python - When using asyncio, how do you allow all running tasks to finish before shutting down the event loop

Question

I have the following code:

@asyncio.coroutine
def do_something_periodically():
    while True:
        asyncio.async(my_expensive_operation())
        yield from asyncio.sleep(my_interval)
        if shutdown_flag_is_set:
            print("Shutting down")
            break

I run this function until complete. The problem occurs when shutdown is set - the function completes and any pending tasks are never run.

This is the error:

task: <Task pending coro=<report() running at script.py:33> wait_for=<Future pending cb=[Task._wakeup()]>>

How do I schedule a shutdown correctly?

To give some context, I'm writing a system monitor which reads from /proc/stat every 5 seconds, computes the cpu usage in that period, and then sends the result to a server. I want to keep scheduling these monitoring jobs until I receive sigterm, when I stop scheduling, wait for all current jobs to finish, and exit gracefully.

Answer 1

You can retrieve unfinished tasks and run the loop again until they finished, then close the loop or exit your program.

pending = asyncio.all_tasks()
loop.run_until_complete(asyncio.gather(*pending))

• pending is a list of pending tasks.
• asyncio.gather() allows to wait on several tasks at once.

If you want to ensure all the tasks are completed inside a coroutine (maybe you have a "main" coroutine), you can do it this way, for instance:

async def do_something_periodically():
    while True:
        asyncio.create_task(my_expensive_operation())
        await asyncio.sleep(my_interval)
        if shutdown_flag_is_set:
            print("Shutting down")
            break

    await asyncio.gather(*asyncio.all_tasks())

Also, in this case, since all the tasks are created in the same coroutine, you already have access to the tasks:

async def do_something_periodically():
    tasks = []
    while True:
        tasks.append(asyncio.create_task(my_expensive_operation()))
        await asyncio.sleep(my_interval)
        if shutdown_flag_is_set:
            print("Shutting down")
            break

    await asyncio.gather(*tasks)