[Python-Dev] PEP 567 v3 (original) (raw)

Nathaniel Smith njs at pobox.com
Thu Jan 18 03:40:39 EST 2018

On Thu, Jan 18, 2018 at 12:03 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

On Wed, 17 Jan 2018 20:53:42 -0500 Yury Selivanov <yselivanov.ml at gmail.com> wrote:

Proposed by Antoine Pitrou, this could enable transparent cross-process use of Context objects, so the Offloading execution to other threads example would work with a ProcessPoolExecutor too. Enabling this is problematic because of the following reasons: 1. ContextVar objects do not have _module_ and _qualname_ attributes, making straightforward pickling of Context objects impossible. This is solvable by modifying the API to either auto detect the module where a context variable is defined, or by adding a new keyword-only "module" parameter to ContextVar constructor. 2. Not all context variables refer to picklable objects. Making a ContextVar picklable must be an opt-in. This is a red herring. If a value isn't picklable, pickle will simply raise as it does in other contexts. You should't need to opt in for anything here.

The complication is that Contexts collect ContextVars from all over the process. So if people are going to pickle Contexts, we need some mechanism to make sure that we don't end up in a situation where it seems to work and users depend on it, and then they import a new library and suddenly pickling raises an error (because the new library internally uses a ContextVar that happens not to be pickleable).

-n

-- Nathaniel J. Smith -- https://vorpus.org

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