第五章  3  函数——变量作用域.ipynb

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   "source": [
    "# 5.3 变量作用域"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "- 不同作用域内同名变量之间互不影响\n",
    "- 在函数内定义的局部变量只在该函数内可见\n",
    "- 在函数内部使用global定义的全局变量当函数结束后仍然存在并且可以访问\n",
    "- global的两种情况：\n",
    "    - 一变量已在函数外定义，在函数内需要修改这个变量的值，并将修改的结果反映到函数之外。\n",
    "    - 在函数内部直接使用global将一变量声明为全局变量"
   ]
  },
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   "execution_count": 3,
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "3 4\n",
      "3\n"
     ]
    },
    {
     "ename": "NameError",
     "evalue": "name 'y' is not defined",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mNameError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-3-b41eae1ec6e6>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[0;32m      7\u001b[0m \u001b[0mdemo\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      8\u001b[0m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 9\u001b[1;33m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0my\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mNameError\u001b[0m: name 'y' is not defined"
     ]
    }
   ],
   "source": [
    "def demo():\n",
    "    global x  #声明或创建全局变量，必须在使用x之前执行\n",
    "    x = 3\n",
    "    y = 4\n",
    "    print(x,y)\n",
    "x = 5\n",
    "demo()\n",
    "print(x)\n",
    "print(y)"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "- 如果在某个作用域内有为变量赋值的操作，那么该变量将被认为是该作用域内的局部变量"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "ename": "UnboundLocalError",
     "evalue": "local variable 'x' referenced before assignment",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mUnboundLocalError\u001b[0m                         Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-10-9b0b4d2bdda1>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[0;32m      4\u001b[0m     \u001b[0mx\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mx\u001b[0m \u001b[1;33m+\u001b[0m \u001b[1;36m1\u001b[0m  \u001b[1;31m#赋值语句，x将被认为是该作用域内的局部变量\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      5\u001b[0m     \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 6\u001b[1;33m \u001b[0mdemo\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m<ipython-input-10-9b0b4d2bdda1>\u001b[0m in \u001b[0;36mdemo\u001b[1;34m()\u001b[0m\n\u001b[0;32m      1\u001b[0m \u001b[0mx\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;36m10\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      2\u001b[0m \u001b[1;32mdef\u001b[0m \u001b[0mdemo\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 3\u001b[1;33m     \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m)\u001b[0m  \u001b[1;31m#引发异常，因为x变量现在还不存在\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m      4\u001b[0m     \u001b[0mx\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mx\u001b[0m \u001b[1;33m+\u001b[0m \u001b[1;36m1\u001b[0m  \u001b[1;31m#赋值语句，x将被认为是该作用域内的局部变量\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      5\u001b[0m     \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mUnboundLocalError\u001b[0m: local variable 'x' referenced before assignment"
     ]
    }
   ],
   "source": [
    "x = 10\n",
    "def demo():\n",
    "    print(x)  #引发异常，因为x变量现在还不存在\n",
    "    x = x + 1  #赋值语句，x将被认为是该作用域内的局部变量\n",
    "    print(x)\n",
    "demo()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "- 若局部变量与全局变量有相同的名字，那么该局部变量会在自己的作用域内暂时隐藏同名的全局变量。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "5\n",
      "3\n",
      "5\n"
     ]
    }
   ],
   "source": [
    "def demo():\n",
    "    x = 3\n",
    "    print(x)\n",
    "x = 5\n",
    "print(x)\n",
    "demo()\n",
    "print(x)"
   ]
  }
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