[Async-sig] question re: loop.shutdown_asyncgens()

[Yury Selivanov]

One of the design decisions about `loop.close()` is that it doesn't
do a single event loop iteration, making its behaviour highly predictable.
To make `loop.close()` to run `loop.shutdown_asyncgens()` (which is a
coroutine), we would have needed to change that.

One of the ways we want to mitigate this problem in Python 3.7 is to
add a new function to bootstrap asyncio and run top-level coroutines:
`asyncio.run()`.  You can read more about it here: [1].

I'm working on a new PEP that will summarize asyncio changes in 3.7.
I don't have a concrete ETA for it, but I'll try to get the first draft out
by mid September.

[1] https://github.com/python/asyncio/pull/465

Thanks,
Yury

On Jul 27, 2017, 11:24 PM -0400, Chris Jerdonek <chris.jerdonek at gmail.com>, wrote:
> I have a question about PEP 525 (Asynchronous Generators) which I'm
> sure has a simple answer, but I didn't see it in the PEP or final
> discussion:
> https://mail.python.org/pipermail/python-dev/2016-September/146265.html
>
> Basically, why is the API such that loop.shutdown_asyncgens() must be
> called manually? For example, why can't it be called automatically as
> part of close(), which seems like it would be a friendlier API and
> more helpful to the common case?
>
> I was trying asynchronous iterators in my code and getting the following error:
>
> Exception ignored in: <generator object Queue.get at 0x7f950a667678
> Traceback (most recent call last):
> File "/usr/local/lib/python3.6/asyncio/queues.py", line 169, in get
> getter.cancel() # Just in case getter is not done yet.
> File "/usr/local/lib/python3.6/asyncio/base_events.py", line
> 574, in call_soon
> self._check_closed()
> File "/usr/local/lib/python3.6/asyncio/base_events.py", line
> 357, in _check_closed
> raise RuntimeError('Event loop is closed')
> RuntimeError: Event loop is closed
>
> Calling loop.shutdown_asyncgens() made the error go away, but it seems
> a little obscure that by adding an asynchronous iterator somewhere in
> your code, you have to remember to check that that line is present
> before loop.close() is called (and the exception message doesn't
> provide a good hint).
>
> Is there any disadvantage to always calling loop.shutdown_asyncgens()
> (i.e. even if it's not needed)? And why might someone need to call it
> at a different time?
>
> Thanks,
> --Chris
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