Thank you very much Kyle for your answer, I am moving this conversation to the more proper python-list for whoever wants to chime in. I summarize here the key points of my original question (full question on the quoted email):
I have an application that listens on two websockets through the async library https://websockets.readthedocs.io/ and I have to perform the same function on the result, no matter where the message came from. I have implemented a rather cumbersome solution with async Queues: https://pastebin.com/BzaxRbtF, but i think there has to be a more async-friendly option I am missing. Now I move on to the comments that Kyle made On Tue, Jun 23, 2020 at 12:32 AM Kyle Stanley <aeros...@gmail.com> wrote: > I believe asyncio.wait() with "return_when=FIRST_COMPLETED" would > perform the functionality you're looking for with the > "asyncio.on_first_return()". For details on the functionality of > asyncio.wait(), see > https://docs.python.org/3/library/asyncio-task.html#asyncio.wait. > > I understand that I can create two coroutines that call the same > function, but it would be much cleaner (because of implementation issues) > if I can simply create a coroutine that yields the result of whichever > connection arrives first. > > You can use an asynchronous generator that will continuously yield the > result of the first recv() that finishes (I'm assuming you mean > "yields" literally and want multiple results from a generator, but I > might be misinterpreting that part). > Yes, I want to have multiple results: the connections listening forever, returning a result for each message received. > > Here's a brief example, using the recv() coroutine function from the > pastebin linked: > > ``` > import asyncio > import random > > async def recv(message: str, max_sleep: int): > sleep_time = max_sleep * random.random() > await asyncio.sleep(sleep_time) > return f'{message} awaited for {sleep_time:.2f}s' > > async def _start(): > while True: > msgs = [ > asyncio.create_task(recv("Messager 1", max_sleep=1)), > asyncio.create_task(recv("Messager 2", max_sleep=1)) > ] > done, _ = await asyncio.wait(msgs, > return_when=asyncio.FIRST_COMPLETED) > result = done.pop() > yield await result > > async def main(): > async for result in _start(): > print(result) > > asyncio.run(main()) > ``` I forgot to mention thatI did try to use asyncio.wait with `FIRST_COMPLETED`; however, the problem is that it seems to evict the not-completed coroutines, so the messenger that arrives second does not send the message. To check it, I have run that script without the random sleep. just msgr1 waits 1s and msgr2 waits 2s, so msgr1 always ends first. I expect a result like this (which I am currently getting with queues): Messenger 1 waits for 1.0s Messenger 1 waits for 1.0s Messenger 2 waits for 2.0s Messenger 1 waits for 1.0s Messenger 1 waits for 1.0s Messenger 2 waits for 2.0s Messenger 1 waits for 1.0s ... but instead I got this: Messenger 1 waits for 1.0s Messenger 1 waits for 1.0s Messenger 1 waits for 1.0s Messenger 1 waits for 1.0s Messenger 1 waits for 1.0s ... > Note that in the above example, in "msgs", you can technically pass > the coroutine objects directly to asyncio.wait(), as they will be > implicitly converted to tasks. However, we decided to deprecate that > functionality in Python 3.8 since it can be rather confusing. So > creating and passing the tasks is a better practice. > Thanks for that info, I am still trying to grasp the best practices surrounding mostly the explicitness in async. > > Again, it's quite likely I am not seeing something obvious, but I didn't > know where else to ask. > > If you're not mostly certain or relatively inexperienced with the > specific area that the question pertains to, I'd recommend asking on > python-list first (or another Python user community). python-ideas is > primarily intended for new feature proposals/suggestions. Although if > you've tried other resources and haven't found an answer, it's > perfectly fine to ask a question as part of the suggestion post. > > > Original question, as posted in python-ideas: > On Mon, Jun 22, 2020 at 6:24 PM Pablo Alcain <pabloalc...@gmail.com> > wrote: > > > > Hey everyone. I have been looking into asyncio lately, and even though I > have had my fair share of work, I still have some of it very shaky, so > first of all forgive me if what I am saying here is already implemented and > I totally missed it (so far, it looks *really* likely). > > > > Basically this is the situation: I have an application that listens on > two websockets through the async library > https://websockets.readthedocs.io/ and I have to perform the same > function on the result, no matter where the message came from. I understand > that I can create two coroutines that call the same function, but it would > be much cleaner (because of implementation issues) if I can simply create a > coroutine that yields the result of whichever connection arrives first. > > > > I have implemented a rather cumbersome solution with async Queues (as I > would do in threading), in which each connection puts its message in a > queue and an adapter class awaits the first element of the queue on > "receive". Here I attach a pastebin with the minimal working example: > https://pastebin.com/BzaxRbtF > > > > However, it looks like a more async-friendly solution should exist, > something like > > > > ``` > > async def _start(): > > msg1 = recv("Messager 1", sleep_time=1) > > msg2 = recv("Messager 2", sleep_time=2) > > while True: > > result = await asyncio.on_first_return(msg1, msg2) > > print(result) > > ``` > > > > (I understand that this implementation would not work because the event > loop doesn't know that it is "the same task repeated", but it's just to > tell you the general idea) > > > > Again, it's quite likely I am not seeing something obvious, but I didn't > know where else to ask. > > > > Thank you very much, > > Pablo > > > > _______________________________________________ > > Python-ideas mailing list -- python-id...@python.org > > To unsubscribe send an email to python-ideas-le...@python.org > > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > > Message archived at > https://mail.python.org/archives/list/python-id...@python.org/message/XBR5QPXRBCCJELDVEWMKRBPTNG4SJM64/ > > Code of Conduct: http://python.org/psf/codeofconduct/ > -- https://mail.python.org/mailman/listinfo/python-list
