How to do async task without waiting for it to finish in python?

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I have a program like this

task_2_running = False

async def task2():
    task_2_running = True
    await do_something()
    task_2_running = False

async def main():
    while True:
        x = await task1()
        do_something(x)
        if some_condition(x) and not task_2_running:
            #Do task2() but don't wait until it finished

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

How do I call task2() so the loop will continue without waiting for task2() to finish, and task2() will still finish sometime in the future?

python-asyncio python
1 Answers

You could use asyncio.create_task, a sample as next:

test.py:

import time

task_2_running = False

async def task1():
    time.sleep(3)

async def task2():
    print("task2 run")
    task_2_running = True
    await asyncio.sleep(5)
    task_2_running = False
    print("task2 finish")

async def main():
    while True:
        print("main run")
        x = await task1()
        if not task_2_running:
            print("create task2")
            #Do task2() but don't wait until it finished
            asyncio.create_task(task2())
        await asyncio.sleep(1)

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

Execution:

$ python3 test.py
main run
create task2
task2 run
main run
create task2
task2 run
task2 finish
main run
create task2
task2 run
^C

From the execution, you could see after task2 run, the main run did not wait for task2 finish, just continue to run next loop to print another main run.

BTW, asyncio.create_task just for python3.7 and higher, for < python3.7, you may need to use asyncio.ensure_future(), see this:

async def coro():
    ...

# In Python 3.7+
task = asyncio.create_task(coro())
...

# This works in all Python versions but is less readable
task = asyncio.ensure_future(coro())
...
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