understanding asyncio already running forever loop and pending tasks

Question

I'm having problems understanding how to pend a new task to an already running event loop.

This code:

import asyncio
import logging

@asyncio.coroutine
def blocking(cmd):
    while True:
        logging.info("in blocking coroutine")
        yield from asyncio.sleep(0.01)
        print("ping")

def main():
    logging.info("in main funciton")
    loop = asyncio.get_event_loop()
    logging.info("new loop created")
    logging.info("loop running forever")
    loop.run_forever()
    asyncio.async(blocking("ls"))

logging.basicConfig(level = logging.INFO)
main()

Changing run_forever() to run_until_complete(asyncio.async(blocking("ls")) works fine. But I'm really confused - why I can't pend a task on the already running loop?

Answer 1

The problem is that the call to loop.run_forever() blocks; it starts the event loop, and won't return until you explicitly stop the loop - hence the forever part of run_forever. Your program never explicitly stops the event loop, so your asyncio.async(blocking("ls")) call is never reached.

Using asyncio.async to add a new task to an already running loop is fine, you just need to make sure the function is actually called from inside a coroutine or callback inside the event loop. Here are some examples:

Schedule blocking to run as soon as the event loop starts:

def main():
    logging.info("in main funciton")
    loop = asyncio.get_event_loop()
    logging.info("new loop created")
    logging.info("loop running forever")
    asyncio.async(blocking("ls"))
    loop.run_forever()

Schedule blocking from a callback executed by the event loop:

def start_blocking():
    asyncio.async(blocking("ls"))

def main():
    logging.info("in main funciton")
    loop = asyncio.get_event_loop()
    logging.info("new loop created")
    logging.info("loop running forever")
    loop.call_soon(start_blocking)  # Calls start_blocking once the event loop starts
    loop.run_forever()