How to add a task to event loop while it is already running in asyncio library in python3.7?

Question

# !/usr/bin/python3

import asyncio
import aiohttp
from threading import Thread


event_loop = asyncio.get_event_loop() # getting the event_loop
Thread(target=event_loop.run_forever).start() # creating one more thread(to add and then to execute tasks when it is necessary)


async def fetch(session, url): # exm/doc code from aiohttp lib.
    async with session.get(url) as response:
        return await response.text()


async def main(): # exm/doc code from aiohttp lib.
    async with aiohttp.ClientSession() as session:
        html = await fetch(session, 'http://python.org')
        print(html)


event_loop.create_task(main()) # doesn't work when the event_loop is already running... Is it the only way to add task before running the event_loop in asyncio?

The goal is to add task when it is necessary. For example, the main thread is for listening to a server and the second one is for responding. P.s this is only a small part of all code/

Am I doing something wrong or the asyncio library does not support this?

there is no reason to use Threading to make a call to run_forever for your event_loop. also since you are using Python 3.7 the preferred method of running your program should be asyncio.run(main()) — gold_cy
– gold_cy, Commented Jul 17, 2020 at 11:47

user4815162342 · Accepted Answer · 2020-07-17 18:01:21Z

2

Am I doing something wrong

Yes. Since asyncio is not thread-safe, you cannot interact with the event loop from outside the thread that runs the event loop. The correct way to create the task is:

asyncio.run_coroutine_threadsafe(main(), event_loop)

See also the call_soon_threadsafe on the event loop.

answered Jul 17, 2020 at 18:01

user4815162342

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