thread_for_async.py

# run_alt_loop() / run_coro_on_other_thread() by Ben Darnell
# https://github.com/tornadoweb/tornado/issues/2798#issuecomment-575909528
import asyncio
import threading
import concurrent

from tornado.ioloop import IOLoop
from tornado.web import Application, RequestHandler

# Not asyncio.Future because of multiple threads.
alt_ioloop_fut = concurrent.futures.Future()

def run_alt_loop():
    asyncio.set_event_loop(asyncio.SelectorEventLoop())
    ioloop = IOLoop()
    alt_ioloop_fut.set_result(ioloop)
    ioloop.start()

alt_thread = threading.Thread(target=run_alt_loop)
alt_thread.daemon = True
alt_thread.start()
alt_ioloop = alt_ioloop_fut.result()

def run_coro_on_other_thread(f, *args, **kw):
    fut = concurrent.futures.Future()

    async def wrapper():
        try:
            res = await f(*args, **kw)
            fut.set_result(res)
        except Exception as e:
            fut.set_exception(e)

    alt_ioloop.add_callback(wrapper)
    return fut.result()


async def aio_func(msg):
    print("started async...")
    await asyncio.sleep(2)
    print("still going", flush=True)
    await asyncio.sleep(2)
    print("async done", flush=True)
    return "async %s!" % msg


class MainHandler(RequestHandler):
    def get(self):
        msg = self.get_argument("msg")
        resp = run_coro_on_other_thread(aio_func, msg)
        self.set_header("Content-Type", "text/plain")
        self.write(resp.encode() + b"\n")


if __name__ == "__main__":
    app = Application([('/', MainHandler)])
    app.listen(8080)
    print("listening on port 8080", flush=True)
    IOLoop.current().start()