Python asyncio: starting a loop without created task

Question

Could somebody explain me why I can't execute my tasks if I start the loop without any added tasks before? (Python 3.7)

import asyncio
import threading    

def run_forever(loop):
    loop.run_forever()

async def f(x):
    print("%s executed" % x)

# init is called first
def init():
    print("init started")

    loop = asyncio.new_event_loop()

    # loop.create_task(f("a1")) # <--- first commented task

    thread = threading.Thread(target=run_forever, args=(loop,))
    thread.start()

    loop.create_task(f("a2")) # <--- this is not being executed

    print("init finished")

If I leave comment on # loop.create_task(f("a1")) the execution is:

init started
init finished

Uncommented execution is:

init started
init finished
a1 executed
a2 executed

Why so? I wanted to create a loop and to add tasks in the future.

user4815162342 · Accepted Answer · 2019-03-15 23:08:19Z

2

Unless explicitly stated otherwise, asyncio API is not thread-safe. This means that calling loop.create_task() from a thread other than the one that runs the event loop will not properly synchronize with the loop.

To submit the task to the event loop from a foreign thread, you need to invoke asyncio.run_coroutine_threadsafe instead:

asyncio.run_coroutine_threadsafe(f("a2"), loop)

This will wake up the loop to alert it that a new task has arrived, and it also returns a concurrent.futures.Future which you can use to obtain the result of the coroutine.

answered Mar 15, 2019 at 23:08

user4815162342

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1 Comment

IgorZ Over a year ago

Thank you very much! I got it.

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