python modules

generator/gen.py

@@ -4,7 +4,7 @@ def flatten(nested):
     try:
         for sublist in nested:
             for element in flatten(sublist):
-                ##这里的语句会被执行吗
+                ##这里的语句会被执行吗 YES
                 print('element',element)
                 yield element
     except TypeError:
@@ -27,4 +27,42 @@ def error():
 lies = [1,2,[3,4]]
 gen = flatten(lies)
 for i in gen:
-    print('print', i)
+    print('print', i)
+
+
+##八皇后问题。generator的形式。
+def conflict(state, nextX):
+    nextY = len(state)
+    for i in range(nextY):
+        if abs(state[i] - nextX) in (0, nextY - i):
+            return True
+    return False
+
+def queens(num=8, state=()):
+    for pos in range(num):
+        if not conflict(state, pos):
+           if len(state) == num-1:
+               ##base case
+              yield (pos,)
+           else:
+               ##recursive case
+              for result in queens(num, state + (pos,)):
+                  ## result is only a tuple 
+                  ## iterator queens has only one element !!!! 
+                  yield (pos,) + result
+'''
+    1 -> (4,3,2,1)
+    2 -> (4,3,2)
+    3 -> (4,3)
+    4 -> (4,)
+    当下一次next(genQueen)的时候， next 应该是从 最深层的yield后面开始执行，不然就是逻辑错误。。。
+    也就是所generator function被调用的时候，系统是会 保留其调用的堆栈的，尽管经过一层层的yield。
+    也就是说yield是可以穿越调用栈，但是不用回退堆栈的！！！！ 是不是像C语言找那个的 setjump 的作用呢？？？
+'''
+
+genQueen = queens(5)
+print( next(genQueen) )
+print( next(genQueen) )
+print( next(genQueen) )
+print( next(genQueen) )
+print( len( list(genQueen) ) )

modules/index.py

@@ -0,0 +1,19 @@
+'''
+    tell the python interpreter to find your module
+    >>> import sys
+    >>> sys.path.append("absolute_path/your_module_dir")
+
+    import-only-once !!!! 模块的缓存
+
+    >>> import importlib
+    >>> hello = importlib.reload(hello)
+'''
+
+def greet():
+    print('hello world')
+
+
+if __name__ == "__main__":
+    ## 不是被别的模块import
+    ## 而是被当做程序执行
+    greet()