Invalid ParamSpec usage in function with added kwargs not reported as error

[sterliakov]

Bug Report

According to PEP-612,

Placing keyword-only parameters between the *args and **kwargs is forbidden.

However, mypy doesn't reject such usage.

To Reproduce

from __future__ import annotations

from typing import TypeVar, ParamSpec, Protocol

_P = ParamSpec('_P')
_R = TypeVar('_R', covariant=True)

class BetterCallable(Protocol[_P, _R]):
    def __call__(self, *args: _P.args, x: int, **kwargs: _P.kwargs) -> _R: ...

Playground

Also consider this code quoted from PEP-612:

from __future__ import annotations
from typing import TypeVar, ParamSpec, Callable, Concatenate

P = ParamSpec('P')

def add(f: Callable[P, int]) -> Callable[Concatenate[str, P], None]:

  def foo(s: str, *args: P.args, **kwargs: P.kwargs) -> None:  # Accepted
    pass

  def bar(*args: P.args, s: str, **kwargs: P.kwargs) -> None:  # Rejected
    pass

  return foo                                                   # Accepted

And here's another playground.

Expected Behavior

In first case the __call__ definition should be flagged as error.

In second case the behaviour should match the inline comments.

Actual Behavior

In first case no errors are found.

In second case foo is rejected due to named arguments (this violates PEP-612, but is easily fixable with __s arg name instead, as usual, error message can be improved here). while bar is accepted, also violating the PEP:

main.py:14: error: Incompatible return value type (got "Callable[[Arg(str, 's'), **P], None]", expected "Callable[[str, **P], None]")  [return-value]
main.py:14: note: This is likely because "foo" has named arguments: "s". Consider marking them positional-only
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: master, 1.0.1
• Mypy command-line flags: --strict and without it
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10, 3.11

[sterliakov]

This is the cause of #13966 - the invalid usage is not reported there.

[nickzoic]

In general mypy doesn't seem to be picking up on errors around the 'rejected' behaviours, just silently ignoring the annotion?

Eg: the following code, case f1 is typed correctly but f2 through f5 are all incorrect:

from typing import Callable, ParamSpec

P1 = ParamSpec('P1')
def f1(x : Callable[P1,None]):
    def f11(*args: P1.args, **kwargs: P1.kwargs) -> None:
        pass

P2 = ParamSpec('P2')
def f2(x : Callable[P2,None]):
    def f21(a: P2.args, b: P2.kwargs) -> None:
        pass

P3 = ParamSpec('P3')
def f3(x : Callable[P3,None]):
    def f31(*args: P3.args) -> None:
        pass

P4 = ParamSpec('P4')
def f4(x : Callable[P4,None]):
    def f41(**kwargs: P4.kwargs) -> None:
        pass

P5 = ParamSpec('P5')
def f5(x : Callable[P5,None]):
    def f51(*args: P5.args, a: int, **kwargs: P5.kwargs) -> None:
        pass

this code generates no issues in mypy 1.0.1 but pyright 1.1.339 correctly flags errors for each of cases 2 .. 5:

/tmp/paramspec_types_2.py
  /tmp/paramspec_types_2.py:10:16 - error: "args" member of ParamSpec is valid only when used with *args parameter
  /tmp/paramspec_types_2.py:10:28 - error: "kwargs" member of ParamSpec is valid only when used with **kwargs parameter
  /tmp/paramspec_types_2.py:15:20 - error: "args" and "kwargs" members of ParamSpec must both appear within a function signature
  /tmp/paramspec_types_2.py:20:23 - error: "args" and "kwargs" members of ParamSpec must both appear within a function signature
  /tmp/paramspec_types_2.py:25:29 - error: Keyword parameter "a" cannot appear in signature after ParamSpec args parameter (reportGeneralTypeIssues)
5 errors, 0 warnings, 0 informations 

[flying-sheep]

See here for a discussion to make this happen: https://discuss.python.org/t/type-hints-for-kwarg-overrides/47196