[FIXED] understanding asyncio await behaviour

Issue

I have the following example code:

from datetime import datetime
import asyncio

async def h():
    print("h() has started")
    await asyncio.sleep(5)
    print("h() has ended")

async def main():
    print("{}: start of program".format(datetime.now()))
    await h()
    print("{}: end of program".format(datetime.now()))

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

I would have expected (and would like the output of code) to be something along the following lines:

2020-03-03 17:31:25.379742: start of program
h() has started
2020-03-03 17:31:30.384977: end of program
h() has ended

however I have the following output:

2020-03-03 17:31:25.379742: start of program
h() has started
h() has ended
2020-03-03 17:31:30.384977: end of program

Is there a specific reason as to why this is happening and how I can go about achieving the desired result?

Solution

The whole point of awaiting a coroutine such as h() is to wait for it to complete, and (where that makes sense) access its return value. For example, line = await stream.readline() suspends the current coroutine until readline() finishes, and then provides the result to assign to line.

If you want h() to run in the background, you can use asyncio.create_task instead. But it still won't result in the desired output, because run_until_complete(main()) only waits for main() to finish, disregarding additional tasks it may have spawned. (It also returns the return value of the coroutine, much like a sync equivalent of await.)

If you want to wait for additional tasks, you have to be explicit about it. Perhaps something like:

async def main():
    bgtasks = []
    print("{}: start of program".format(datetime.now()))
    bgtasks.append(asyncio.create_task(h()))
    print("{}: end of program".format(datetime.now()))
    return bgtasks

async def run_everything():
    more_tasks = await main()
    await asyncio.gather(*more_tasks)

if __name__ == '__main__':
    asyncio.run(run_everything())

Answered By - user4815162342