how to run gather and a coroutine task concurrently

Question

I have approximately the following code

import asyncio
.
.
.
async def query_loop()
    while connected:
        result = await asyncio.gather(get_value1, get_value2, get_value3)
        if True in result:
            connected = False

async def main():
    await query_loop()

asyncio.run(main())

The get_value - functions query a device, receive values, and publish them to a server. If no problems occur they return False, else True.

Now I need to implement, that the get_value2-function checks if it received the value 7. In this case I need the program to wait for 3 min before sending a special command to the device. But in the mean time, and also afterwards the query_loop should continue.

Has anybody an idea how to do that ? thanks in advance!

user4815162342 · Accepted Answer · 2020-08-15 12:12:53Z

0

If I understand you correctly, you want to modify get_value2 so that it reacts to a value received from device by spawning additional work in the background, i.e. do something without the loop in query_loop having to wait for that new work to finish.

You can use asyncio.create_task() to spawn a background task. In fact, you can always combine create_task() and await to runs things in the background; asyncio.gather is just a utility function that does it for you. In this case query_loop remains unchanged, and get_value2 gets modified like this:

async def get_value2():
    ...
    value = await receive_value_from_device()
    if value == 7:
        # schedule send_command() to run, but don't wait for it
        asyncio.create_task(special_command())
    ...
    return False

async def special_command():
    await asyncio.sleep(180)
    await send_command_to_device(...)

Note that if get_value1 and others are async functions, the correct invocation of gather must call them, so it should be await asyncio.gather(get_value1(), get_value2(), get_value3()) (note the extra parentheses).

answered Aug 15, 2020 at 12:12

user4815162342

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Art3mis Over a year ago

ok thanks for your answer it was very helpful! But one last question: you never await the task "asyncio.create_task(special_command()). Are sheduled task executed automatically even if they are not called with await ?

user4815162342 Over a year ago

@Art3mis Yes, that's what create_task does, you can think of it as "await in the background".

Art3mis Over a year ago

so now I have the problem, that the task special_command is invoked on every iteration of the loop. But i only need it to run once in a time..

user4815162342 Over a year ago

@Art3mis special_command is only invoked when the value 7 is received from device. If you need to additionally restrict it by time, you can use a global variable (or any other shared-state variable) to prevent multiple special_commands from running at once. The answer was intended to show how to spawn a background task, not to cover all the minutiae of your use case.

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