问题11：如何进行反向迭代 & 如何实现反向迭代

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

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

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

# 此循环，实例化时自动调用__iter__()方法，而不是__reversed__()方法；
for x in FloatRange(1.0, 3.0, 0.5):
    print(x)

# 此循环，只有定义了__reversed__()方法后，才能直接使用reversed；
for x in reversed(FloatRange(1.0, 4.0, 0.5)):
    print(x)