There are several changes:

1. __len__ is no longer there, because old-style iterable protocol does not need it, see

   cpython/Objects/iterobject.c

   Line 76 in 84bc6a4

   return NULL;

2. Foo is now fully runnable, so people can see what __getitem__ should look like
3. any("__iter__" in B.__dict__ for B in C.__mro__) is simplified to be hasattr, much more common and much more pythonic (initial request by @JosephSBoyle)
4. I've added a comment about why we need return NotImplemented here, see https://docs.python.org/3/library/abc.html#abc.ABCMeta.__subclasshook__
5. self.__iter__() is much better as iter(self), calling magic methods directly is not something we should promote in this case. Now it is in sync with Foo.get_iterator()

• Issue: Subtle pattern in the __subclasshook__ docs isn't explained #100407