生成器

任何包含yield语句的函数称为生成器。
   它的行为和普通的函数有很大的差别，它不是像return那样返回值，而是每次产生
   多个值。每次产生一个值(使用yield语句),函数就会被冻结:即函数停在那点等待
   被激活。函数被激活后就从停止的那点开始执行。

例子:

def flatten(nested):
    for sublist in nested:
        for element in sublist:
            yield element

nested = [[1,2], [3, 4], [5]]

print list(flatten(nested))

递归生成器:

# recursion yield example
def flatten2(nested):
    try:
        # Don't iteration of the string similar objects
        try: nested + ''
        except TypeError: pass
        else: raise TypeError
        for sublist in nested:
            for element in flatten2(sublist):
                yield element
    except TypeError:
        yield nested

print list(flatten2([[[1],2], 3, 4, [5, [6, 7]], 8]))

print list(flatten2(['foo', ['bar', ['baz']]]))