Make a Python asyncio call from a Flask route

You can incorporate some async functionality into Flask apps without having to completely convert them to asyncio.

import asyncio
from flask import Flask

async def abar(a):
    print(a)

loop = asyncio.get_event_loop()
app = Flask(__name__)

@app.route("/")
def notify():
    loop.run_until_complete(abar("abar"))
    return "OK"

if __name__ == "__main__":
    app.run(debug=False, use_reloader=False)

This will block the Flask response until the async function returns, but it still allows you to do some clever things. I've used this pattern to perform many external requests in parallel using aiohttp, and then when they are complete, I'm back into traditional flask for data processing and template rendering.

import aiohttp
import asyncio
import async_timeout
from flask import Flask

loop = asyncio.get_event_loop()
app = Flask(__name__)

async def fetch(url):
    async with aiohttp.ClientSession() as session, async_timeout.timeout(10):
        async with session.get(url) as response:
            return await response.text()

def fight(responses):
    return "Why can't we all just get along?"

@app.route("/")
def index():
    # perform multiple async requests concurrently
    responses = loop.run_until_complete(asyncio.gather(
        fetch("https://google.com/"),
        fetch("https://bing.com/"),
        fetch("https://duckduckgo.com"),
        fetch("http://www.dogpile.com"),
    ))

    # do something with the results
    return fight(responses)

if __name__ == "__main__":
    app.run(debug=False, use_reloader=False)

A simpler solution to your problem (in my biased view) is to switch to Quart from Flask. If so your snippet simplifies to,

import asyncio
from quart import Quart

async def abar(a):
    print(a)

app = Quart(__name__)

@app.route("/")
async def notify():
    await abar("abar")
    return "OK"

if __name__ == "__main__":
    app.run(debug=False)

As noted in the other answers the Flask app run is blocking, and does not interact with an asyncio loop. Quart on the other hand is the Flask API built on asyncio, so it should work how you expect.

Also as an update, Flask-Aiohttp is no longer maintained.

Your mistake is to try to run the asyncio event loop after calling app.run(). The latter doesn't return, it instead runs the Flask development server.

In fact, that's how most WSGI setups will work; either the main thread is going to busy dispatching requests, or the Flask server is imported as a module in a WSGI server, and you can't start an event loop here either.

You'll instead have to run your asyncio event loop in a separate thread, then run your coroutines in that separate thread via asyncio.run_coroutine_threadsafe(). See the Coroutines and Multithreading section in the documentation for what this entails.

Here is an implementation of a module that will run such an event loop thread, and gives you the utilities to schedule coroutines to be run in that loop:

import asyncio
import itertools
import threading

__all__ = ["EventLoopThread", "get_event_loop", "stop_event_loop", "run_coroutine"]

class EventLoopThread(threading.Thread):
    loop = None
    _count = itertools.count(0)

    def __init__(self):
        self.started = threading.Event()
        name = f"{type(self).__name__}-{next(self._count)}"
        super().__init__(name=name, daemon=True)

    def __repr__(self):
        loop, r, c, d = self.loop, False, True, False
        if loop is not None:
            r, c, d = loop.is_running(), loop.is_closed(), loop.get_debug()
        return (
            f"<{type(self).__name__} {self.name} id={self.ident} "
            f"running={r} closed={c} debug={d}>"
        )

    def run(self):
        self.loop = loop = asyncio.new_event_loop()
        asyncio.set_event_loop(loop)
        loop.call_later(0, self.started.set)

        try:
            loop.run_forever()
        finally:
            try:
                shutdown_asyncgens = loop.shutdown_asyncgens()
            except AttributeError:
                pass
            else:
                loop.run_until_complete(shutdown_asyncgens)
            try:
                shutdown_executor = loop.shutdown_default_executor()
            except AttributeError:
                pass
            else:
                loop.run_until_complete(shutdown_executor)
            asyncio.set_event_loop(None)
            loop.close()

    def stop(self):
        loop, self.loop = self.loop, None
        if loop is None:
            return
        loop.call_soon_threadsafe(loop.stop)
        self.join()

_lock = threading.Lock()
_loop_thread = None

def get_event_loop():
    global _loop_thread

    if _loop_thread is None:
        with _lock:
            if _loop_thread is None:
                _loop_thread = EventLoopThread()
                _loop_thread.start()
                # give the thread up to a second to produce a loop
                _loop_thread.started.wait(1)

    return _loop_thread.loop

def stop_event_loop():
    global _loop_thread
    with _lock:
        if _loop_thread is not None:
            _loop_thread.stop()
            _loop_thread = None

def run_coroutine(coro):
    """Run the coroutine in the event loop running in a separate thread

    Returns a Future, call Future.result() to get the output

    """
    return asyncio.run_coroutine_threadsafe(coro, get_event_loop())

You can use the run_coroutine() function defined here to schedule asyncio routines. Use the returned Future instance to control the coroutine:

Get the result with Future.result(). You can give this a timeout; if no result is produced within the timeout, the coroutine is automatically cancelled.
You can query the state of the coroutine with the .cancelled(), .running() and .done() methods.
You can add callbacks to the future, which will be called when the coroutine has completed, or is cancelled or raised an exception (take into account that this is probably going to be called from the event loop thread, not the thread that you called run_coroutine() in).

For your specific example, where abar() doesn't return any result, you can just ignore the returned future, like this:

@app.route("/")
def notify():
    run_coroutine(abar("abar"))
    return "OK"

Note that before Python 3.8 that you can't use an event loop running on a separate thread to create subprocesses with! See my answer to Python3 Flask asyncio subprocess in route hangs for backport of the Python 3.8 ThreadedChildWatcher class for a work-around for this.

Make a Python asyncio call from a Flask route

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