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Wed Aug 30 10:36:20 EDT 2017 · https://mail.python.org/pipermail/python-dev/2017-August/149086.html

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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[Python-Dev] PEP 550 v4