isinstance with self.__class__ infers 'object' as the type

[JukkaL]

An unexpected type is revealed by this program:

from typing import Any

class A:
 def f(self, o: Any) -> None:
 isinstance(o, self.__class__) and reveal_type(o) # Revealed type is 'builtins.object'

The issue was introduced by #2997. The most precise type would be A. Mypy used to infer Any as the type before #2997.

[JukkaL]

A slightly more straightforward way to repro:

from typing import Any

class A:
 def f(self, o: Any) -> None:
 if isinstance(o, self.__class__):
 reveal_type(o) # builtins.object

[elazarg]

It should be fixable by changing typeshed:

class object:
 @property
 def __class__(self: _T) -> Type[_T]: ...

Seems to work (at least for this particular case).

 reveal_type(o) # Revealed type is 'tmp.A*'

[pkch]

@elazarg I think that's a good fix. It should probably be combined with #3056.

[JelleZijlstra]

@elazarg's proposed typeshed change isn't quite right because __class__ is writable. Though I suppose if you assign to __class__ there's little hope for typechecking your code. I'll try adding a setter.

[gvanrossum]

This should be reopened since python/typeshed#1549 was reverted in python/typeshed#1632.

[ilevkivskyi]

@gvanrossum I think the original PR can be simply re-applied, since the issue that caused reverting it should be fixed now.