span>"</span>)</span><br></pre></td></tr></table></figure><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">x: <span class="number">2</span></span><br><span class="line"><<span class="class"><span class="keyword">class</span> '<span class="title">str</span>'></span></span><br><span class="line"><span class="class"><span class="title">x</span>:</span> <span class="number">2</span>, y: <span class="number">3</span></span><br></pre></td></tr></table></figure><p>对于bool(x)，为 False 的情况<strong>只有</strong>：“”，0，None</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">print(bool(<span class="string">""</span>))<span class="comment"># False</span></span><br></pre></td></tr></table></figure><h1 id="Quiz"><a href="#Quiz" class="headerlink" title="Quiz"></a>Quiz</h1><p>👉 What are the primitive types in Python?</p><p>👉 What are the results?</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">fruit = <span class="string">"Apple"</span></span><br><span class="line">print(fruit[<span class="number">1</span>])</span><br><span class="line">print(fruit[<span class="number">1</span>:<span class="number">-1</span>])</span><br></pre></td></tr></table></figure><p>👉 What are the results?</p><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">print(<span class="number">10</span> % <span class="number">3</span>)</span><br><span class="line">print(bool(<span class="string">"False"</span>))</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html">
<p><a href="https://codewithmosh.com/" target="_blank" rel="noopener">Mosh的课程网址</a></p>
<h1 id="Primitive-Types"><a href="#Primitive-Types"
</summary>
<category term="Python" scheme="http://yoursite.com/categories/Python/"/>
<category term="job notes-python" scheme="http://yoursite.com/tags/job-notes-python/"/>
</entry>
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