[Python-Dev] PEP 492: async/await in Python; version 4

Ben Darnell ben at bendarnell.com
Thu May 7 01:06:38 CEST 2015

On Tue, May 5, 2015 at 1:39 PM, Paul Moore <p.f.moore at gmail.com> wrote:

> It would probably be helpful to have a concrete example of a basic
> event loop that did *nothing* but schedule tasks. No IO waiting or
> similar, just scheduling. I have a gut feeling that event loops are
> more than just asyncio, but without examples to point to it's hard to
> keep a focus on that fact. And even harder to isolate "what is an
> event loop mechanism" from "what is asyncio specific". For example,
> asyncio.BaseEventLoop has a create_connection method. That's
> *obviously* not a fundamental aspect of a generic event loop, But
> call_soon (presumably) is. Having a documented "basic event loop"
> interface would probably help emphasise the idea than event loops
> don't have to be asyncio. (Actually, what *is* the minimal event loop
> interface that is needed for the various task/future mechanisms to
> work, independently of asyncio? And what features of an event loop etc
> are needed for the PEP, if it's being used outside of asyncio?)

Twisted has a pretty good taxonomy of event loop methods, in the interfaces
at the bottom of this page:
and the comparison matrix at

The asyncio event loops implement most of these (not the exact interfaces,
but the same functionality). Tornado implements FDSet, Time, and part of
Threads in the IOLoop itself, with the rest of the functionality coming
from separate classes. (You may wonder then why Twisted and asyncio put
everything in the core event loop? It's necessary to abstract over certain
platform differences, which is one big reason why Tornado has poor support
for Windows).


> I guess the other canonical event loop use case is GUI system message
> dispatchers.
> >> You can argue that the syntax is needed to help
> >> make async more accessible - but if that's the case then the
> >> terminology debates and confusion are clear evidence that it's not
> >> succeeding in that goal.
> >
> > Perhaps, but arguing about the nitty-gritty details of something doesn't
> > automatically lead to a clearer understanding of the higher level
> concept.
> > Discussing how turning a steering wheel in a car might help you grasp how
> > cars turn, but it isn't a requirement to get "turn the wheel left to make
> > the car go left".
> Fair point. If only I could avoid driving into walls :-)
> >> Of course, that's based on my perception of
> >> one of the goals of the PEP as being "make coroutines and asyncio more
> >> accessible", If the actual goals are different, my conclusion is
> >> invalid.
> >
> > I think the goal is "make coroutines easier to use" and does not directly
> > relate to asyncio.
> OK. But in that case, some examples using a non-asyncio toy "just
> schedule tasks" event loop might help.
> >> Well, twisted always had defer_to_thread. Asyncio has run_in_executor,
> >> but that seems to be callback-based rather than coroutine-based?
> >
> > Yep.
> ... and so you can't use it with async/await?
> >> Many people use requests for their web access. There are good reasons
> >> for this. Are you saying that until someone steps up and writes an
> >> async implementation of requests, I have to make a choice - requests
> >> or asyncio?
> >
> > I believe so; you need something to implement __await__. This is true in
> any
> > language that implements co-routines.
> >
> >> Unfortunately, I can't see myself choosing asyncio in that
> >> situation. Which again means that asyncio becomes "something that the
> >> average user can't use". Which in turn further entrenches it as a
> >> specialist-only tool.
> >
> > You forgot to append "... yet" to that statement. Just because something
> > isn't available out of the box without some effort to support doesn't
> mean
> > it will never happen, else there would be absolutely no Python 3 users
> out
> > there.
> Fair point. Yuri mentioned aiohttp, as well. The one difference
> between this and Python 2/3, is that here you *have* to have two
> separate implementations. There's no equivalent of a "shared source"
> async and synchronous implementation of requests. So the typical
> "please support Python 3" issue that motivates projects to move
> forward doesn't exist in the same way. It's not to say that there
> won't be async versions of important libraries, it's just hard to see
> how the dynamics will work. I can't see myself raising an issue on
> cx_Oracle saying "please add asyncio support", and I don't know who
> else I would ask...
> > Co-routine-based asynchronous programming is new to Python, so as a
> > community we don't have it as something everyone learns over time. If you
> > don't come from an area that supports it then it will be foreign to you
> and
> > not make sense without someone giving you a good tutorial on it. But
> > considering C#, Dart, and Ecmascript 6 (will) have co-routine support --
> and
> > those are just the languages I can name off the top of my head -- using
> the
> > exact same keywords suggests to me that it isn't *that* difficult of a
> topic
> > to teach people. This is just one of those PEPs where you have to trust
> the
> > people with experience in the area are making good design decisions for
> > those of us who aren't in a position to contribute directly without more
> > experience in the domain.
> That's also a fair point, and it seems to me that there *is*
> reasonably general feeling that the experts can be trusted on the
> basic principles. There's also a huge amount of bikeshedding, but
> that's pretty much inevitable :-)
> But I do think that unless someone does something to offer some
> non-asyncio examples of coroutine-based asynchronous programming in
> Python, the link in people's minds between async and asyncio will
> become more and more entrenched. While asyncio is the only real event
> loop implementation, saying "async can be used for things other than
> asyncio" is a rather theoretical point.
> Is there anyone who feels they could write a stripped down but working
> example of a valid Python event loop *without* the asyncio aspects? Or
> is that what David Beazley's talk does? (I got the impression from
> what you said that he was aiming at async IO rather than just a non-IO
> event loop). Can asyncio.Future and asyncio.Task be reused with such
> an event loop, or would those need to be reimplemented as well?
> Writing your own event loop seems like a plausible exercise. Writing
> your own version of the whole
> task/future/coroutine/queue/synchronisation mechanisms seems like a
> lot to expect. And the event loop policy mechanism says that it works
> with loops that implement asyncio.BaseEventLoop (which as noted
> includes things like create_connection, etc).
> Paul
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