Incorrect type narrowing for inspect.isawaitable

[heckad]

Bug Report
https://mypy-play.net/?mypy=1.4.0&python=3.10&gist=5ab30e161d5e83a16538d790c9c2deb9

Expected Behavior

No errors

Actual Behavior

Error

[AlexWaygood]

Mypy's behaviour is correct here, as inspect.isawaitable is annotated using TypeGuard in typeshed, and TypeGuard doesn't apply type narrowing in the negative case (only in the positive case). This is specified in PEP 674 and is implemented correctly by mypy.

There has been discussion of a stricter version of TypeGuard that could apply type narrowing in the negative case (python/typing#996, python/typing#926), which you might be interested in. However, that would be a new feature in the typing system, so can't be done currently.

[AlexWaygood]

To make your code pass mypy in the meantime, you can do this:

 import inspect
 from typing import Awaitable

 async def f(t: Awaitable[int] | int) -> int:
     if inspect.isawaitable(t):
         return await t
     else:
+        assert isinstance(t, int)
         return t