Mypy Error with Decorator Handling Both Sync and Async Functions

[FelixSiegel]

Hi everyone,

I have a somewhat complex decorator setup, and I'm running into a Mypy type-checking issue when using it on async functions.

What the Decorator Does

• It can wrap both synchronous and asynchronous functions.
• It executes the function within a try-except block.
• If the function executes successfully, it returns its normal return value.
• If an exception occurs, it logs the error and returns None.

Here’s the 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, RetType]
    ) -> Callable[Param, RetType | None]: ...
    
    @overload
    @classmethod
    def catch_errors(
        cls, func: Callable[Param, Awaitable[RetType]]
    ) -> Callable[Param, Awaitable[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:
                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:
                try:
                    return func(*args, **kwargs)
                except Exception as err:
                    # do some error logging
                    return None
            return wrapper

The Problem

When I decorate an async function like this:

@Logger.catch_errors
async def foo() -> int:
    return 1

And then call it like this:

var = await foo()

Mypy gives me the following error:

Incompatible types in "await" (actual type "Coroutine[Any, Any, int] | None", expected type "Awaitable[Any]")

What I Tried

• I checked whether the return type annotations were correct.
• I played around with Awaitable and Coroutine in the overload signatures.
• I searched online but couldn’t find a clear solution for this specific case.

Question

How can I fix this type issue so that Mypy correctly understands the return type when decorating async functions? Any insights would be greatly appreciated!

[Daverball]

You need to swap the two overloads, the non-async overload is broader than the async one and will match all async functions as well. So you always end up with RetType | None as the return type, which in case of your async function will be Coroutine[Any, Any, int] | None.

[FelixSiegel]

Thank you very much, that worked and now makes sense to me after your explanation!