no error when calling `Callable` with `ParamSpec` using `Concatenate` with not enough args

[DetachHead]

from typing import Callable, Concatenate, ParamSpec

P = ParamSpec("P")


def decorator(fn: Callable[Concatenate[str, P], None]) -> None:
    fn("value") # runtime TypeError: foo() missing 1 required positional argument: 's2'


@decorator
def foo(s: str, s2: str) -> None:
    ...

playground

[AlexWaygood]

I think mypy should indeed emit an error on your code snippet, but not the error you think. Your use of ParamSpec here is invalid -- the ParamSpec P in the decorator function will go unsolved, since it is only used once in the function signature.

[KotlinIsland]

@AlexWaygood It is true that the ParamSpec is only used once, but there is still an issue that the P could represent any signature. Look at this playground

[AlexWaygood]

@AlexWaygood It is true that the ParamSpec is only used once, but there is still an issue that the P could represent any signature. Look at this playground

Yes, that example illustrates the type-safety issue much more clearly. So that other people do not have to click through to see the snippet, here it is copied out. No mypy errors are emitted, but TypeError will always be raised if you try to call the foo function at runtime:

from typing import Callable, Concatenate, ParamSpec

P = ParamSpec("P")


def decorator(fn: Callable[Concatenate[str, P], None]) -> Callable[P, None]:
    def inner(*args: P.args, **kwargs: P.kwargs) -> None:
        fn("value")
    return inner
        


@decorator
def foo(s: str, s2: str) -> None:
    ...

[sterliakov]

Seems like this has nothing to do with Concatenate. The following is green on master, with and without --strict:

from typing_extensions import ParamSpec
from typing import Callable

_P = ParamSpec("_P")

def run(predicate: Callable[_P, None], *args: _P.args, **kwargs: _P.kwargs) -> None:
    predicate()

[sterliakov]

@AlexWaygood Since this is a major problem (not detecting a relatively common problem of forgetting to pass args&kwargs), I'll try to ping someone here once again: are there any blockers that prevent #17323 (green, small, mergeable) from being merged? Is there anyone I can ping to get that patch reviewed? Something wrong there from my side, making the review or the patch implausible/undesired/inefficient/whatever?

[AlexWaygood]

Unfortunately I'm only a triager here so do not have power to merge. I could review, but I also have pretty limited time to work on mypy right now, sadly. I'll post on the mypy maintainer Discord, however

[hauntsaninja]

Thanks sterliakov and DetachHead!