Make a Python asyncio call from a Flask route

Question

I want to execute an async function every time the Flask route is executed. Why is the abar function never executed?

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 also tried putting the blocking call in a separate 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

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)