False-negative: ParamSpec.args annotating non-*args

Per the docs, "P.args ... should only be used to annotate *args":

from typing import ParamSpec

P = ParamSpec('P')


def f(args: P.args) -> None:
    g(*args)


def g(a: str) -> None:
    print(type(a))
    
    

f(42)

Gist:
https://mypy-play.net/?mypy=latest&python=3.11&gist=28f03ed1b30e9707edfd080095b6a868

The code above (in particular, the signature for function f) isn't legal. A P.args annotation can be used only within a context where P is bound to a scope (a generic function or class), only with a *args parameter, and only when paired with a corresponding **kwargs: P.kwargs parameter. See PEP 612 for details.

Mypy should flag the illegal use of P.args as an error. (That makes this a false negative, not a false positive.)

@erictraut

@erictraut, thanks for jumping in so quickly!

I've read this in the docs

P.args represents the tuple of positional parameters in a given call

and failed to read the rest of it :)

[...] and should only be used to annotate *args

Indeed, then it shall be a false negative.

Reject invalid ParamSpec locations (python#18278)

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False-negative: ParamSpec.args annotating non-*args #14622

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False-negative: ParamSpec.args annotating non-*args #14622

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erictraut commented on Feb 6, 2023

ikonst commented on Feb 6, 2023

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