108 lines
3.8 KiB
ReStructuredText
108 lines
3.8 KiB
ReStructuredText
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: Draft
|
||
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:
|
||
|
||
|
||
|
||
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already been superseded by alternatives.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 optimisations 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>`_.
|
||
|
||
Since this field is private and makes no guarantees on backwards compatibility,
|
||
this PEP proposes to repeal :pep:`509` immediately within the next CPython
|
||
version. A function for fast cached dictionary lookups may be provided as an
|
||
alternative feature to replace directly accessing ``ma_version_tag``.
|
||
|
||
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.
|
||
|
||
|
||
Special Thanks
|
||
==============
|
||
|
||
Thanks to C.A.M. Gerlach for edits and wording changes to this document.
|
||
|
||
|
||
Copyright
|
||
=========
|
||
|
||
This document is placed in the public domain or under the
|
||
CC0-1.0-Universal license, whichever is more permissive.
|
||
|
||
|