Metaclass confusion with properties

I'm using python 3.8 and mypy 0.740. The script below prints "21" and "42" when run, but fails to type check with mypy:

class meta (type):
    pass

class value (metaclass=meta):
    pass


class submeta (meta):
    @property
    def attr(cls) -> int:
        return 42

class subvalue (metaclass=submeta):
    @property
    def attr(self) -> int:
        return 21

o = subvalue()
a = o.attr
print(a)
a = subvalue.attr
print(a)

The mypy diagnostics say:

meta-test.py:24: error: Incompatible types in assignment (expression has type "Callable[[subvalue], int]", variable has type "int")
Found 1 error in 1 file (checked 1 source file)

The error is incorrect, "subvalue.attr" has type "int", as it resolves to the property defined on the metaclass, not to the property defined on the class.

This is fairly esoteric as written, but is the structure used by PyObjC to expose Objective-C classes. PyObjC uses metaclasses to expose class methods because ObjC classes can have instance- and class-methods with the same name.

Riproduci la diagnostica usando l’esempio meta-test.py descritto nell’issue con Python 3.8 e mypy 0.740. Inizia tracciando come mypy risolve le proprietà su subvalue rispetto alla sua metaclasse submeta. Il lavoro è completo quando subvalue.attr viene inferito come int senza l’errore incompatible-assignment segnalato, mentre l’accesso all’istanza rimane corretto.