Incorrect type for metaclass __dict__

[tim-mitchell]

Bug Report

Given a metaclass called Meta, mypy considers the Meta.__dict__ attribute to have type def (self: builtins.type) -> types.MappingProxyType[builtins.str, Any] when the actual type is types.MappingProxyType[builtins.str, Any].

To Reproduce
Run mypy on this file.

class Meta(type):
    def __call__(cls, *args, **kw):
        return super().__call__(*args, **kw)

reveal_type(Meta.__dict__)
call = Meta.__dict__['__call__']  # mypy raises error here

source: https://gist.github.com/mypy-play/843cafceeef590efc6d3705b44c29c19
playground: https://mypy-play.net/?mypy=latest&python=3.11&gist=843cafceeef590efc6d3705b44c29c19

Expected Behavior

reveal_type(Meta.__dict__) returns types.MappingProxyType[builtins.str, Any]
No errors reported by mypy for this file.

Actual Behavior

mypy_test.py:6: note: Revealed type is "def (self: builtins.type) -> types.MappingProxyType[builtins.str, Any]"
mypy_test.py:7: error: Value of type "Callable[[type], MappingProxyType[str, Any]]" is not indexable  [index]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.7.0
• Mypy command-line flags: --check-untyped-defs
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

[erictraut]

I think mypy is doing the correct thing here given the way __dict__ is defined in builtins.pyi. It's defined as a property, and an access to a property through a class returns the property object.

class type:
    ...
    @property
    def __dict__(self) -> types.MappingProxyType[str, Any]: ...  # type: ignore[override]

FWIW, pyright's behavior is the same as mypy's.

I suspect that __dict__ was modeled as a property in typeshed because it is read-only. If the type system were to include support for a ReadOnly qualifier like the one proposed in draft PEP 705, typeshed could model this better.

[tim-mitchell]

re-raised as python/typeshed#11033