python 自定义__iter__

class FloatRange:
    def __init__(self, start, end, step=0.1):
        self.start = start
        self.end = end
        self.step = step

    def __iter__(self):
        t = self.start
        while t <= self.end:
            yield t
            t += self.step

    def __reversed__(self):
        t = self.end
        while t >= self.start:
            yield t
            t -= self.step

for x in reversed(FloatRange(1.0, 4.0, 0.5)):
    print x


for x in FloatRange(1.0, 4.0, 0.2):
    print x

reversed 方法起什么作用