[Python-ideas] concurrency-local namespaces

[Guido van Rossum]

On Wed, Apr 22, 2015 at 11:12 AM, Eric Snow <ericsnowcurrently at gmail.com>
wrote:

> On Wed, Apr 22, 2015 at 11:46 AM, Guido van Rossum <guido at python.org>
> wrote:
> > Hey Nathaniel, can you bring this up in the PEP 492 thread? It sounds
> like
> > it would be super great if the async with statement would (optionally?)
> > notify the context manager of switching to a different task and back.
>
> Interesting idea.  Keep in mind that decimal.localcontext() makes use
> of thread-local namespaces.  So we may still be better off if we can
> adapt that concept to coroutines.  If I've understood correctly,
> Tasks.current_task() should give us what we need.  I'm just not clear
> on a couple things.  Where does the loop comes from?  Can more than
> one loop be running at the same time?  I probably need to go dig into
> the asyncio docs.
>

Usually there's only one loop, and you get it using
asyncio.get_event_loop(). A loop is tied to a thread, so that function gets
the current thread's loop. You could have another loop running in another
thread, but it requires an explicit choice to set that up (by default you
only get a loop for the main thread). Each loop will manages a completely
distinct set of tasks (and it would require some dedication and ill will to
mix up tasks between loops).

You'd probably need to either have a separate asyncio-aware context for
decimal, or you'd have to add asyncio-awareness to the existing context
class. Both seem reasonable options; the latter seems a bit more
user-friendly since users can convert existing thread-based code to
coroutines without having to worry about decimal context managers across
I/O waits.

-- 
--Guido van Rossum (python.org/~guido)
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