24.11.2016, 23:23, Nathaniel Smith kirjoitti:
On Nov 23, 2016 11:29 PM, "Alex Grönholm" <alex.gronh...@nextday.fi
<mailto:alex.gronh...@nextday.fi>> wrote:
>
> 23.11.2016, 01:34, Nathaniel Smith kirjoitti:
>>
>> On Tue, Nov 22, 2016 at 2:22 PM, Alex Grönholm
<alex.gronh...@nextday.fi <mailto:alex.gronh...@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 aTypeErrorto pass anything other than an/awaitable/object to
anawaitexpression.
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. So far I just haven't encountered anything that would produce
an error.
>> > Native async generators don't support "yield from" either, so I
didn't try
>> > to add that until there's a new PEP that defines its exact semantics.
>>
>> Yeah, one of the motivations for async_generator has been to figure
out what the semantics for native generators should be :-). The native
async generators in 3.6 use the same implementation trick as
async_generator (not entirely sure if Yury got it from me, or if it's
an independent invention), and the yield from implementation is partly
motivated as a proof of concept / testing ground for hopefully getting
it into 3.7. The yield from semantics are pretty straightforward
though, I think.
>>
>> > As for
>> > aclose/asend/athrow support, I will have to look into that.
>> >
>> > Could we synchronize the implementations so that they're
compatible with
>> > each other? I would've preferred there to be only a single
implementation
>> > (asyncio_extras was published first, but admittedly I didn't
advertise it
>> > that well) but now it'd be best if we work together, wouldn't you
agree?
>>
>> Makes sense to me :-). I think the obvious approach would be for
async_extras to just depend on async_generator, since AFAICT
async_generator is pretty much complete, and like you say, there's not
much point in carrying around two copies of the same thing. But if you
have another suggestion I'd be interested to hear it...
>
> I may consider that at some point, but I'm not yet comfortable with
those extended capabilities. Meanwhile I will add any missing
functionality (asend, athrow etc.) to mine.
You could just not import yield_from_ and then you don't have any
extended capabilities... but up to you, obviously.
> One thing I noticed is that there seems to be no way to detect async
generator functions in your implementation. That is something I would
want to have before switching.
Good point. That should be pretty trivial to add.
-n
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