I think we're getting way beyond the rationale Pau Freixes requested... On Mon, Jun 12, 2017 at 12:05 PM, Andrew Svetlov <andrew.svetlov@gmail.com> wrote:
Agree in general but current asyncio still may shoot your leg. The solution (at least for my unittest example) might be in adding top level functions for running asyncio code (asyncio.run() and asyncio.run_forever() as Yury Selivanov proposed in https://github.com/python/asyncio/pull/465) After this we could raise a warning in `asyncio.get_event_loop()` if the loop was not set explicitly by `asyncio.set_event_loop()`.
On Mon, Jun 12, 2017 at 9:50 PM Guido van Rossum <guido@python.org> wrote:
Honestly I think we're in agreement. There's never a use for one loop running while another is the default. There are some rare use cases for multiple loops running but before the mentioned commit it was up to the app to ensure to switch the default loop when running a loop. The commit took the ability to screw up there out of the user's hand.
On Mon, Jun 12, 2017 at 9:57 AM, Andrew Svetlov <andrew.svetlov@gmail.com
wrote:
Yes, but with one exception: default event loop created on module import stage might co-exist with a loop created for test. It leads to mystic hangs, you know. Please recall code like: class A: mongodb = motor.motor_asyncio.AsyncIOMotorClient()
On Mon, Jun 12, 2017 at 7:37 PM Guido van Rossum <guido@python.org> wrote:
Yes, but not co-existing, I hope!
On Mon, Jun 12, 2017 at 9:25 AM, Andrew Svetlov < andrew.svetlov@gmail.com> wrote:
Unit tests at least. Running every test in own loop is crucial fro tests isolation.
On Mon, Jun 12, 2017 at 7:04 PM Guido van Rossum <guido@python.org> wrote:
Multiple loops in the same thread is purely theoretical -- the API allows it but there's no use case. It might be necessary if a platform has a UI-only event loop that cannot be extended to do I/O -- the only solution to do background I/O might be to alternate between two loops. (Though in that case I would still prefer a thread for the background I/O.)
On Mon, Jun 12, 2017 at 8:49 AM, Pau Freixes <pfreixes@gmail.com> wrote:
> And what about the rationale of having multiple loop instances in > the same thread switching btw them. Im still trying to find out what > patterns need this... Do you have an example? > > Btw thanks for the first explanation > > El 12/06/2017 17:36, "Guido van Rossum" <guido@python.org> escribió: > >> In theory it's possible to create two event loops (using >> new_event_loop()), then set one as the default event loop (using >> set_event_loop()), then run the other one (using run_forever() or >> run_until_complete()). To tasks running in the latter event loop, >> get_event_loop() would nevertheless return the former. >> >> On Mon, Jun 12, 2017 at 4:39 AM, Pau Freixes <pfreixes@gmail.com> >> wrote: >> >>> Sorry a bit of topic, but I would like to figure out why older >>> python >>> versions, prior this commit [1], the get_event_loop is not >>> considered >>> deterministic >>> >>> does anybody know the reason behind this change? >>> >>> >>> [1] https://github.com/python/cpython/commit/ >>> 600a349781bfa0a8239e1cb95fac29c7c4a3302e >>> >>> On Fri, Jun 9, 2017 at 6:07 PM, Ben Darnell <ben@bendarnell.com> >>> wrote: >>> > On Fri, Jun 9, 2017 at 11:51 AM Cory Benfield <cory@lukasa.co.uk> >>> wrote: >>> >> >>> >> >>> >> >>> >> My concern with multiple loops boils down to the fact that >>> urllib3 >>> >> supports being used in a multithreaded context where each >>> thread can >>> >> independently make forward progress on one request. To >>> establish that with a >>> >> synchronous codebase you either need one event loop per thread >>> or you need >>> >> to spawn a background thread on startup that owns the only >>> event loop in the >>> >> process. >>> > >>> > >>> > Yeah, one event loop per thread is probably the way to go for >>> integration >>> > with synchronous codebases. A dedicated event loop thread may >>> perform better >>> > but libraries that spawn threads are problematic. >>> > >>> >> >>> >> >>> >> Generally speaking I’ve not had positive results with libraries >>> spawning >>> >> their own threads in Python. In my experience this has tended >>> to lead to >>> >> programs that deadlock mysteriously or that fail to terminate >>> in the face of >>> >> a Ctrl+C. So I tend to prefer to have users spawn their own >>> threads, which >>> >> would make me want a “one-event-loop-per-thread” model: hence, >>> needing a >>> >> loop parameter to pass around prior to 3.6. >>> > >>> > >>> > You can avoid the loop parameter on older versions of asyncio >>> (at least as >>> > long as the default event loop policy is used) by manually >>> setting your >>> > event loop as current before calling run_until_complete (and >>> resetting it >>> > afterwards). >>> > >>> > Tornado's run_sync() method is equivalent to asyncio's >>> run_until_complete(), >>> > and Tornado supports multiple IOLoops in this way. We use this >>> to expose a >>> > synchronous version of our AsyncHTTPClient: >>> > https://github.com/tornadoweb/tornado/blob/ >>> 62e47215ce12aee83f951758c96775a43e80475b/tornado/httpclient.py#L54 >>> > >>> > -Ben >>> > >>> >> >>> >> >>> >> I admit that my concerns here regarding libraries spawning >>> their own >>> >> threads may be overblown: after my series of negative >>> experiences I >>> >> basically never went back to that model, and it may be that the >>> problems >>> >> were more user-error than anything else. However, I feel >>> comfortable saying >>> >> that libraries spawning their own Python threads is definitely >>> subtle and >>> >> hard to get right, at the very least. >>> >> >>> >> Cory >>> >> _______________________________________________ >>> >> Async-sig mailing list >>> >> Async-sig@python.org >>> >> https://mail.python.org/mailman/listinfo/async-sig >>> >> Code of Conduct: https://www.python.org/psf/codeofconduct/ >>> > >>> > >>> > _______________________________________________ >>> > Async-sig mailing list >>> > Async-sig@python.org >>> > https://mail.python.org/mailman/listinfo/async-sig >>> > Code of Conduct: https://www.python.org/psf/codeofconduct/ >>> > >>> >>> >>> >>> -- >>> --pau >>> _______________________________________________ >>> Async-sig mailing list >>> Async-sig@python.org >>> https://mail.python.org/mailman/listinfo/async-sig >>> Code of Conduct: https://www.python.org/psf/codeofconduct/ >>> >> >> >> >> -- >> --Guido van Rossum (python.org/~guido) >> >
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-- Thanks, Andrew Svetlov
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-- Thanks, Andrew Svetlov
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-- Thanks, Andrew Svetlov
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