> If this patch goes ahead, I think the ABC documentation should clarify which methods are checked for None and which aren’t. 

That seems fair. 

Also, as you pointed out on #25958, at least one other ABC has the same problem as Iterable: you can block the "in" operator by setting __contains__=None, but you'll still be a Container. So, do we want to go through all of the existing ABCs and make sure they all do this negative check, instead of just Iterable?

> Also, what is the point of the odd __getitem__() method in test_enumerate.py? Maybe you should use assertRaisesRegex() to check that the intended TypeError is actually raised.

If an implementation doesn't raise a TypeError there, that's a failure. If it raises one with a different (possibly less helpful) message, I think that's just a quality-of-implementation issue, isn't it?