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

[Chris Jerdonek]

Thanks, Yury. Have you also considered including recommended setup /
cleanup boilerplate in a place where it's easy for asyncio users to find,
like in the asyncio docs here?
https://docs.python.org/3/library/asyncio-eventloop.html#run-an-event-loop

One example of a Python module using this approach is itertools:
https://docs.python.org/3/library/itertools.html#itertools-recipes

Currently, even the example snippet provided for loop.shutdown_asyncgens():
https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.AbstractEventLoop.shutdown_asyncgens
is incomplete because it doesn't execute shutdown_asyncgens() in a
try-finally like you do in your latest patch posted on PR #465.

Also, even if run() is added to Python 3.7, Python 3.6 users would still
need / benefit from being able to find blessed boilerplate in a central
place.

Thanks,
--Chris


On Thu, Jul 27, 2017 at 8:40 PM, Yury Selivanov <yselivanov at gmail.com>
wrote:

> 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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