Asyncio with threads explained

asyncio_with_threads.py

import time
import asyncio
import threading


async def coro(name: str) -> str:
    await asyncio.sleep(0.001)
    print(f'[Coroutine] Called with {name=}')
    return name


def call_coro_from_thread() -> None:
    print('[Second] I\'m going to call a coroutine in the first thread now!')
    asyncio.run_coroutine_threadsafe(coro('foo'), loop=loop)
    print('[Second] Done. Closing second thread.')


async def main() -> None:
    for _ in range(3):
        print('[First] Event loop is running')
        await asyncio.sleep(0.1)
    print('[First] Closing event loop and first thread')

loop: asyncio.AbstractEventLoop = asyncio.get_event_loop()

threads: list[threading.Thread] = [
    threading.Thread(target=loop.run_until_complete, args=(main(),)),
    threading.Thread(target=call_coro_from_thread),
]

print('[Main] Hello. I\'m going to launch all threads.')

for th in threads:
    th.start()

print('[Main] thread is here and it is still free!')

time.sleep(0.3)

print('[Main] Closing all threads.')

for th in threads:
    th.join()

print('[Main] Goodbye!')

You would see:

$ python asyncio_with_threads.py
[Main] Hello. I'm going to launch all threads.
[First] Event loop is running
[Second] I'm going to call a coroutine in the first thread now!
[Main] thread is here and it is still free!
[Second] Done. Closing second thread.
[Coroutine] Called with name='foo'
[First] Event loop is running
[First] Event loop is running
[Main] Closing all threads.
[First] Closing event loop and the first thread
[Main] Goodbye!