对迭代器的理解

1. 定义了__iter()__方法的类 是可迭代对象 ，需要返回一个迭代器对象的引用
2. 定义了__iter()__,和__next()__方法的类是迭代器,__iter()__可以返回self，__next()__返回要迭代的元素，迭代完成抛出StopIteration异常以终止for循环
3. eg:

class Reverse:
    """Iterator for looping over a sequence backwards."""
    def __init__(self, data):
        self.data = data
        self.index = len(data)

    def __iter__(self):
        return self

    def __next__(self):
        if self.index == 0:
            raise StopIteration
        self.index = self.index - 1
        return self.data[self.index]

>>> rev = Reverse('spam')
>>> iter(rev)
<__main__.Reverse object at 0x00A1DB50>
>>> for char in rev:
...     print(char)
...
m
a
p
s