[Python-Dev] PEP 550 v4

[Kevin Conway]

> Can Execution Context be implemented outside of CPython

I know I'm well late to the game and a bit dense, but where in the pep is
the justification for this assertion? I ask because we buy something to
solve the same problem in Twisted some time ago:
https://bitbucket.org/hipchat/txlocal . We were able to leverage
generator/coroutine decorators to preserve state without modifying the
runtime.

Given that this problem only exists in runtime that multiplex coroutines on
a single thread and the fact that coroutine execution engines only exist in
user space, why doesn't it make more sense to leave this to a library that
engines like asyncio and Twisted are responsible for standardising on?

On Wed, Aug 30, 2017, 09:40 Yury Selivanov <yselivanov.ml at gmail.com> wrote:

> On Wed, Aug 30, 2017 at 9:44 AM, Yury Selivanov <yselivanov.ml at gmail.com>
> wrote:
> [..]
> >> FYI, I've been sketching an alternative solution that addresses these
> kinds
> >> of things. I've been hesitant to post about it, partly because of the
> >> PEP550-based workarounds that Nick, Nathaniel, Yury etc. have been
> >> describing, and partly because that might be a major distraction from
> other
> >> useful discussions, especially because I wasn't completely sure yet
> about
> >> whether my approach has some fatal flaw compared to PEP 550 ;).
> >
> > We'll never know until you post it. Go ahead.
>
> The only alternative design that I considered for PEP 550 and
> ultimately rejected was to have a the following thread-specific
> mapping:
>
>   {
>      var1: [stack of values for var1],
>      var2: [stack of values for var2]
>   }
>
> So the idea is that when we set a value for the variable in some
> frame, we push it to its stack.  When the frame is done, we pop it.
> This is a classic approach (called Shallow Binding) to implement
> dynamic scope.  The fatal flow that made me to reject this approach
> was the CM protocol (__enter__).  Specifically, context managers need
> to be able to control values in outer frames, and this is where this
> approach becomes super messy.
>
> Yury
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