PEP: 685
Title: Comparison of extra names for optional distribution dependencies
Author: Brett Cannon <brett@python.org>
PEP-Delegate: Paul Moore <p.f.moore@gmail.com>
Discussions-To: https://discuss.python.org/t/14141
Status: Final
Type: Standards Track
Topic: Packaging
Created: 08-Mar-2022
Post-History: `08-Mar-2022 <https://discuss.python.org/t/14141>`__
Resolution: https://discuss.python.org/t/pep-685-comparison-of-extra-names-for-optional-distribution-dependencies/14141/55


.. canonical-pypa-spec::


Abstract
========

This PEP specifies how to normalize `distribution extra <Provides-Extra_>`_
names when performing comparisons.
This prevents tools from either failing to find an extra name, or
accidentally matching against an unexpected name.


Motivation
==========

The `Provides-Extra`_ core metadata specification states that an extra's
name "must be a valid Python identifier".
:pep:`508` specifies that the value of an ``extra`` marker may contain a
letter, digit, or any one of ``.``, ``-``, or ``_`` after the initial character.
There is no other `PyPA specification
<https://packaging.python.org/en/latest/specifications/>`_
which outlines how extra names should be written or normalized for comparison.
Due to the amount of packaging-related code in existence,
it is important to evaluate current practices by the community and
standardize on one that doesn't break most existing code, while being
something tool authors can agree to following.

The issue of there being no consistent standard was brought forward by an
`initial discussion <https://discuss.python.org/t/7614>`__
noting that the extra ``adhoc-ssl`` was not considered equal to the name
``adhoc_ssl`` by pip 22.


Rationale
=========

:pep:`503` specifies how to normalize distribution names::

    re.sub(r"[-_.]+", "-", name).lower()

This collapses any run of the characters ``-``, ``_`` and ``.``
down to a single ``-``.
For example, ``---`` ``.`` and ``__`` all get converted to just ``-``.
This does **not** produce a valid Python identifier, per
the core metadata 2.2 specification for extra names.

`Setuptools 60 performs normalization <https://github.com/pypa/setuptools/blob/b2f7b8f92725c63b164d5776f85e67cc560def4e/pkg_resources/__init__.py#L1324-L1330>`__
via::

    re.sub(r'[^A-Za-z0-9-.]+', '_', name).lower()

The use of an underscore/``_`` differs from PEP 503's use of a hyphen/``-``,
and it also normalizes characters outside of those allowed by :pep:`508`.
Runs of ``.`` and ``-``, unlike PEP 503, do **not** get normalized to one ``_``,
e.g. ``..`` stays the same. To note, this is inconsistent with this function's
docstring, which *does* specify that all non-alphanumeric characters
(which would include ``-`` and ``.``) are normalized and collapsed.

For pip 22, its
"extra normalisation behaviour is quite convoluted and erratic" [pip-erratic]_
and so its use is not considered.

.. [pip-erratic] Tzu-ping Chung on Python Discourse <https://discuss.python.org/t/7614/10


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

When comparing extra names, tools MUST normalize the names being compared
using the semantics outlined in :pep:`PEP 503 <0503#normalized-names>`
for names::

    re.sub(r"[-_.]+", "-", name).lower()

The `core metadata`_ specification will be updated such that the allowed
names for `Provides-Extra`_ matches what :pep:`508` specifies for names.
This will bring extra naming in line with that of the Name_ field.
Because this changes what is considered valid, it will lead to a core
metadata version increase to ``2.3``.

For tools writing `core metadata`_,
they MUST write out extra names in their normalized form.
This applies to the `Provides-Extra`_ field and the
:pep:`extra marker <0508#extras>` when used in the `Requires-Dist`_ field.

Tools generating metadata MUST raise an error if a user specified
two or more extra names which would normalize to the same name.
Tools generating metadata MUST raise an error if an invalid extra
name is provided as appropriate for the specified core metadata version.
If a project's metadata specifies an older core metadata version and
the name would be invalid with newer core metadata versions,
tools reading that metadata SHOULD warn the user.
Tools SHOULD warn users when an invalid extra name is read and SHOULD
ignore the name to avoid ambiguity.
Tools MAY raise an error instead of a warning when reading an
invalid name, if they so desire.


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

Moving to :pep:`503` normalization and :pep:`508` name acceptance
allows for all preexisting, valid names to continue to be valid.

Based on research looking at a collection of wheels on PyPI [pypi-results]_,
the risk of extra name clashes is limited to 73 instances when considering
all extras names on PyPI, valid or not (not just those within a single package)
while *only* looking at valid names leads to only 3 clashes:

* ``dev-test``: ``dev_test``, ``dev-test``, ``dev.test``
* ``dev-lint``: ``dev-lint``, ``dev.lint``, ``dev_lint``
* ``apache-beam``: ``apache-beam``, ``apache.beam``

By requiring tools writing core metadata to only record the normalized name,
the issue of preexisting, invalid extra names should diminish over time.

.. [pypi-results] Paul Moore on Python Discourse https://discuss.python.org/t/14141/17


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

It is possible that for a distribution that has conflicting extra names, a
tool ends up installing dependencies that somehow weaken the security
of the system.
This is only hypothetical and if it were to occur,
it would probably be more of a security concern for the distributions
specifying such extras names rather than the distribution that pulled
them in together.


How to Teach This
=================

This should be transparent to users on a day-to-day basis.
It will be up to tools to educate/stop users when they select extra
names which conflict.


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

No reference implementation is provided aside from the code above,
but the expectation is the `packaging project`_ will provide a
function in its ``packaging.utils`` module that will implement extra name
normalization.
It will also implement extra name comparisons appropriately.
Finally, if the project ever gains the ability to write out metadata,
it will also implement this PEP.


Transition Plan
===============

There is a risk that a build tool will produce core metadata
conforming to version 2.3 and thus this PEP but which is consumed by a
tool that is unaware of this PEP (if that tool chooses to attempt to
read a core metadata version it does not directly support).
In such a case there is a chance that a user may specify an extra
using an non-normalized name which worked previously but which fails
now.

As such, consumers of this PEP should be prioritized more than
producers so that users can be notified that they are specifying extra
names which are not normalized (and thus may break in the future).


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

Using setuptools 60's normalization
-----------------------------------

Initially, this PEP proposed using setuptools ``safe_extra()`` for normalization
to try to minimize backwards-compatibility issues.
However, after checking various wheels on PyPI,
it became clear that standardizing **all** naming on :pep:`508` and
:pep:`503` semantics was easier and better long-term,
while causing minimal backwards compatibility issues.


Open Issues
===========

N/A


Copyright
=========

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


.. _core metadata: https://packaging.python.org/en/latest/specifications/core-metadata/
.. _Name: https://packaging.python.org/en/latest/specifications/core-metadata/#name
.. _packaging project: https://packaging.pypa.io
.. _Provides-Extra: https://packaging.python.org/en/latest/specifications/core-metadata/#provides-extra-multiple-use
.. _Requires-Dist: https://packaging.python.org/en/latest/specifications/core-metadata/#requires-dist-multiple-use
