python-peps/peps/pep-0699.rst

PEP: 699
Title: Remove private dict version field added in PEP 509
Author: Ken Jin <kenjin@python.org>
Discussions-To: https://discuss.python.org/t/pep-699-remove-private-dict-version-field-added-in-pep-509/19724
Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 03-Oct-2022
Python-Version: 3.12
Post-History: `05-Oct-2022 <https://discuss.python.org/t/pep-699-remove-private-dict-version-field-added-in-pep-509/19724>`__
Replaces: 509
Resolution: https://discuss.python.org/t/pep-699-remove-private-dict-version-field-added-in-pep-509/19724/13



Abstract
========

:pep:`509` introduced a private ``ma_version_tag`` field for dictionaries to
allow optimizations in CPython and extension libraries. This PEP proposes to
rescind :pep:`509` and declare the field an implementation detail, as it has
already been superseded by alternatives. This will further allow the field to
be reused for future optimization.


Motivation
==========

:pep:`509` introduced the ``ma_version_tag`` field to dictionaries. This 64-bit
field stored a version counter that updates every time a dictionary was
modified. The original proposal was to use this version counter as a
guard for optimizations.

Since CPython 3.11, this field has become unused by internal optimization
efforts. :pep:`659`-specialized instructions use other methods of verifying
that certain optimizations are safe.

To enable further optimizations in CPython, this PEP proposes that the
``ma_version_tag`` field no longer conform to the :pep:`509` specification.
This will allow the CPython developers to store other optimization information,
such as dictionary write watchers.


Rationale
=========
This PEP does not specify what the field may be used for in the future. This is
intentional, as implementation details are subject to change, and the field
should be used only for internal consumption by CPython.


Specification
=============

This specification rescinds that in :pep:`509`. The ``ma_version_tag`` field of
the Python :class:`dict` class is declared to be an internal implementation
detail and may be removed altogether, or may have a different representation.
C extensions should not rely on this field.


Backwards Compatibility
=======================

Certain extensions use ``ma_version_tag`` for fast dictionary or globals
lookups. For example,
`Cython uses the field for fast dynamic module variable lookups <https://github.com/cython/cython/blob/169876872f3cb6198971a1db07e5b8a9d12b3dac/Cython/Utility/ObjectHandling.c#L1556>`_.

This PEP proposes to emit a compiler warning when accessing ``ma_version_tag``.
After two consecutive version releases with warnings, ``ma_version_tag``
will be removed, in line with :pep:`387`.

The biggest user the author could find for this field was Cython.
Discussions with a Cython maintainer indicated that
`removing support for it from Cython is trivial <https://github.com/faster-cpython/ideas/issues/461#issuecomment-1250358596>`_.


Security Implications
=====================

:pep:`509` was concerned with integer overflow. However, this PEP does not
introduce any additional security concerns.


Rejected Ideas
==============

A possible alternative is to preserve the field for backwards compatibility.
This PEP rejects that alternative as future optimizations will consume more
memory, and the field was always considered private and undocumented aside
from the PEP, with no backward compatibility guarantees. Dictionaries in Python
are commonly used, and any increase in their memory consumption will adversely
affect Python’s performance.


Reference Implementation
========================

A reference implementation can be found in
`python/cpython#101193 <https://github.com/python/cpython/pull/101193>`_.


Special Thanks
==============

Thanks to C.A.M. Gerlach for edits and wording changes to this document.
Thanks also to Mark Shannon and Kumar Aditya for providing possible
implementations.

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.