PEP 641: Using an underscore in the version portion of Python 3.10 compatibility tags (#1667)

pep-0641.rst

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+PEP: 641
+Title: Using an underscore in the version portion of Python 3.10 compatibility tags
+Author: Brett Cannon <brett@python.org>,
+        Steve Dower <steve.dower@python.org>,
+        Barry Warsaw <barry@python.org>
+BDFL-Delegate:
+Discussions-To:
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 2020-10-20
+Python-Version: 3.10
+Post-History: 2020-10-20
+Resolution:
+
+Abstract
+========
+
+Using the tag system outlined in :pep:`425` (primarily used for wheel
+file names), each release of Python specifies compatibility tags
+(e.g. ``cp39``, ``py39`` for CPython 3.9). For CPython 3.10, this PEP
+proposes using ``3_10`` as the version portion of the tags
+(instead of ``310``).
+
+
+Motivation
+==========
+
+Up to this point, the version portion of compatibility tags used in
+e.g. wheel file names has been a straight concatenation of the major
+and minor versions of Python, both for the CPython interpreter tag and
+the generic, interpreter-agnostic interpreter tag (e.g. ``cp39`` and
+``py39``, respectively). This also applies to the ABI tag
+(e.g. ``cp39``). Thanks to both the major and minor versions being
+single digits, it has been unambiguous what which digit in e.g. ``39``
+represented.
+
+But starting with Python 3.10, ambiguity comes up as ``310`` does not
+clearly delineate whether the Python version is ``3.10``, ``31.0``, or
+``310`` as the major-only version of Python. Thus using ``3_10`` to
+separate major/minor portions as allowed by :pep:`425` disambiguates
+the Python version being supported.
+
+
+Rationale
+=========
+
+Using ``3_10`` instead of another proposed separator is a restriction
+of :pep:`425`, thus the only options are ``3_10`` or ``310``.
+
+
+Specification
+=============
+
+The ``SOABI`` configure variable and
+``sysconfig.get_config_var('py_version_nodot')`` will be updated to
+use ``3_10`` appropriately.
+
+
+Backwards Compatibility
+=======================
+
+Tools relying on the 'packaging' project [2]_ already expect a
+version specification of ``3_10`` for Python 3.10. Keeping the version
+specifier as ``310`` would require backing that change out and
+updating dependent projects (e.g. pip).
+
+Switching to ``3_10`` will impact any tools that implicitly rely on
+the convention that the minor version is a single digit. However,
+these are broken regardless of any change here.
+
+For tools assuming the major version is only the first digit, they
+will require updating if we switch to ``3_10``.
+
+In non-locale ASCII, ``_`` sorts after any digit, so lexicographic
+sorting matching a sort by Python version of a wheel file name will be
+kept.
+
+Since PEP 515 (Python 3.6), underscores in numeric literals are ignored.
+This means that ``int("3_10")`` and ``int("310")`` produce the same result,
+and ordering based on conversion to an integer will be preserved.
+**However**, this is still a bad way to sort tags, and the point is raised
+here simply to show that this proposal does not make things worse.
+
+Security Implications
+=====================
+
+There are no known security concerns.
+
+
+How to Teach This
+=================
+
+As use of the interpreter tag is mostly machine-based and this PEP
+disambiguates, there should not be any special teaching consideration
+required.
+
+
+Reference Implementation
+========================
+
+A pull request [1]_ already exists adding support to CPython 3.10.
+Support for reading wheel files with this proposed PEP is already
+implemented.
+
+
+Rejected Ideas
+==============
+
+Not making the change
+---------------------
+It was considered to not change the tag and stay with ``310``. The
+argument was it's less work and it won't break any existing
+tooling. But in the end it was thought that the disambiguation is
+better to have.
+
+
+Open Issues
+===========
+
+How far should we take this?
+----------------------------
+Other places where the major and minor version are used could be
+updated to use an underscore as well (e.g. ``.pyc`` files, the import
+path to the zip file for the stdlib). It is not known how useful it
+would be to make this pervasive.
+
+
+References
+==========
+
+.. [1] Reference implementation
+   (https://github.com/python/cpython/pull/20333)
+
+.. [2] The 'packaging' project
+   (https://pypi.org/project/packaging/)
+
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.
+
+
+
+..
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