On 24.08.2016 18:35, Guido van Rossum wrote:
On Wed, Aug 24, 2016 at 8:17 AM, Yury Selivanov
<yselivanov...@gmail.com <mailto:yselivanov...@gmail.com>> wrote:
On 2016-08-23 10:38 PM, Rajiv Kumar wrote:
I was playing with your implementation to gain a better
understanding of the operation of asend() and friends. Since I
was explicitly trying to "manually" advance the generators, I
wasn't using asyncio or other event loop. This meant that the
first thing I ran into with my toy code was the RuntimeError
("cannot iterate async generator without finalizer set").
As you have argued elsewhere, in practice the finalizer is
likely to be set by the event loop. Since the authors of event
loops are likely to know that they should set the finalizer,
would it perhaps be acceptable to merely issue a warning
instead of an error if the finalizer is not set? That way
there isn't an extra hoop to jump through for simple examples.
In my case, I just called
sys.set_asyncgen_finalizer(lambda g: 1)
to get around the error and continue playing :) (I realize
that's a bad thing to do but it didn't matter for the toy cases)
Yeah, maybe warning would be sufficient. I just find it's highly
unlikely that a lot of people would use async generators without a
loop/coroutine runner, as it's a very tedious process.
Heh, I had the same reaction as Rajiv. I think the tediousness is
actually a good argument that there's no reason to forbid this. I
don't even think a warning is needed. People who don't use a coroutine
runner are probably just playing around (maybe even in the REPL) and
they shouldn't get advice unasked.
I also was irritated as Yury said there were absolutely no changes after
python-ideas. He said he might consider a clearer warning for those
examples at the beginning of the PEP to make them work for the reader.
Would it be possible to print a warning only when an async generator
is being finalized and doesn't run straight to the end without
suspending or yielding? For regular generators we have a similar
exception (although I don't recall whether we actually warn) -- if you
call close() and it tries to yield another value it is just GC'ed
without giving the frame more control. For an async generator there
are two cases: either it tries to yield another value (the first time
this happens you can throw an error back into it) or it tries to await
-- in that case you can also throw an error back into it, and if the
error comes out unhandled you can print the error (in both cases
actually).
It's probably to specify all this behavior using some kind of default
finalizer (though you don't have to implement it that way).
Does a default finalizer solve the "event loop does not know its AGs"
problem?
Hopefully there will be other discussion as well, otherwise I'll have
to accept the PEP once this issue is cleared up. :-)
--
--Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
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