[issue39046] collections.abc.Reversible should not be a subclass of Hashable

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

I don't know that this is easily solved. By design, issubclass(X, Hashable) checks whether X defines a __hash__ method (or at least has a class attribute __hash__ that isn't None). And because everything ultimately derives from object, which *does* have a __hash__ method (that just hashes the object's address), everything appears hashable, *except* if it explicitly sets __hash__ = None.

You'll find that many classes defined in collections.abc are hashable (e.g. Iterable, Iterator, Sequence, Collection).

But not Set and Mapping. This is because those override __eq__, and there's some deep magic somewhere that sets __hash__ = None in the class dict if __eq__ is overridden.

You could try the following: add an explicit __hash__ = None to all the collection ABCs (in _collections_abc.py) that you think shouldn't define __hash__, and see if all the tests pass. 

However, even if they do, I suspect that this may break stuff. E.g. a class that inherits from Iterable but doesn't override __hash__ or __eq__ would suddenly no longer be usable as a dict key.

Since this is only a problem with abstract classes like Iterable or Reversible, maybe we should just ignore the problem?

----------
nosy: +gvanrossum

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39046>
_______________________________________