[Python-Dev] PEP 492: async/await in Python; version 5

Rajiv Kumar rajiv.kumar at gmail.com
Wed May 6 00:36:53 CEST 2015

I wrote a little example[1] that has a bare-bones implementation of Go
style channels via a custom event loop. I used it to translate the prime
sieve example from Go[2] almost directly to Python. The code uses "message
= await channel.receive()" to mimic Go's "message <- channel". Instead of
using "go func()" to fire off a goroutine, I add the PEP492 coroutine to my
simple event loop.

It's not an efficient implementation - really just a proof of concept that
you can use async/await in your own code without any reference to asyncio.
I ended up writing it as I was thinking about how PEP 342 style coroutines
might look like in an async/await world.

In the course of writing this, I did find that it would be useful to have
the PEP document how event loops should advance the coroutines (via
.send(None) for example). It would also be helpful to have the semantics of
how await interacts with different kinds of awaitables documented. I had to
play with Yury's implementation to see what it does if the __await__ just
returns iter([1,2,3]) for example.

- Rajiv

[1] https://gist.github.com/vrajivk/c505310fb79d412afcd5#file-sieve-py

[2] https://golang.org/doc/play/sieve.go

On Tue, May 5, 2015 at 2:54 PM, Paul Moore <p.f.moore at gmail.com> wrote:

> On 5 May 2015 at 22:38, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> > n 2015-05-05 5:01 PM, Paul Moore wrote:
> >>
> >> On 5 May 2015 at 21:00, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> >>>
> >>> On 2015-05-05 3:40 PM, Jim J. Jewett wrote:
> >>>>
> >>>> On Tue May 5 18:29:44 CEST 2015, Yury Selivanov posted an updated
> >>>> PEP492.
> >>>>
> >>>> Where are the following over-simplifications wrong?
> >>>>
> >>>> (1)  The PEP is intended for use (almost exclusively) with
> >>>> asychronous IO and a scheduler such as the asynchio event loop.
> >>>
> >>> Yes. You can also use it for UI loops.  Basically, anything
> >>> that can call your code asynchronously.
> >>
> >> Given that the stdlib doesn't provide an example of such a UI loop,
> >> what would a 3rd party module need to implement to provide such a
> >> thing? Can any of the non-IO related parts of asyncio be reused for
> >> the purpose, or must the 3rd party module implement everything from
> >> scratch?
> >
> > The idea is that you integrate processing of UI events to
> > your event loop of choice.  For instance, Twisted has
> > integration for QT and other libraries [1].  This way you
> > can easily combine async network (or OS) calls with your
> > UI logic to avoid "callback hell".
> We seem to be talking at cross purposes. You say the PEP is *not*
> exclusively intended for use with asyncio. You mention UI loops, but
> when asked how to implement such a loop, you say that I integrate UI
> events into my event loop of choice. But what options do I have for
> "my event loop of choice"? Please provide a concrete example that
> isn't asyncio. Can I use PEP 492 with Twisted (I doubt it, as Twisted
> doesn't use yield from, which is Python 3.x only)? I contend that
> there *is* no concrete example that currently exists, so I'm asking
> what I'd need to do to write one. You pointed at qamash, but that
> seems to be subclassing asyncio, so isn't "something that isn't
> asyncio".
> Note that I don't have a problem with there being no existing
> implementation other than asyncio. I'd just like it if we could be
> clear over exactly what we mean when we say "the PEP is not tied to
> asyncio". It feels like the truth currently is "you can write your own
> async framework that uses the new features introduced by the PEP". I
> fully expect that *if* there's a need for async frameworks that aren't
> fundamentally IO multiplexors, then it'll get easier to write them
> over time (the main problem right now is a lack of good tutorial
> examples of how to do so). But at the moment, asyncio seems to be the
> only game in town (and I can imagine that it'll always be the main IO
> multiplexor, unless existing frameworks like Twisted choose to compete
> rather than integrate).
> Paul
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