Python3 Asyncio call from Flask route

Question

I want to execute an async function everytime the flask route is executed. Currently my abar function is never executed. Can you tell me why? Thank you very much:

import asyncio
from flask import Flask

async def abar(a):
    print(a)

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

@app.route("/")
def notify():
    asyncio.ensure_future(abar("abar"), loop=loop)
    return "OK"

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

I tried it also to put the one blocking call in a seperate thread. But it is still not calling the abar function.

import asyncio
from threading import Thread
from flask import Flask

async def abar(a):
    print(a)

app = Flask(__name__)

def start_worker(loop):
    asyncio.set_event_loop(loop)
    try:
        loop.run_forever()
    finally:
        loop.close()

worker_loop = asyncio.new_event_loop()
worker = Thread(target=start_worker, args=(worker_loop,))

@app.route("/")
def notify():
    asyncio.ensure_future(abar("abar"), loop=worker_loop)
    return "OK"

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

Answer 1

For same reason you won't see this print:

if __name__ == "__main__":
    app.run(debug=False, use_reloader=False)
    print('Hey!')
    loop.run_forever()

loop.run_forever() is never called since as @dirn already noted app.run is also blocking.

Running global blocking event loop - is only way you can run asyncio coroutines and tasks, but it's not compatible with running blocking Flask app (or with any other such thing in general).

If you want to use asynchronous web framework you should choose one created to be asynchronous. For example, probably most popular now is aiohttp:

from aiohttp import web


async def hello(request):
    return web.Response(text="Hello, world")


if __name__ == "__main__":
    app = web.Application()
    app.router.add_get('/', hello)
    web.run_app(app)  # this runs asyncio event loop inside

Upd:

About your try to run event loop in background thread. I didn't investigate much, but it seems problem somehow related with tread-safety: many asyncio objects are not thread-safe. If you change your code this way, it'll work:

def _create_task():
    asyncio.ensure_future(abar("abar"), loop=worker_loop)

@app.route("/")
def notify():
    worker_loop.call_soon_threadsafe(_create_task)
    return "OK"

But again, this is very bad idea. It's not only very inconvenient, but I guess wouldn't make much sense: if you're going to use thread to start asyncio, why don't just use threads in Flask instead of asyncio? You will have Flask you want and parallelization.

If I still didn't convince you, at least take a look at Flask-aiohttp project. It has close to Flask api and I think still better that what you're trying to do.

Answer 2

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)

Answer 3

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.