关于内置iter__

而 documentation doesn't make it very clear ,这是因为 __iter__ 必须 (不是 应该 )返回an 迭代器 ,而不是 可迭代的 :

% python
Python 3.6.3 (default, Oct  3 2017, 21:45:48) 
[GCC 7.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo:
...     def __iter__(self):
...         return []
... 
>>> iter(Foo())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: iter() returned non-iterator of type 'list'