"Incompatible types" for asyncio.wait() + run_in_executor()

[neilconway]

Sample program:

import asyncio
from typing import Any, Dict


async def message_reader(socket: Any) -> Any:
    return await socket.recv()


def do_work() -> Dict[str, Any]:
    return {}

async def foo(socket: Any) -> None:
    loop = asyncio.get_event_loop()
    work_task = loop.run_in_executor(None, do_work)
    recv_task = asyncio.ensure_future(message_reader(socket))
    pending = {work_task, recv_task}

    done, pending = await asyncio.wait(pending)

Error:

$ mypy --strict wait_mypy.py
wait_mypy.py:19: error: Incompatible types in assignment (expression has type "Set[Future[Any]]", variable has type "Set[Iterable[Any]]")
wait_mypy.py:19: error: Argument 1 to "wait" has incompatible type "Set[Iterable[Any]]"; expected "Iterable[Union[Generator[Any, None, <nothing>], Awaitable[<nothing>]]]"
wait_mypy.py:19: error: Argument 1 to "wait" has incompatible type "Set[Iterable[Any]]"; expected "Iterable[Union[Generator[Any, None, Any], Awaitable[Any]]]"
$ mypy --version
mypy 0.570

[gvanrossum]

To debug this please use reveal_type() on various sub-expressions to figure out what's going on.

[JelleZijlstra]

What's going on here is that one of the two tasks is inferred to be a typing.AwaitableGenerator, and the other is an asyncio.Future. In typeshed, both of these classes inherit from Iterable (possibly indirectly) and Awaitable, with the Iterable base first. Apparently, mypy infers the first common base class to be the common type if there are multiple common base classes. That's a reasonable thing to do, but in the case of futures, inferring Awaitable is probably much more commonly the right thing to do.

To fix this, I'll move the Awaitable base classes first. I still get an error samples/asyncwait.py:20: error: Incompatible types in assignment (expression has type "Set[Future[Any]]", variable has type "Set[Awaitable[Any]]") on @neilconway's code, but that's a separate issue that's easily worked around by using a different variable name.

[neilconway]

@JelleZijlstra Awesome, thank you for looking into this!