Non-overlapping comparison with Dict and MutableMapping subclass

[Dreamsorcerer]

Trivial reproducer (with --strict-equality):

class A(MutableMapping[str, Any]): pass
def something() -> A: pass
something() == {}

Outputs:

error: Non-overlapping equality check (left operand type: "A", right operand type: "Dict[<nothing>, <nothing>]")  [comparison-overlap]

It seems that it believes a dictionary cannot be equal to a class which inherits from MutableMapping.

[Dreamsorcerer]

No longer reproducible.

(Missed the CLI argument).

[JelleZijlstra]

Mypy seems correct here. dict.__eq__ never returns True for a non-dict, and there's no __eq__ in A's MRO that would make it return equal.

[Dreamsorcerer]

You're right, it works fine with __eq__() defined. Now if I can just remember what code I was looking at when I filed the issue...

[Dreamsorcerer]

Found it: https://github.com/aio-libs/aiohttp-session/blob/master/tests/test_encrypted_cookie_storage.py#L115

The test seems to pass, even though there is no __eq__() defined.

[Dreamsorcerer]

OK, it seems like dict.__eq__ calls __iter__(), so it does work with non-dict types.

This works at runtime:

from typing import Any, MutableMapping

class A(MutableMapping[str, Any]):
    def __delitem__(self, *args):
        ...
    def __getitem__(self, *args):
        ...
    def __iter__(self):
        return iter(())
    def __len__(self):
        ...
    def __setitem__(self, *args):
        ...

print(A() == {})  # True

[JelleZijlstra]

Looks like Mapping defines an __eq__ that allows two mappings to compare equal.

[Dreamsorcerer]

OK, so it's just a typeshed issue? If so, I'll look at making a fix tomorrow.

[Dreamsorcerer]

python/typeshed#10297