Inheritance issues with generic collections

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?

I'll look over your patch.
I'll see if I can release a 3.5.1 version -- it looks like 3.5.2 is still a while away.
It's probably an oversight that Iterable isn't abstract; or perhaps there was a technical issue.

Updated the typing gpackage to 3.5.1. Please verify that it's to your liking.

Updated the typing gpackage to 3.5.1.

Thanks!

As for the abstract methods, adding __iter__ to Iterable seems to have no ill effects.

Container, Awaitable, and AsyncIterable and their derivatives are also affected — in other words everything that doesn't inherit from Sized or Hashable.

One option is to just copy everything over during class construction:

 class GenericMeta(TypingMeta, abc.ABCMeta):

     def __new__(cls, name, bases, namespace,
                 tvars=None, args=None, origin=None, extra=None):
+        if extra and origin is None:
+            for methodname in getattr(extra, '__abstractmethods__', ()):
+                if methodname in extra.__dict__:
+                    namespace.setdefault(methodname, extra.__dict__[methodname])
+
         self = super().__new__(cls, name, bases, namespace, _root=True)

I would like to go slow here. I have other priorities (see #203 (comment)). I will eventually get to this but I'm not sure I want to rush anything into 3.5.2 beyond what's already on deck.

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Inheritance issues with generic collections #202

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Inheritance issues with generic collections #202

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gvanrossum commented on Apr 16, 2016

gvanrossum commented on Apr 16, 2016

bintoro commented on Apr 16, 2016

gvanrossum commented on Apr 18, 2016

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