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.
