support ParamSpec

[carljm]

[danielkelshaw]

Just a quick example that doesn't work:

import typing as tp


def my_decorator[**P, T](fn: tp.Callable[P, T]) -> tp.Callable[P, T]:

    def wrapped(*args: P.args, **kwargs: P.kwargs) -> T:
        print('in decorator..')
        return fn(*args, **kwargs)
    
    return wrapped


@my_decorator
def my_adder(a: int, b: int) -> int:
    return a + b

print(my_adder(1, 3))

This gives the errors:

error: lint:invalid-type-form: The first argument to `Callable` must be either a list of types, ParamSpec, Concatenate, or `...`
 --> main.py:4:42
  |
4 | def my_decorator[**P, T](fn: tp.Callable[P, T]) -> tp.Callable[P, T]:
  |                                          ^
5 |
6 |     def wrapped(*args: P.args, **kwargs: P.kwargs) -> T:
  |

error: lint:invalid-type-form: The first argument to `Callable` must be either a list of types, ParamSpec, Concatenate, or `...`
 --> main.py:4:64
  |
4 | def my_decorator[**P, T](fn: tp.Callable[P, T]) -> tp.Callable[P, T]:
  |                                                                ^
5 |
6 |     def wrapped(*args: P.args, **kwargs: P.kwargs) -> T:
  |

Found 2 diagnostics

[dhruvmanila]

I think it would be useful to not error in this case even without the support of ParamSpec, not sure how hard that would be.

[sharkdp]

I think it would be useful to not error in this case even without the support of ParamSpec, not sure how hard that would be.

See astral-sh/ruff#18001

[Dr-Irv]

Here's another example where ParamSpec is not accepted as an argument to Protocol. This example works fine with pyright, mypy and pyrefly:

from typing import Protocol, TypeVar, ParamSpec

# Define a TypeVar for the return type of the callable
R = TypeVar('R', covariant=True)

# Define a ParamSpec to capture the parameters of the callable
P = ParamSpec('P')

class LoggableFunction(Protocol[P, R]):
    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> R:
        ...