Question regarding Coroutine vs Awaitable

[ojii]

Given this code:

from typing import *
from collections import deque

Task = Coroutine[Any, None, Any]
tasks: Deque[Task] = deque()


async def foo() -> None:
    print('foo!')


def add_task(task: Task) -> None:
    tasks.append(task)


def run() -> None:
    task = tasks.popleft()
    try:
        task.send(None)
    except StopIteration:
        pass


add_task(foo())
run()

Versions:

$ mypy --version
mypy 0.560-51e044c4ecf2a52fc6c41ee63019723e0d3061e1-dirty
$ python --version
Python 3.6.4

Running mypy --strict script.py I get script.py:24: error: Argument 1 to "add_task" has incompatible type "Awaitable[None]"; expected "Coroutine[Any, None, Any]", but changing Task to Awaitable[None] gives me script.py:19: error: "Awaitable[None]" has no attribute "send". The code runs fine though.

What would be the proper type hints in this scenario?

[JelleZijlstra]

I think this is a bug in mypy. Adding reveal_type(foo()) to your example gives bin/coro.py:24: error: Revealed type is 'typing.Awaitable[builtins.None]', which I think is wrong; foo() returns a Coroutine.

This might be fairly easy to fix—if you are able to, feel free to submit a PR to fix the issue.

[ojii]

This might be fairly easy to fix

The tricky thing is that Coroutine takes 3 arguments (like Generator) but Awaitable only one. How would the send and yield types of Coroutine be filled?

Edit: I tried to have a look around the codebase and see if I can figure out what would need changing but it's all a bit too complicated for me, so unfortunately I think I cannot fix this myself at the moment.

[AndreLouisCaron]

This problem is similar to #3569. I ran the fix in #5052 against this script and the error disappears.