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Bad error message for arguments between ParamSpec P.args and P.kwargs #13966
Description
Bug Report
ParamSpec P.args and P.kwargs are not defined when using explicit keyword argument.
To Reproduce
This code would be used to type anyio.run().
from typing import ParamSpec, Awaitable, TypeVar, Callable, Any
P = ParamSpec("P")
T_Retval = TypeVar("T_Retval")
def run(
func: Callable[P, Awaitable[T_Retval]],
*args: P.args,
backend: str = "asyncio",
backend_options: dict[str, Any] | None = None,
**kwargs: P.kwargs,
) -> T_Retval:
passbut it works without explicit keyword arguments.
from typing import ParamSpec, Awaitable, TypeVar, Callable, Any
P = ParamSpec("P")
T_Retval = TypeVar("T_Retval")
def run(
func: Callable[P, Awaitable[T_Retval]],
*args: P.args,
**kwargs: P.kwargs,
) -> T_Retval:
passExpected Behavior
It should be able to type and recognize that run() passes the rest of the arguments to the function a().
async def a(b: str, c: int) -> None:
pass
run(a, "", c=0)Actual Behavior
main.py:10: error: Name "P.args" is not defined
main.py:13: error: Name "P.kwargs" is not defined
Your Environment
- Mypy version used: 0.981
- Mypy command-line flags: none
- Mypy configuration options from
mypy.ini(and other config files): none - Python version used: 3.10
Activity
erictraut commentedon Oct 30, 2022
With
ParamSpec, you are not allowed to define parameters between*argsand**kwargs. PEP 612 is very clear on this point:Mypy's error message in this case should probably be improved to indicate as such.
If you move the parameters
backendandbackend_optionsbefore*args, it will type check without errors. That assumes you wantbackendandbackend_optionsto be positional + keyword parameters and not keyword-only parameters. There's no way to combineParamSpecwith keyword-only parameters because of the constraint I mentioned above.hauntsaninja commentedon Oct 31, 2022
mypy 0.990 will have a less atrocious error message here (but definitely could still be made more helpful)
Add another invalid ParamSpec test case
Add another invalid ParamSpec test case (#13968)
ilevkivskyi commentedon Aug 16, 2023
Yeah, the error message is still not very good:
[-]ParamSpec P.args and P.kwargs are not defined when using explicit keyword argument[/-][+]Bad error message for arguments between ParamSpec P.args and P.kwargs[/+]Reject invalid ParamSpec locations (python#18278)