Async monitor decorator makes type checking complex

[kojiromike]

How do you use Sentry?

Sentry Saas (sentry.io)

Version

1.44.1

Steps to Reproduce

1. Create two simple synchronous functions that returns a type, where one is decorated and the second calls the first:

from sentry_sdk.crons.decorator import monitor

@monitor(monitor_slug="test")
def foo() -> int:
    return 1

def bar() -> int:
    return foo()

2. Run mypy on both and see if the behavior is the same in v1.44.0 and v1.44.1

Expected Result

The functions are synchronous and there is no type failure in v1.44.1.

Actual Result

When testing upgrading to 1.44.1, we got a type failure from mypy that was not there using the 1.44.0 sdk version. Our function returns a dictionary, not an int, but the essentials are the same as above.

We see

Incompatible return value type (got "dict[str, list[dict[str, Any]]] | Awaitable[dict[str, list[dict[str, Any]]]]", expected "dict[str, list[dict[str, Any]]]") [return-value]

But since our function is synchronous, mypy should not be confused by the Awaitable. I think this is ultimately due to inspect.iscoroutinefunction being a runtime check that mypy does not use for type elimination.

[szokeasaurusrex]

@kojiromike This indeed appears to be because of how we typed the monitor after the async changes.

The problem is that we have defined the monitor to return Callable[P, Union[R, Awaitable[R]]]. We instead need to add overloads to specify that monitor returns Callable[P, R] when it is passed a Callable[P, R], and that it returns Callable[P, Awaitable[R]] when it receives a Callable[P, Awaitable[R]].

I will work on a fix now. Hopefully, we will have it ready soon!

[kojiromike]

NB this appears to be very similar to the situation reported python/mypy#15520, and largely handled in https://peps.python.org/pep-0742/. (Though I guess only in python ^3.13.0 IIRC?)