Type checking problems. "Awaitable" vs "Future"

[dettonijr]

import threading
import tornado.ioloop
import time
from tornado.concurrent import Future

ioloop = tornado.ioloop.IOLoop.current()

def callback(future: Future[int]) -> None:
    print(f"Callback {future.result()} {threading.current_thread().name}")
    ioloop.stop()

def function() -> int:
    print(f"Function {threading.current_thread().name}")
    time.sleep(1)
    return 1

future = ioloop.run_in_executor(None, function)
ioloop.add_future(future, callback)
ioloop.start()

It does what's supposed to, runs a callback in the main thread after the the function runs in the executor. And everything is typed. However, using mypy:

$ mypy script.py 
script.py:17: error: Argument 1 to "add_future" of "IOLoop" has incompatible type "Awaitable[int]"; expected "Union[asyncio.futures.Future[int], concurrent.futures._base.Future[int]]"

I see the type annotations were all added for both functions on the same commit afe7dc5

Is there a reason run_in_executor returns Awaitable[_T] and not Future[_T]?

[bdarnell]

run_in_executor returns Awaitable because that's what the corresponding asyncio method is typed to return. If I'm following the various threads correctly (starting with python/typeshed#3999), now that https://bugs.python.org/issue40782 has been fixed in cpython, we need a typeshed PR to change the return type of AbstractEventLoop.run_in_executor from Awaitable to Future, and then we can make the same change in Tornado.

[bdarnell]

Typeshed now has this method returning a Future, so this is a change we could make next time we upgrade mypy/typeshed.