Help with "ParamSpec is unbound" mypy error in decorator for sync + async functions

[FelixSiegel]

Hi everyone,

I'm currently writing a decorator that should catch errors for both async and non-async functions. While it works as expected, mypy complains about the use of ParamSpec, and I'm not sure why.

Here's my implementation:

from typing import Callable, ParamSpec, TypeVar, Awaitable, overload
import asyncio
import functools

Param = ParamSpec("Param")
RetType = TypeVar("RetType")

class Logger:
    @overload
    @classmethod
    def catch_errors(
        cls, func: Callable[Param, Awaitable[RetType]]
    ) -> Callable[Param, Awaitable[RetType | None]]: ...

    @overload
    @classmethod
    def catch_errors(
        cls, func: Callable[Param, RetType]
    ) -> Callable[Param, RetType | None]: ...

    @classmethod
    def catch_errors(cls, func):
        if asyncio.iscoroutinefunction(func):

            @functools.wraps(func)
            async def async_wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType | None:  # ← mypy error here
                try:
                    return await func(*args, **kwargs)
                except Exception as err:
                    # do some error logging
                    return None
            return async_wrapper

        else:

            @functools.wraps(func)
            def wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType | None:  # ← mypy error here
                try:
                    return func(*args, **kwargs)
                except Exception as err:
                    # do some error logging
                    return None
            return wrapper

The error from mypy:

error: ParamSpec "Param" is unbound  [valid-type]

As noted in the comments, the error appears on both wrapper function definitions where I use *args: Param.args, **kwargs: Param.kwargs.

Does anyone know how to fix this or why mypy doesn’t like the usage of ParamSpec in this context?

Thanks in advance!

[erictraut]

I'm not able to repro the error. Here's your code in the mypy playground. Perhaps you're using an outdated version of mypy?

[FelixSiegel]

Oh right, my bad — I forgot I had an mypy.ini file with the following content:

[mypy]
check_untyped_defs = True
explicit_package_bases = True

Looks like enabling this is what’s causing the error.

[Daverball]

You need to add annotations to your implementation signature as well, since mypy will use those annotations to check the body.

Note the use of Any for the implementation for the Callable return type, this reduces type safety slightly in the implementation, but if you try to use a union of the two overloads instead, you will end up with errors in other places, since mypy can't figure out the relationship between the two overloads.

from typing import Any, Callable, ParamSpec, TypeVar, Awaitable, overload
import asyncio
import functools

Param = ParamSpec("Param")
RetType = TypeVar("RetType")

class Logger:
    @overload
    @classmethod
    def catch_errors(
        cls, func: Callable[Param, Awaitable[RetType]]
    ) -> Callable[Param, Awaitable[RetType | None]]: ...

    @overload
    @classmethod
    def catch_errors(
        cls, func: Callable[Param, RetType]
    ) -> Callable[Param, RetType | None]: ...

    @classmethod
    def catch_errors(
        cls, func: Callable[Param, Any]
    ) -> Callable[Param, Any]:
        if asyncio.iscoroutinefunction(func):

            @functools.wraps(func)
            async def async_wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType | None:  # ← mypy error here
                try:
                    return await func(*args, **kwargs)
                except Exception as err:
                    # do some error logging
                    return None
            return async_wrapper

        else:

            @functools.wraps(func)
            def wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType | None:  # ← mypy error here
                try:
                    return func(*args, **kwargs)
                except Exception as err:
                    # do some error logging
                    return None
            return wrapper

[FelixSiegel]

Alright, yeah — I already tried annotating the function signature using a union to combine both overloads, but as you said, that leads to other complications. So I'll go with the Any type here, since it at least fixes the error.

Thanks a lot for the hint and clarification!