[Python-Dev] PEP 567 v3

[Yury Selivanov]

On Wed, Jan 17, 2018 at 2:24 PM, Guido van Rossum <gvanrossum at gmail.com> wrote:
> Perhaps you can update the PEP with a summary of the rejected ideas from
> this thread?

The Rejected Ideas section of the PEP is now updated with the below:

Token.reset() instead of ContextVar.reset()
-------------------------------------------

Nathaniel Smith suggested to implement the ``ContextVar.reset()``
method directly on the ``Token`` class, so instead of::

    token = var.set(value)
    # ...
    var.reset(token)

we would write::

    token = var.set(value)
    # ...
    token.reset()

Having ``Token.reset()`` would make it impossible for a user to
attempt to reset a variable with a token object created by another
variable.

This proposal was rejected for the reason of ``ContextVar.reset()``
being clearer to the human reader of the code which variable is
being reset.


Make Context objects picklable
------------------------------

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.

Given the time frame of the Python 3.7 release schedule it was decided
to defer this proposal to Python 3.8.


Yury