[Python-Dev] PEP 550 v3

[Guido van Rossum]

On Mon, Aug 21, 2017 at 10:09 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> My latest suggestion to Yury was to see how the PEP reads with it
> called ImplicitContext, such that:
>
> * the active execution context is a stack of implicit contexts
> * ContextKey.set() updates the innermost implicit context
> * Contextkey.get() reads the whole stack of active implicit contexts
> * by default, generators (both sync and async) would have their own
> implicit context, but you could make them use the context of method
> callers by doing "gen.__implicit_context__ = None"
> * by default, coroutines would use their method caller's context, but
> async frameworks would make sure to give top-level tasks their own
> independent contexts
>
> That proposal came from an initial attempt at redrafting the Abstract
> and Rationale sections, where it turns out that one of the things the
> current version of the PEP is somewhat taking for granted is that the
> reader already has a particular understanding of the difference
> between explicit state management (i.e. passing things around as
> function arguments and instance attributes) and implicit state
> management (i.e. relying on process globals and thread locals).
>

I think I like ImplicitContext. Maybe we can go with this as a working
title at least.

I think we should also rethink the form the key framework-facing APIs will
take, and how they are presented in the PEP -- I am now leaning towards
explaining this from the start as an immutable linked list of immutable
mappings, where the OS-thread state gets updated to a new linked list when
it is changed (either by ContextKey.set or by the various stack
manipulations). I think this falls under several Zen-of-Python points:
EIBTI, and "If the implementation is easy to explain, it may be a good
idea."

-- 
--Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
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