Problems using asyncio in Thread

Question

There is no problem using asyncio as follows.

import asyncio


async def main():
    await asyncio.sleep(1)
    aaa = 1
    print (aaa)


loop = asyncio.get_event_loop()
loop.run_until_complete(main())
loop.close()

However, I have to use asyncio within Thread as shown in the simple code below, which results in an error as follows

import asyncio
import threading

async def main():
    await asyncio.sleep(1)
    aaa = 1
    print (aaa)


def test():
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())
    loop.close()


t=threading.Thread(target=test)
t.start()

Error Message (RuntimeError: There is no current event loop in thread 'Thread-1'.)

Exception in thread Thread-1:
Traceback (most recent call last):
  File "D:\Anaconda3\Lib\threading.py", line 932, in _bootstrap_inner
    self.run()
  File "D:\Anaconda3\Lib\threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "D:/Test/Test4/main.py", line 57, in test
    loop = asyncio.get_event_loop()
  File "D:\Anaconda3\Lib\asyncio\events.py", line 639, in get_event_loop
    raise RuntimeError('There is no current event loop in thread %r.'
RuntimeError: There is no current event loop in thread 'Thread-1'.

How can I use asyncio in Thread?

This is very important to me. I've been struggling with this problem for two days.

I would appreciate your help.

Answer 1

Unless you need to work with a pre-existing event loop (not the case here) or support Python 3.6, use asyncio.run instead of manually handling the loop:

def test():
    asyncio.run(main())

This always ensures that there is a valid event loop, and also handles proper cleanup.

This function always creates a new event loop and closes it at the end. It should be used as a main entry point for asyncio programs, and should ideally only be called once.

---

Note that when combining asyncio with threads, the event loop should generally be run in the main loop (see e.g. bpo#34697). Instead of spawning a thread for asyncio, prefer to spawn one for the synchronous parts of your program.

# launch synchronous work in child threads
t=threading.Thread(target=my_sync_tasks)
t.start()

# launch asynchronous work in main thread
asyncio.run(main())
t.join()

Answer 2

When using a thread other than the main thread you need to set the event loop

def test():
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    loop.run_until_complete(main())
    loop.close()