[Async-sig] A possible tiny step towards event loop interoperability

[Ben Darnell]

On Fri, Jun 3, 2016 at 8:48 PM, Guido van Rossum <guido at python.org> wrote:

> OK, that helps. Could you talk me through the crucial
> incompatibilities here? (If that's too  much to ask I'm also happy to
> leave this up to the experts -- I trust that there's more than enough
> brainpower here to come up with a solution that works for all
> concerned.)
>

Coroutines that are chained together with either `yield from` or `await`
are essentially one big generator with one shared coroutine runner. At the
bottom, though, are the actual objects that are yielded by this generator
(in asyncio, this happens in `Future.__iter__`). If the coroutine runner is
asyncio.Task, it will raise if any part of this chain of coroutines yields
anything but None or an asyncio.Future. The Tornado coroutine runner is
more liberal: it accepts its own futures as well as asyncio and concurrent
futures or twisted deferreds.

In other words, this works with tornado, but not with asyncio:

import asyncio
import tornado.gen
import tornado.ioloop
import tornado.platform.asyncio

use_tornado = False

async def f():
    await asyncio.sleep(0.1)
    await tornado.gen.sleep(0.1)

if use_tornado:
    tornado.platform.asyncio.AsyncIOMainLoop().install()
    tornado.ioloop.IOLoop.current().run_sync(f)
else:
    asyncio.get_event_loop().run_until_complete(f())




There were more incompatibilities with the yield-based syntax than with the
await-based syntax. Tornado's coroutine runner allowed you to yield objects
like lists and dicts (yielding a container was a kind of fork/join) that
were not allowed in asyncio (thanks mainly to asyncio's use of `yield
from`). The introduction of `await` has imposed similar restrictions on
both systems, so there is no more yielding of lists or dicts in either
case; explicit wrapper objects must be used. (but there's still a
difference in that asyncio.Task allows None but tornado coroutines do not)





>
> On Fri, Jun 3, 2016 at 5:45 PM, Ben Darnell <ben at bendarnell.com> wrote:
> > On Fri, Jun 3, 2016 at 8:43 PM, Guido van Rossum <guido at python.org>
> wrote:
> >>
> >> Could someone point me to the specific code that's considered the
> >> coroutine runner in asyncio and Tornado? I've been immersed in asyncio
> >> for so long that I don't know which part you're talking about. :-(
> >
> >
> > asyncio.Task and tornado.gen.Runner. Basically the thing that calls
> next()
> > and send() on generator objects.
> >
> >>
> >>
> >> On Fri, Jun 3, 2016 at 5:26 PM, Nathaniel Smith <njs at pobox.com> wrote:
> >> > On Fri, Jun 3, 2016 at 5:14 PM, Ben Darnell <ben at bendarnell.com>
> wrote:
> >> >> I think this could be useful, but's answering the question of "what
> >> >> coroutine runner is this", not "what event loop is this".
> >> >
> >> > Thanks, that's definitely a better way to put it.
> >> >
> >> > -n
> >> >
> >> > --
> >> > Nathaniel J. Smith -- https://vorpus.org
> >> > _______________________________________________
> >> > Async-sig mailing list
> >> > Async-sig at python.org
> >> > https://mail.python.org/mailman/listinfo/async-sig
> >> > Code of Conduct: https://www.python.org/psf/codeofconduct/
> >>
> >>
> >>
> >> --
> >> --Guido van Rossum (python.org/~guido)
> >
> >
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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