[Python-ideas] Letting context managers react to yields inside their scope

[Yury Selivanov]

Hi Nathanial,

Sorry for not replying to your last email promptly.
FWIW I was going to do it later today ;)

My opinion on this subject (and I've implemented lots
of local-context kind of objects in different frameworks) is
that *just* inserting some kind of suspend/resume points
before/after yields does not work.

Why:

1. Greenlets, gevent, eventlet, stackless python: they do
not have 'yield'. Context switches are invisible to the
interpreter.  And those frameworks are popular.  You want
numpy.errstate/decimal.localcontext to work there too.

2. I'm curious how this will be implemented and what's
the performance impact.

3. I think that mechanism should be more generic than
just 'with' staments. What if you want to have a decorator
that applies some context? What if you can't write it as
a generator with @contextlib.contextmanager?

What I would propose:

I think that the right approach here would be to have a
standard protocol; something similar to how we defined
WSGI -- it doesn't require any changes in the interpreter,
it is a protocol.

For instance, we could have a module in the stdlib, with a
class Context:

    class Context:
       @classmethod
       def __suspend_contexts(cls):
          for child in cls.__subclasses__():
             child.__suspend_context()

       @classmethod
def __resume_contexts(cls): ..


To inform context objects that the context is about to change,
asyncio event loop would call Context.__suspend_contexts()

I think that it's up to the event loop/library/framework
to manage context switches properly.  In asyncio, for instance,
you can attach callbacks to Futures. I think it would be
great if such callbacks are executed in the right context.

I would also suggest to think about a universal context --
the one that works both for asyncio coroutines and
threadpools they might use.

This way any framework can implement the context in the
most efficient way.

All in all, I'm in favor of API and a couple functions
added to the stdlib for this.

Thanks,
Yury

On 2015-04-29 4:22 PM, Nathaniel Smith wrote:
> Hi all,
>
> Well, after a few days no-one has responded to my post on another
> thread about this [1], but the more I thought about it the more this
> seemed like a good idea, so I wrote up a little more-formal proposal
> (attached) for letting context managers react to 'yield's that occur
> within their 'with' block.
>
> This should in many ways be treated as a straw man proposal -- there's
> tons I don't know about how async code is written in Python these days
> -- but it seems like a good idea to me and I'd like to hear what
> everyone else thinks :-).
>
> -n
>
> [1] https://mail.python.org/pipermail/python-ideas/2015-April/033176.html
>
>
>
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