[Python-Dev] async/await behavior on multiple calls

[Roy Williams]

@Kevin correct, that's the point I'd like to discuss.  Most other
mainstream languages that implements async/await expose the programming
model with Tasks/Futures/Promises as opposed to coroutines  PEP 492 states
'Objects with __await__ method are called Future-like objects in the rest
of this PEP.' but their behavior differs from that of Futures in this core
way.  Given that most other languages have standardized around async
returning a Future as opposed to a coroutine I think it's worth exploring
why Python differs.

There's a lot of benefits to making the programming model coroutines
without a doubt.  It's absolutely brilliant that I can just call code
annotated with @asyncio.coroutine and have it just work.  Code using the
old @asyncio.coroutine/yield from syntax should absolutely stay the same.
Similarly, since ES7 async/await is backed by Promises it'll just work for
any existing code out there using Promises.

My proposal would be to automatically wrap the return value from an `async`
function or any object implementing `__await__` in a future with
`asyncio.ensure_future()`.  This would allow async/await code to behave in
a similar manner to other languages implementing async/await and would
remain compatible with existing code using asyncio.

What's your thoughts?

Thanks,
Roy

On Tue, Dec 15, 2015 at 3:35 PM, Kevin Conway <kevinjacobconway at gmail.com>
wrote:

> I think there may be somewhat of a language barrier here. OP appears to be
> mixing the terms of coroutines and futures. The behavior OP describes is
> that of promised or async tasks in other languages.
>
> Consider a JS promise that has been resolved:
>
> promise.then(function (value) {...});
>
> promise.then(function (value) {...});
>
> Both of the above will execute the callback function with the resolved
> value regardless of how much earlier the promise was resolved. This is not
> entirely different from how Futures work in Python when using
> 'add_done_callback'.
>
> The code example from OP, however, is showing the behaviour of awaiting a
> coroutine twice rather than awaiting a Future twice. Both objects are
> awaitable but both exhibit different behaviour when awaited multiple times.
>
> A scenario I believe deserves a test is what happens in the asyncio
> coroutine scheduler when a promise is awaited multiple times. The current
> __await__ behaviour is to return self only when not done and then to return
> the value after resolution for each subsequent await. The Task, however,
> requires that it must be a Future emitted from the coroutine and not a
> primitive value. Awaiting a resolved future should result
>
> On Tue, Dec 15, 2015, 14:44 Guido van Rossum <guido at python.org> wrote:
>
>> Agreed. (But let's hear from the OP first.)
>>
>> On Tue, Dec 15, 2015 at 12:27 PM, Andrew Svetlov <
>> andrew.svetlov at gmail.com> wrote:
>>
>>> Both Yury's suggestions sounds reasonable.
>>>
>>> On Tue, Dec 15, 2015 at 10:24 PM, Yury Selivanov
>>> <yselivanov.ml at gmail.com> wrote:
>>> > Hi Roy and Guido,
>>> >
>>> > On 2015-12-15 3:08 PM, Guido van Rossum wrote:
>>> > [..]
>>> >>
>>> >>
>>> >> I don't know how long you have been using async/await, but I wonder if
>>> >> it's possible that you just haven't gotten used to the typical usage
>>> >> patterns? In particular, your claim "anything that takes an
>>> `awaitable` has
>>> >> to know that it wasn't already awaited" makes me sound that you're
>>> just
>>> >> using it in an atypical way (perhaps because your model is based on
>>> other
>>> >> languages). In typical asyncio code, one does not usually take an
>>> awaitable,
>>> >> wait for it, and then return it -- one either awaits it and then
>>> extracts
>>> >> the result, or one returns it without awaiting it.
>>> >
>>> >
>>> > I agree.  Holding a return value just so that coroutine can return it
>>> again
>>> > seems wrong to me.
>>> >
>>> > However, since coroutines are now a separate type (although they share
>>> a lot
>>> > of code with generators internally), maybe we can change them to throw
>>> an
>>> > error when they are awaited on more than one time?
>>> >
>>> > That should be better than letting them return `None`:
>>> >
>>> >     coro = coroutine()
>>> >     await coro
>>> >     await coro  # <- will raise RuntimeError
>>> >
>>> >
>>> > I'd also add a check that the coroutine isn't being awaited by more
>>> than one
>>> > coroutine simultaneously (another, completely different issue, more on
>>> which
>>> > here: https://github.com/python/asyncio/issues/288).  This was fixed
>>> in
>>> > asyncio in debug mode, but ideally, we should fix this in the
>>> interpreter
>>> > core.
>>> >
>>> > Yury
>>> > _______________________________________________
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>>> > Unsubscribe:
>>> >
>>> https://mail.python.org/mailman/options/python-dev/andrew.svetlov%40gmail.com
>>>
>>>
>>>
>>> --
>>> Thanks,
>>> Andrew Svetlov
>>>
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>>
>>
>> --
>> --Guido van Rossum (python.org/~guido)
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