一、正向迭代器实现__iter__方法

二、反向迭代器实现__reversed__方法

三、实例：

from decimal import Decimal

class FloatRange:
    def __init__(self, a, b, step):
        self.a = Decimal(str(a))
        self.b = Decimal(str(b))
        self.step = Decimal(str(step))

    def __iter__(self):
        t = self.a
        while t <= self.b:
            yield t
            t += self.step

    def __reversed__(self):
        t = self.b
        while t >= self.a:
            yield t
            t -= self.step


fr = FloatRange(3.0, 4.0, 0.2)
print(fr, reversed(fr))
for i in fr:
    print(i)
print("*"*20)

for x in reversed(fr):
    print(x)