Shared via mypy Playground

main.py

class X:
    val: object
    
class Y(X):
    @property
    def val(self) -> object:
        return object()
    @val.setter
    def val(self, value):
        self.x = True

class Z(X):
    @property
    def val(self):
        return object()


a: X = Z()
a.val = 3


class A:
    def f(self, a):
        print(a)

class B(A):
    def f(self) -> None:
        print(2)