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throwup238 • today at 5:09 PM • 1 reply • view on HN

So do Python and Javascript. I think most languages with async/await also support noop-ing the yield if the future is already resolved. It’s only when you create a new task/promise that stuff is guaranteed to get scheduled instead of possibly running immediately.

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amluto • today at 6:38 PM

I can't quite parse what you're saying.

Python works like this:

    import asyncio

    async def sleepy() -> None:
        print('Sleepy started')
        await asyncio.sleep(0.25)
        print('Sleepy resumed once')
        await asyncio.sleep(0.25)
        print('Sleepy resumed and is done!')


    async def main():
        sleepy_future = sleepy()
        print('Started a sleepy')

        await asyncio.sleep(2)
        print('Main woke back up.  Time to await the sleepy.')

        await sleepy_future

    if __name__ == "__main__":
        asyncio.run(main())
Running it does this:

    $ python3 ./silly_async.py
    Started a sleepy
    Main woke back up.  Time to await the sleepy.
    Sleepy started
    Sleepy resumed once
    Sleepy resumed and is done!
So there mere act of creating a coroutine does not cause the runtime to run it. But if you explicitly create a task, it does get run:

    import asyncio

    async def sleepy() -> None:
        print('Sleepy started')
        await asyncio.sleep(0.25)
        print('Sleepy resumed once')
        await asyncio.sleep(0.25)
        print('Sleepy resumed and is done!')


    async def main():
        sleepy_future = sleepy()
        print('Started a sleepy')

        sleepy_task = asyncio.create_task(sleepy_future)
        print('The sleepy future is now in a task')

        await asyncio.sleep(2)
        print('Main woke back up.  Time to await the task.')

        await sleepy_task

    if __name__ == "__main__":
        asyncio.run(main())

    $ python3 ./silly_async.py
    Started a sleepy
    The sleepy future is now in a task
    Sleepy started
    Sleepy resumed once
    Sleepy resumed and is done!
    Main woke back up.  Time to await the task.
I personally like the behavior of coroutines not running unless you tell them to run -- it makes it easier to reason about what code runs when. But I do not particularly like the way that Python obscures the difference between a future-like thing that is a coroutine and a future-like thing that is a task.
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