Error on call to asyncio.gather() with arguments of different types

[JelleZijlstra]

In mypy, the following code:

import asyncio


async def f() -> int:
    return 3


async def g() -> str:
    return ''


async def h() -> None:
    x, y = await asyncio.gather(f(), g())  # type: int, str

produces the error:

gather.py: note: In function "h":
gather.py:13: error: Cannot infer type argument 1 of "gather"

(regardless of whether the type error on the last line is present)

This is on the latest release of mypy with the git HEAD version of typeshed (the released mypy has an old version of typeshed that doesn't support gather() at all).

I think this is a typeshed issue, not a mypy issue, but I'm not sure how the type of this function should be expressed instead. The current annotation for gather() is:

def gather(*coros_or_futures: Union[Future[_T], Generator[Any, None, _T], Awaitable[_T]],
           loop: AbstractEventLoop = ..., return_exceptions: bool = False) -> Future[List[_T]]: ...

[gvanrossum]

@sixolet: Here's another example for variadic type variables.

@JelleZijlstra: Since we don't have variadic type variables yet, I think the only way to fix the stub for now is to use Any instead of _T, but this loses type-safety. Or you could add # type: ignore and you'd still lose type-safety but at least we'd have type-safety in case all coroutines passed return the same type of value.

Do you have a preference?

[JelleZijlstra]

I did solve my issue for now with # type: ignore, but don't have a strong opinion on what typeshed should do here. In general it seems better to err on the side of using Any where a more specific type could lead to false positive errors (similar to #285), but I'll leave that judgment to you.

[gvanrossum]

OK, I'll use Any.