`self` not considered positional-only when followed by `**kwargs`

[osandov]

Bug Report

The following example using Concatenate to represent a method passes mypy in strict mode:

https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=5d5a1b27f2838371bee58b2d9c9a7721

from collections.abc import Callable
from typing import Any, Concatenate


def func(x: int) -> None:
    return


class MyClass:
    def method(self, x: int) -> None:
        return


def check[**P](f: Callable[P, None], m: Callable[Concatenate[Any, P], None]) -> None:
    return


check(func, MyClass.method)

However, if the signature has **kwargs, it fails with Argument 2 to "check" has incompatible type ... This is likely because "method of MyClass" has named arguments: "self".

https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=ef01c9fd1251d95be27ce96401ec7fe8

from collections.abc import Callable
from typing import Any, Concatenate


def func(**kwargs: Any) -> None:
    return


class MyClass:
    def method(self, **kwargs: Any) -> None:
        return


def check[**P](f: Callable[P, None], m: Callable[Concatenate[Any, P], None]) -> None:
    return


check(func, MyClass.method)

Explicitly making self positional-only fixes this, but in my case, I don't control the method signature (in my actual use case, the method is argparse.ArgumentParser.add_argument() and the function is copy_method_params() from python/cpython#121693).

To Reproduce

https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=ef01c9fd1251d95be27ce96401ec7fe8

Expected Behavior

There should be no errors.

Actual Behavior

There is an error:

main.py:18: error: Argument 2 to "check" has incompatible type "Callable[[Arg(MyClass, 'self'), KwArg(Any)], None]"; expected "Callable[[Any, KwArg(Any)], None]"  [arg-type]
main.py:18: note: This is likely because "method of MyClass" has named arguments: "self". Consider marking them positional-only
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: master, latest version
• Mypy command-line flags: --strict

[sterliakov]

self is never positional-only unless declared as such. Consider:

from collections.abc import Callable
from typing import Any, Concatenate

class MyClass:
    def method(self, **kwargs: Any) -> None:
        return


def check[**P](func: Callable[Concatenate[Any, P], None], *args: P.args, **kwargs: P.kwargs) -> None:
    func(MyClass(), *args, **kwargs)

check(MyClass.method, self=1)  # E: Argument 1 to "check" has incompatible type "Callable[[Arg(MyClass, 'self'), KwArg(Any)], None]"; expected "Callable[[Any, KwArg(Any)], None]"  [arg-type]
# Runtime: TypeError: MyClass.method() got multiple values for argument 'self'

playground

[osandov]

self is never positional-only unless declared as such.

In that case, why doesn't my first example (https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=5d5a1b27f2838371bee58b2d9c9a7721) produce an error?

[sterliakov]

Because there's no fallback **kwargs or explicit self kw-only argument - your Concatenate strips the first argument, and you're left with one, positional-or-keyword argument x, there's no uncertainty whether self can be also passed as kwarg later.

Concatenate treats that as posonly, and def method(self, /, **kwargs) won't raise the runtime error, so we shouldn't reject it inside check.

[osandov]

Ok, that makes sense now, thanks! I found a solution by replacing the Callable with a Protocol that includes the self parameter.