Issue 40782: AbstactEventLoop.run_in_executor is listed as an async method, but should actually return a Future

As discussed in < https://github.com/python/typeshed/issues/3999#issuecomment-634097968 > the type of `AbstractEventLoop.run_in_executor` is defined at < https://github.com/python/cpython/blob/master/Lib/asyncio/events.py#L286 > as follows:

```
async def run_in_executor(self, executor, func, *args):
        raise NotImplementedError
```

However all concrete implementations of this method are actually not async methods but rather synchronous methods which return a Future object.

Logically this appears to make sense: at base `run_in_executor` is not a coroutine, since it doesn't create an object representing code which will be executed when the object is awaited, rather it returns an object representing code which is running asynchronously elsewhere (on another thread) and which can be awaited to wait for that other thread to complete its task. Which seems to be a perfect match to what a Future object is supposed to be.

As such it seems that the current definition of the method as a coroutine is possibly a mistake.

Alternatively if some feel that it is important to allow concrete implementations to implement it as a coroutine if they need to then perhaps it could be specified to be a method returning an Awaitable, since that would cover both options?

From looking at the commit history of AbstactEventLoop.run_in_executor(), it seems that it was previously be a non-coroutine method prior to the conversion from the `@asyncio.coroutine` decorator to `async def` (PR-4753). See https://github.com/python/cpython/blame/ede157331b4f9e550334900b3b4de1c8590688de/Lib/asyncio/events.py#L305.

The only context for the change I can find is the following conversation between Andrew and Yury: https://github.com/python/cpython/pull/4753#issuecomment-350114336. However, the example provided of `connect_read_pipe()` had already been a coroutine at the time for the BaseEventLoop implementation, which makes sense in that case. So, it's not clear to me as to why `run_in_executor()` was also converted to "async def" when its main implementation is not a coroutine. Furthermore, it's documented as an awaitable rather than a coroutine (https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.run_in_executor).

@Andrew do you have any additional context to provide that I'm potentially missing?

Is there any further movement on this?

I think it makes sense to remove the `async` from the definition in AbstractEventLoop.

If you want to help, you can submit a PR to do it.

https://github.com/python/cpython/pull/21852

New changeset 29f84294d88ec493c2de9d6e8dbc12fae3778771 by James Weaver in branch 'master':
bpo-40782: Change asyncio.AbstractEventLoop.run_in_executor to be a method not a coroutine (GH-21852)
https://github.com/python/cpython/commit/29f84294d88ec493c2de9d6e8dbc12fae3778771

New changeset 1baa8b14ee23ef3040923f53565c8d1bafd28117 by Miss Islington (bot) in branch '3.8':
bpo-40782: Change asyncio.AbstractEventLoop.run_in_executor to be a method not a coroutine (GH-21852)
https://github.com/python/cpython/commit/1baa8b14ee23ef3040923f53565c8d1bafd28117

New changeset d6bdf6d52f0400df1bd1dce24aaad9514015c755 by Miss Islington (bot) in branch '3.9':
bpo-40782: Change asyncio.AbstractEventLoop.run_in_executor to be a method not a coroutine (GH-21852)
https://github.com/python/cpython/commit/d6bdf6d52f0400df1bd1dce24aaad9514015c755