Subclasses cannot use properties to override super-class attributes

[OddBloke]

When subclassing, overriding a class attribute with a property with the same type causes type checking failures. This is (I think?) a fairly common way for subclasses to introduce dynamic behaviour transparently to consumers, so my expectation is that this would not cause failures.

Python Code

from typing import List


class Super:

    x: List[str] = []
    reveal_type(x)


class Sub(Super):

    @property
    def x(self) -> List[str]:
        return []
    reveal_type(x)

mypy Output

$ mypy reproducer.py
reproducer.py:7: error: Revealed type is 'builtins.list[builtins.str]'
reproducer.py:13: error: Signature of "x" incompatible with supertype "Super"
reproducer.py:15: error: Revealed type is 'def (self: reproducer.Sub) -> builtins.list[builtins.str]'

Versions

$ python3 --version  # This is the packaged version in Ubuntu 19.04
Python 3.7.2+
$ mypy --version
mypy 0.710+dev.b29a4330d2d7d1032c30a1990e7f1c477db070fd

[JelleZijlstra]

This is unsafe because the superclass allows writing to x (y = Super(); y.x = [] works), but the subclass doesn't (y = Sub(); y.x = [] is an AttributeError).

[OddBloke]

Aha, right, that makes sense. Thanks!

[ilevkivskyi]

Mypy however still complains if I make the property settable. I looked at the code and it is indeed buggy (several corner cases with is_property and is_settable_property are not considered).

Generally, the way how we check incompatibility with supertype is fragile and uses a lot of code duplication with checkmember.py. I would say we should simplify this significantly by just re-using checkmember.py. See also #5136

[ilevkivskyi]

Oh, this is actually a duplicate of #4125