現在你可能注意到大多數容器對象都可以用?for?遍歷: ~~~ for element in [1, 2, 3]: print(element) for element in (1, 2, 3): print(element) for key in {'one':1, 'two':2}: print(key) for char in "123": print(char) for line in open("myfile.txt"): print(line) ~~~ 這種形式的訪問清晰、簡潔、方便。迭代器的用法在 Python 中普遍而且統一。在后臺，?for?語句在容器對象中調用?iter()?。該函數返回一個定義了?next()?方法的迭代器對象，它在容器中逐一訪問元素。沒有后續的元素時，?next()?拋出一個?StopIteration?異常通知?for?語句循環結束。以下是其工作原理的示例: ~~~ >>> s = 'abc' >>> it = iter(s) >>> it >>> next(it) 'a' >>> next(it) 'b' >>> next(it) 'c' >>> next(it) Traceback (most recent call last): File "", line 1, in ? next(it) StopIteration ~~~ 了解了迭代器協議的后臺機制，就可以很容易的給自己的類添加迭代器行為。定義一個?__iter__()方法，使其返回一個帶有?next()?方法的對象。如果這個類已經定義了?next()?，那么?__iter__()只需要返回?self: ~~~ 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) >>> for char in rev: ... print(char) ... m a p s ~~~