[Async-sig] async generator confusion or bug?

[Nathaniel Smith]

I actually thought that async generators already guarded against this using
their ag_running attribute. If I try running Dima's example with
async_generator, I get:

sending user-1
received user-1
sending user-2
sending user-0
Traceback (most recent call last):
[...]
ValueError: async generator already executing

The relevant code is here:
https://github.com/njsmith/async_generator/blob/e303e077c9dcb5880c0ce9930d560b282f8288ec/async_generator/impl.py#L273-L279

But I added this in the first place because I thought it was needed for
compatibility with native async generators :-)

-n

On Jun 26, 2017 6:54 PM, "Yury Selivanov" <yselivanov at gmail.com> wrote:

> (Posting here, rather than to the issue, because I think this actually
> needs more exposure).
>
> I looked at the code (genobject.c) and I think I know what's going on
> here.  Normally, when you work with an asynchronous generator (AG) you
> interact with it through "asend" or "athrow" *coroutines*.
>
> Each AG has its own private state, and when you await on "asend" coroutine
> you are changing that state.  The state changes on each "asend.send" or
> "asend.throw" call.  The normal relation between AGs and asends is 1 to 1.
>
>   AG - asend
>
> However, in your example you change that to 1 to many:
>
>      asend
>     /
>   AG - asend
>     \
>      asend
>
> Both 'ensure_future' and 'gather' will wrap each asend coroutine into an
> 'asyncio.Task'. And each Task will call "asend.send(None)" right in its
> '__init__', which changes the underlying *shared* AG instance completely
> out of order.
>
> I don't see how this can be fixed (or that it even needs to be fixed), so
> I propose to simply raise an exception if an AG has more than one asends
> changing it state *at the same time*.
>
> Thoughts?
>
> Yury
>
> > On Jun 26, 2017, at 12:25 PM, Dima Tisnek <dimaqq at gmail.com> wrote:
> >
> > Hi group,
> >
> > I'm trying to cross-use an sync generator across several async functions.
> > Is it allowed or a completely bad idea? (if so, why?)
> >
> > Here's MRE:
> >
> > import asyncio
> >
> >
> > async def generator():
> >    while True:
> >        x = yield
> >        print("received", x)
> >        await asyncio.sleep(0.1)
> >
> >
> > async def user(name, g):
> >    print("sending", name)
> >    await g.asend(name)
> >
> >
> > async def helper():
> >    g = generator()
> >    await g.asend(None)
> >
> >    await asyncio.gather(*[user(f"user-{x}", g) for x in range(3)])
> >
> >
> > if __name__ == "__main__":
> >    asyncio.get_event_loop().run_until_complete(helper())
> >
> >
> > And the output it produces when ran (py3.6.1):
> >
> > sending user-1
> > received user-1
> > sending user-2
> > sending user-0
> > received None
> > received None
> >
> >
> > Where are those None's coming from in the end?
> > Where did "user-0" and "user-1" data go?
> >
> > Is this a bug, or am I hopelessly confused?
> > Thanks!
> > _______________________________________________
> > Async-sig mailing list
> > Async-sig at python.org
> > https://mail.python.org/mailman/listinfo/async-sig
> > Code of Conduct: https://www.python.org/psf/codeofconduct/
>
>
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