迭代器

自定义迭代器：

class countDown:
    def __init__(self,step):    #实例化
        self.step = step

    def __next__(self):         #返回下个元素
        if self.step<=0:
            raise StopIteration
        self.step -= 1
        return self.step

    def __iter__(self):         #返回迭代器实例
        return self

for element in countDown(4):
    print(element)

输出实例：

3
2
1
0