[Python-Dev] async/await in Python; v2 (original) (raw)

Andrew Svetlov andrew.svetlov at gmail.com
Wed Apr 22 20:32:40 CEST 2015

For now I can use mix asyncio.coroutines and async def functions, I mean I can write await f() inside async def to call asyncio.coroutine f and vise versa: I can use yield from g() inside asyncio.coroutine to call async def g(): ....

If we forbid to call async def from regualr code how asyncio should work? I'd like to push async def everywhere in asyncio API where asyncio.coroutine required.

On Wed, Apr 22, 2015 at 8:13 PM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:

Hi Rajiv,

On 2015-04-22 12:53 PM, Rajiv Kumar wrote:

I'd like to suggest another way around some of the issues here, with apologies if this has already been discussed sometime in the past. From the viewpoint of a Python programmer, there are two distinct reasons for wanting to suspend execution in a block of code: 1. To yield a value from an iterator, as Python generators do today. 2. To cede control to the event loop while waiting for an asynchronous task to make progress in a coroutine. As of today both of these reasons to suspend are supported by the same underlying mechanism, i.e. a "yield" at the end of the chain of "yield from"s. PEPs 492 and 3152 introduce "await" and "cocall", but at the bottom of it all there's effectively still a yield as I understand it. I think that the fact that these two concepts use the same mechanism is what leads to the issues with coroutine-generators that Greg and Yury have raised. With that in mind, would it be possible to introduce a second form of suspension to Python to specifically handle the case of ceding to the event loop? I don't know what the implementation complexity of this would be, or if it's even feasible. But roughly speaking, the syntax for this could use "await", and code would look just like it does in the PEP. The semantics of "await " would be analogous to "yield from " today, with the difference that the Task would go up the chain of "await"s to the outermost caller, which would typically be asyncio, with some modifications from its form today. Progress would be made via anext instead of next. I think that what you propose is a great idea. However, its implementation will be far more invasive than what PEP 492 proposes. I doubt that we'll be able to make it in 3.5 if we choose this route. BUT: With my latest proposal to disallow for..in loops and iter()/list()-like builtins, the fact that coroutines are based internally on generators is just an implementation detail. There is no way users can exploit the underlying generator object. Coroutine-objects only provide 'send()' and 'throw()' methods, which they would also have with your implementation idea. This gives us freedom to consider your approach in 3.6 if we decide to add coroutine-generators. To make this work we might want to patch inspect.py to make isgenerator() family of functions to return False for coroutines/coroutine-objects. Thanks a lot for the feedback! Yury

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-- Thanks, Andrew Svetlov

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