Allow union-with-callable attributes to be overridden by methods

[brianschubert]

Fixes #12569

Before

from typing import Callable

class Parent:
    f1: Callable[[str], str]
    f2: Callable[[str], str] | str

class Child(Parent):
   def f1(self, x: str) -> str: return x  # ok
   def f2(self, x: str) -> str: return x  # Signature of "f2" incompatible with supertype "Parent"

After

from typing import Callable

class Parent:
    f1: Callable[[str], str]
    f2: Callable[[str], str] | str

class Child(Parent):
   def f1(self, x: str) -> str: return x  # ok
   def f2(self, x: str) -> str: return x  # ok

[hauntsaninja]

Nice! Could you add a test case with this error code enabled: https://github.com/python/mypy/pull/16399/files

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

packaging (https://github.com/pypa/packaging)
+ src/packaging/specifiers.py:248: error: Unused "type: ignore" comment  [unused-ignore]

steam.py (https://github.com/Gobot1234/steam.py)
- steam/models.py:239: error: Signature of "url" incompatible with supertype "_IOMixin"  [override]
- steam/models.py:239: note:      Superclass:
- steam/models.py:239: note:          str | _ReadOnlyProto[str]
- steam/models.py:239: note:      Subclass:
- steam/models.py:239: note:          str
- steam/reaction.py:236: error: Signature of "url" incompatible with supertype "_IOMixin"  [override]
- steam/reaction.py:236: note:      Superclass:
- steam/reaction.py:236: note:          str | _ReadOnlyProto[str]
- steam/reaction.py:236: note:      Subclass:
- steam/reaction.py:236: note:          str
- steam/reaction.py:273: error: Signature of "url" incompatible with supertype "_IOMixin"  [override]
- steam/reaction.py:273: note:      Superclass:
- steam/reaction.py:273: note:          str | _ReadOnlyProto[str]
- steam/reaction.py:273: note:      Subclass:
- steam/reaction.py:273: note:          str
- steam/reaction.py:312: error: Signature of "url" incompatible with supertype "_IOMixin"  [override]
- steam/reaction.py:312: note:      Superclass:
- steam/reaction.py:312: note:          str | _ReadOnlyProto[str]
- steam/reaction.py:312: note:      Subclass:
- steam/reaction.py:312: note:          str

[hauntsaninja]

Thanks!

I kind of want to try ignore_pos_arg_names=False. This is something we probably want to eventually do anyway and should be fairly harmless in this case. But it's likely wiser to be consistent.

[hauntsaninja]

I was thinking about this PR some more and I think it introduces some unsoundness. We should have a Liskov error in the following:

import typing

class X:
    @property
    def x(self) -> typing.Optional[bool]: ...

    @x.setter
    def x(self, value: typing.Optional[bool]) -> None: ...

class Y(X):
    @property
    def x(self) -> bool: ...

    @x.setter
    def x(self, value: bool) -> None: ...

Input to setter should be wider, output of getter should be narrower.

[brianschubert]

Hmm, I think that setup is currently caught by mutable-override, is it not? Is the behavior after this PR different from what mypy does for this setup?

class X:
    x: bool | None

class Y(X):
    x: bool

[hauntsaninja]

Oh looks like I must have messed up my test. It does error with mutable-override, sorry for the noise!

[hauntsaninja]

I think what happened is I messed up the branches I tested. I think I probably did this, which mypy doesn't error on:

class AA:
    @property
    def a(self) -> int: ...
    @a.setter
    def a(self, value: int | str) -> None: ...

class BB(AA):
    @property
    def a(self) -> int: ...
    @a.setter
    def a(self, value: int) -> None: ...

This would be unsound, except that mypy doesn't support this at all and will error if you attempt to set a to str. This is the (very popular) issue for that: #3004