How to Achieve 'Fire and Forget' Behavior in Python 3.5's Async/Await?

"Fire and Forget" Python Async/Await

In asynchronous programming, it can be useful to perform non-critical operations without waiting for their completion. In Tornado, this "fire and forget" behavior can be achieved by omitting the yield keyword in a coroutine. However, in Python 3.5's async/await syntax, this approach results in a runtime warning and the non-execution of the desired operation.

Solution: asyncio.Task

According to Python's documentation for asyncio.Task, it allows for the execution of a coroutine "in the background." By using asyncio.ensure_future to spawn a task, the execution is not blocked, and the function returns immediately, similar to the "fire and forget" behavior.

import asyncio

async def async_foo():
    print("async_foo started")
    await asyncio.sleep(1)
    print("async_foo done")

async def main():
    asyncio.ensure_future(async_foo())  # fire and forget async_foo()

    # Perform other actions

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

Output:

async_foo started
Do some actions 1
async_foo done
Do some actions 2
Do some actions 3

Handling Pending Tasks

If tasks are still executing after the event loop has completed, a warning may be displayed. To prevent this, all pending tasks can be awaited once the event loop finishes:

# Let's also finish all running tasks:
pending = asyncio.Task.all_tasks()
loop.run_until_complete(asyncio.gather(*pending))

Cancelling Tasks

In some cases, it may be necessary to cancel tasks that are not expected to be completed. This can be achieved using task.cancel():

# Let's also cancel all running tasks:
pending = asyncio.Task.all_tasks()
for task in pending:
    task.cancel()
    # Now we should await task to execute it's cancellation.
    # Cancelled task raises asyncio.CancelledError that we can suppress:
    with suppress(asyncio.CancelledError):
        loop.run_until_complete(task)

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