if __name__ == "__main__":
    a = [1, 2, 3, 4]
    # python自带的reversed函数搞定了反向迭代，其本质时调用了__reversed__方法
    for i in reversed(a):
        print(i)

    for i in a.__reversed__():
        print(i)

    # 如果想要实现类的反向迭代，可以在实现__iter__()方法的同时搞一个__reversed__()方法
    class Countdown:
        def __init__(self, start):
            self.start = start

        def __iter__(self):
            n = self.start
            while n > 1:
                yield n
                n -= 1

        def __reversed__(self):
            n = 1
            while n <= self.start:
                yield n
                n += 1

    a = Countdown(10)
    for i in reversed(a):
        print(i)