[Async-sig] [ANN] async_generator v1.2 released

[Nathaniel Smith]

On Fri, Nov 25, 2016 at 12:03 AM, Alex Grönholm
<alex.gronholm at nextday.fi> wrote:
> 24.11.2016, 23:23, Nathaniel Smith kirjoitti:
>
> On Nov 23, 2016 11:29 PM, "Alex Grönholm" <alex.gronholm at nextday.fi> wrote:
>>
>> 23.11.2016, 01:34, Nathaniel Smith kirjoitti:
>>>
>>> On Tue, Nov 22, 2016 at 2:22 PM, Alex Grönholm <alex.gronholm at nextday.fi>
>>> wrote:
>>> > I'm not sure where asyncio_extras's async generator implementation
>>> > assumes
>>> > you're using asyncio. Could you elaborate on that?
>>>
>>> If I'm reading it right, it assumes that the only two things that might
>>> be yielded to the coroutine runner are either (a) the special yield wrapper,
>>> or (b) an awaitable object like an asyncio.Future. This works on asyncio,
>>> because that's all the asyncio runner supports, but it doesn't work with
>>> (for example) curio. async_generator (like native async generators) allows
>>> arbitrary objects to be yielded to the coroutine runner.
>>
>> You are misreading the code. It is in no way limited to what asyncio
>> accepts. It doesn't even import asyncio in the asyncyield or generator
>> modules. The only parts of the library that depend on PEP 3156 event loops
>> are the ones that involve executors and threads.
>
> I didn't say that it imported asyncio. I said that it assumes the only
> things that will be yielded are the things that asyncio yields. This is the
> line that I'm worried about:
>
>
> https://github.com/agronholm/asyncio_extras/blob/aec412e1b7034ca3cad386c381e655ce3547fee3/asyncio_extras/asyncyield.py#L40
>
> The code awaits the value yielded by the coroutine, but there's no guarantee
> that this value is awaitable. It's an arbitrary Python object representing a
> message sent to the coroutine runner. It turns out that asyncio only uses
> awaitable objects for its messages, so this code can get away with this on
> asyncio, but if you try using this code with curio then I'm pretty sure
> you're going to end up doing something like "await (3,)" and then blowing
> up.
>
> PEP 492 clearly states the following:
>
> It is a TypeError to pass anything other than an awaitable object to an
> await expression.
>
> That (3,) is not an awaitable, so the example is invalid. That said, I will
> re-examine this part of the implementation and correct it if necessary.

I feel like I'm running out of ideas for how to explain this, and
starting to just repeat myself :-(. There is nothing that says the
return value from coro.__next__() must be awaitable. Coroutines can
yield arbitrary objects. (3,) is not awaitable, but it's a perfectly
valid thing to be yielded from a coroutine.

> So far I just haven't encountered anything that would produce an error.

That's because you've only tested on asyncio, not curio. I promise!

-n

-- 
Nathaniel J. Smith -- https://vorpus.org