[Async-sig] async/sync library reusage

[Pau Freixes]

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 at bendarnell.com> wrote:
> On Fri, Jun 9, 2017 at 11:51 AM Cory Benfield <cory at 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
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>
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-- 
--pau