I have a problem with how __extra__ is currently inherited:

>>> class CustomMapping(typing.Mapping): ...
... 
>>> issubclass(dict, CustomMapping)
True

>>> class CustomDict(typing.Dict): ...
... 
>>> issubclass(dict, CustomDict)
True

This can't be right.

My initial impression is that __extra__ should only be propagated on parameterization, not when subclassing, and that the concrete types List, Dict, etc. should have their own extras (list, dict, and so on). Thoughts? A tentative patch is here.

Another thing: The module distributed on PyPI still has the issue where a generic doesn't include itself in the subtype's MRO during parameterization. This was fixed for Python 3.5.1 in aab2c59. It would be great if we could get a more recent revision on PyPI as well.

3.5.1:

>>> issubclass(Iterable[int], Iterable)
True

3.5.0:

>>> issubclass(Iterable[int], Iterable)
False

And finally, a question: The collections ABC replacements do not provide the abstract (or any other) methods of the originals. In particular, Iterable is technically not abstract at all. Is this intentional or just a temporary state of affairs?