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Document arbitrary equality character restrictions in dependency specifiers #1445

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29 changes: 29 additions & 0 deletions docs/specifiers.rst
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Expand Up @@ -85,6 +85,35 @@ rejected; call :meth:`~packaging.specifiers.SpecifierSet.to_range` on both
sides for those.


Arbitrary equality
------------------

The ``===`` operator performs case-insensitive string comparison without
version normalization. It is intended as an escape hatch for legacy versions
that do not follow the `Version Specifiers Specification`_. Prefer standard
version comparisons, such as ``==``, when possible.

When using arbitrary equality in dependency specifiers, use only ASCII
letters, digits, ``-``, ``_``, ``.``, ``*``, ``+``, and ``!`` in the version
string. These are the characters allowed by the `dependency specifier
grammar`_; other characters may not work, even if a standalone
:class:`~packaging.specifiers.Specifier` accepts them.

In particular, a comma separates specifiers when constructing a
:class:`~packaging.specifiers.SpecifierSet` from a string. It is not part of
an arbitrary equality version in that form:

.. doctest::

>>> from packaging.specifiers import Specifier, SpecifierSet
>>> Specifier("===legacy,").version
'legacy,'
>>> next(iter(SpecifierSet("===legacy,"))).version
'legacy'

.. _dependency specifier grammar: https://packaging.python.org/en/latest/specifications/dependency-specifiers/#grammar


Reference
---------

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