Incompatible override when overriding non-overload with overload

from typing import overload, Union

class A:
    def f(self, x: Union[int, str]) -> None: ...
    
class B(A):
    @overload
    def f(self, x: int) -> None: ...
    @overload
    def f(self, x: str) -> None: ...
    def f(self, x: Union[int, str]) -> None: ...

https://mypy-play.net/?mypy=latest&python=3.10&gist=37c2291afd7ddd170576b1de208ba77c

produces:

main.py:7: error: Signature of "f" incompatible with supertype "A"
main.py:7: note:      Superclass:
main.py:7: note:          def f(self, x: Union[int, str]) -> None
main.py:7: note:      Subclass:
main.py:7: note:          @overload
main.py:7: note:          def f(self, x: int) -> None
main.py:7: note:          @overload
main.py:7: note:          def f(self, x: str) -> None
Found 1 error in 1 file (checked 1 source file)

But as far as I can see this is safe. Mypy accepts a union as an argument to B.f:

arg: Union[int, str]
B().f(arg)

Of course, the overloads are useless in this toy example, but the error also comes up if you vary the return type in the overloads. This came up in python/typeshed#7510.

Possibly a duplicate of #495

This can also go the other way around - something I encountered while working with Tkinter and subclassing widgets:

from __future__ import annotations

from typing import Any, overload

# defined within the tkinter itself (roughly)
class BaseWidget:
    @overload
    def configure(self, arg: int) -> int:
        ...

    @overload
    def configure(self, arg: str) -> str:
        ...

    def configure(self, arg: int | str) -> int | str:
        ...

    config = configure  # alias for the 'configure' method

# my subclass
class SubWidget(BaseWidget):
    # overwrite 'configure' with a pass-through function that runs some extra code first
    # NOTE: 'Any' usage to not have to replicate the overloads, this type-checks just fine
    def configure(self, *args: Any, **kwargs: Any) -> Any:
        ...  # some extra code
        super().configure(*args, **kwargs)

    # ..., but the alias complains
    config = configure  # error: Incompatible types in assignment (expression has type "Callable[[SubWidget, VarArg(Any), KwArg(Any)], Any]", base class "BaseWidget" defined the type as overloaded function)

Current workaround:

    def config(self, *args: Any, **kwargs: Any) -> Any:
        self.configure(*args, **kwargs)

Handle edge case in overload subtyping (python#22007)

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Incompatible override when overriding non-overload with overload #12379

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Incompatible override when overriding non-overload with overload #12379

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JelleZijlstra commented on Mar 19, 2022

DevilXD commented on Mar 26, 2022

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