'Final' (PEP 591) shouldn't apply to '__private' symbols

[gvanrossum]

Bug Report

Since __private symbols are not shared between classes, I think we shouldn't complain if a symbol (either variable or method) that's declared Final or @final is redefined in a subclass. If you agree, we should probably add something to PEP 591 about this.

To Reproduce

from typing import Final, final

class Parent:
    __foo: Final[int] = 0

    @final
    def __bar(self) -> None:
        pass

class Child(Parent):
    __foo: Final[int] = 1  # error: Cannot override final attribute "__foo" (previously declared in base class "Parent")

    @final  # error: Cannot override final attribute "__bar" (previously declared in base class "Parent")
    def __bar(self) -> None:
        pass

Expected Behavior

No errors.

Your Environment

• Mypy version used: 0.800+dev.37d2a5ff7a245e58af18a84e04581ccddbfb8414
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): nothing
• Python version used: 3.9
• Operating system and version: Windows 10

[hauntsaninja]

That makes sense to me. I think mypy in general is just missing mangling logic (see #9446, #8267, I also had to implement custom logic in stubtest)

[gvanrossum]

Heh, that's too bad. This was reported for pyright and they just fixed it. :-/