How to schedule and cancel tasks with asyncio

Question

I am writing a client-server application. While connected, client sends to the server a "heartbeat" signal, for example, every second. On the server-side I need a mechanism where I can add tasks (or coroutines or something else) to be executed asynchronously. Moreover, I want to cancel tasks from a client, when it stops sending that "heartbeat" signal.

In other words, when the server starts a task it has kind of timeout or ttl, in example 3 seconds. When the server receives the "heartbeat" signal it resets timer for another 3 seconds until task is done or client disconnected (stops send the signal).

Here is an example of canceling a task from asyncio tutorial on pymotw.com. But here the task is canceled before the event_loop started, which is not suitable for me.

import asyncio async def task_func(): print('in task_func') return 'the result' event_loop = asyncio.get_event_loop() try: print('creating task') task = event_loop.create_task(task_func()) print('canceling task') task.cancel() print('entering event loop') event_loop.run_until_complete(task) print('task: {!r}'.format(task)) except asyncio.CancelledError: print('caught error from cancelled task') else: print('task result: {!r}'.format(task.result())) finally: event_loop.close()

are you averse to using something like celery to do your async? — Keith Bailey
– Keith Bailey, Commented Oct 13, 2016 at 9:03
@KeithBailey actually, celery may help; but it would be much better if there is a solution with asyncio which is a part of standard library, not a 3rdparty lib — Sergey Belash
– Sergey Belash, Commented Oct 13, 2016 at 9:09
I have a rough idea how to do it in celery, in asyncio it looks like you should be able to do it using the queues type (theres an asyncio one and a multiprocessing one). Which allow you to define queues and consumers and then remove items from queues. I don't believe it would let you stop running tasks. — Keith Bailey
– Keith Bailey, Commented Oct 13, 2016 at 10:03
There is a task wrapper in Asyncio via ensure_future(). It also has a cancel method. — songololo
– songololo, Commented Oct 13, 2016 at 12:17
@shongololo I knew about it: pymotw.com/3/asyncio/tasks.html#creating-tasks-from-coroutines but can't understand how to use it if the event loop is already run_forever. May you provide an example? — Sergey Belash
– Sergey Belash, Commented Oct 13, 2016 at 12:51

songololo · Accepted Answer · 2019-12-08 13:48:38Z

You can use asyncio Task wrappers to execute a task via the ensure_future() method.

ensure_future will automatically wrap your coroutine in a Task wrapper and attach it to your event loop. The Task wrapper will then also ensure that the coroutine 'cranks-over' from await to await statement (or until the coroutine finishes).

In other words, just pass a regular coroutine to ensure_future and assign the resultant Task object to a variable. You can then call Task.cancel() when you need to stop it.

import asyncio async def task_func(): print('in task_func') # if the task needs to run for a while you'll need an await statement # to provide a pause point so that other coroutines can run in the mean time await some_db_or_long_running_background_coroutine() # or if this is a once-off thing, then return the result, # but then you don't really need a Task wrapper... # return 'the result' async def my_app(): my_task = None while True: await asyncio.sleep(0) # listen for trigger / heartbeat if heartbeat and my_task is None: my_task = asyncio.ensure_future(task_func()) # also listen for termination of hearbeat / connection elif not heartbeat and my_task: if not my_task.cancelled(): my_task.cancel() else: my_task = None run_app = asyncio.ensure_future(my_app()) event_loop = asyncio.get_event_loop() event_loop.run_forever()

Note that tasks are meant for long-running tasks that need to keep working in the background without interrupting the main flow. If all you need is a quick once-off method, then just call the function directly instead.

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