[Python-ideas] async/await in Python

[Yury Selivanov]

Yann,

On 2015-04-21 3:28 AM, Yann Kaiser wrote:
> On Tue, 21 Apr 2015 at 00:07 Yury Selivanov <yselivanov.ml at gmail.com> wrote:
[...]
>> If someone smarter than me can figure out a way to do this
>> in a non-confusing way that won't require duplicating genobject.c
>> and fourth of ceval.c I'd be glad to update the PEP.
>
> Well the above strategy could be imitated, multiplexing the yield channel
> between yields meant for the event loop and those meant for the
> asynchronous iteration protocol. It could be done with an additional
> parameter to YIELD_VALUE (I'm assuming await compiles down to it or
> YIELD_FROM ?), although it would be redundant for functions not using both
> concepts at the same time. Still, it's just one byte.
> YIELD_FROM... my head explosion warnings are also being set off.
>
> As Greg pointed out, the presence of yield in an async function would have
> it return an object also supporting __aiter__ and __anext__.
>

Yann, I think that there are no fundamental reasons
preventing us from having a coroutine-generator hybrid.

The semantics will probably be like that:

def foo(): # generator function
    yield 1

foo() # generator object

async def bar(): # coroutine-generator function
    async yield 1 # or just 'yield 1'

bar() # coroutine-generator object

Maybe it's possible to heavily hack genobject.c and
ceval.c (YIELD & YIELD_FROM implementations) to support this.

But I think that the amount of hacks will be so huge, that
we'll need a pair of new opcodes + new asyncgenobject.c, to
at least avoid performance penalties on regular generators.

Anyways, I think it's outside of the scope of PEP 492.

I think that the best thing we should do now, is to make
sure that we *can* have coroutine-generators in the future.
I think that PEP 492, in its current state leaves a room
for that.

If you want to continue the discussion on this topic, I
suggest you to clone the reference implementation and
implement your concept.  But there is no guarantee that
it will land to PEP 492 though.

Thanks,
Yury