python concurrency task python-3.4 python-asyncio
Updated Tue, 20 Sep 2022 11:18:21 GMT

How to properly create and run concurrent tasks using python's asyncio module?

I am trying to properly understand and implement two concurrently running Task objects using Python 3's relatively new asyncio module.

In a nutshell, asyncio seems designed to handle asynchronous processes and concurrent Task execution over an event loop. It promotes the use of await (applied in async functions) as a callback-free way to wait for and use a result, without blocking the event loop. (Futures and callbacks are still a viable alternative.)

It also provides the asyncio.Task() class, a specialized subclass of Future designed to wrap coroutines. Preferably invoked by using the asyncio.ensure_future() method. The intended use of asyncio tasks is to allow independently running tasks to run 'concurrently' with other tasks within the same event loop. My understanding is that Tasks are connected to the event loop which then automatically keeps driving the coroutine between await statements.

I like the idea of being able to use concurrent Tasks without needing to use one of the Executor classes, but I haven't found much elaboration on implementation.

This is how I'm currently doing it:

import asyncio
print('running async test')
async def say_boo():
    i = 0
    while True:
        await asyncio.sleep(0)
        print(' {0}'.format(i))
        i += 1
async def say_baa():
    i = 0
    while True:
        await asyncio.sleep(0)
        print('...baa {0}'.format(i))
        i += 1
# wrap in Task object
# -> automatically attaches to event loop and executes
boo = asyncio.ensure_future(say_boo())
baa = asyncio.ensure_future(say_baa())
loop = asyncio.get_event_loop()

In the case of trying to concurrently run two looping Tasks, I've noticed that unless the Task has an internal await expression, it will get stuck in the while loop, effectively blocking other tasks from running (much like a normal while loop). However, as soon the Tasks have to (a)wait, they seem to run concurrently without an issue.

Thus, the await statements seem to provide the event loop with a foothold for switching back and forth between the tasks, giving the effect of concurrency.

Example output with internal await:

running async test 0
...baa 0 1
...baa 1 2
...baa 2

Example output without internal await: 0 1 2 3 4


Does this implementation pass for a 'proper' example of concurrent looping Tasks in asyncio?

Is it correct that the only way this works is for a Task to provide a blocking point (await expression) in order for the event loop to juggle multiple tasks?


2022 UPDATE: Please note that the asyncio API has changed fairly substantially since this question was asked. See the newly marked as correct answer which now shows the correct use of the API given Python 3.10. I still recommend the answer from @dano for broader knowledge of how this works under the hood.


The functions asyncio.ensure_future and asyncio.get_event_loop are deprecated in Python 3.10.

You can run the two coroutines say_boo and say_baa concurrently through asyncio.create_task:

async def main():
    boo = asyncio.create_task(say_boo())
    baa = asyncio.create_task(say_baa())
    await boo
    await baa

You can also use asyncio.gather

async def main():
    await asyncio.gather(say_boo(), say_baa())

Comments (1)

  • +0 – For the benefit of new views of this question I am marking this question as correct given the more recent changes to the API. — Sep 05, 2022 at 08:25