Marker version comparison fails when the marker variable is on the RHS of a term.

[jsirois]

As far as I can tell, the markers spec¹ does not limit the position of marker variables in term expressions and they are allowed to appear on the right hand side. It's true the examples in the spec exclusively use the LHS, but the very explicit grammar²³ write up doubles down on allowing marker variables on either side of a term.

For example, although normally spelled python_version >= '3.9', you should be able to say '3.9' <= python_version to the same end. Now you ~can run that example, but an issue similar to #633 is hidden. I expose the issue in the == '3.13.*' example below though:

:; pex packaging
Pex 2.55.2 ephemeral hermetic environment with 1 requirement and 1 activated distribution.
Python 3.13.7 (main, Aug 15 2025, 11:40:50) [GCC 14.2.0] on linux
Type "help", "pex", "copyright", "credits" or "license" for more information.
>>> pex()
Running from ephemeral loose PEX directory: /tmp/tmpixmwdi3m
Requirements:
  packaging
Activated Distributions:
  packaging-25.0-py3-none-any.whl
>>> from packaging.markers import Marker
>>> Marker("python_version >= '3.9'").evaluate()
True
>>> Marker("'3.9' <= python_version").evaluate()
True
>>> Marker("python_full_version == '3.13.*'").evaluate()
True
>>> Marker("'3.13.*' == python_full_version").evaluate()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/jsirois/.cache/pex/installed_wheels/0/29572ef2b1f17581046b3a2227d5c611fb25ec70ca1ba8554b24b0e69331a484/packaging-25.0-py3-none-any.whl/packaging/markers.py", line 347, in evaluate
    return _evaluate_markers(
        self._markers, _repair_python_full_version(current_environment)
    )
  File "/home/jsirois/.cache/pex/installed_wheels/0/29572ef2b1f17581046b3a2227d5c611fb25ec70ca1ba8554b24b0e69331a484/packaging-25.0-py3-none-any.whl/packaging/markers.py", line 239, in _evaluate_markers
    groups[-1].append(_eval_op(lhs_value, op, rhs_value))
                      ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jsirois/.cache/pex/installed_wheels/0/29572ef2b1f17581046b3a2227d5c611fb25ec70ca1ba8554b24b0e69331a484/packaging-25.0-py3-none-any.whl/packaging/markers.py", line 187, in _eval_op
    return spec.contains(lhs, prereleases=True)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jsirois/.cache/pex/installed_wheels/0/29572ef2b1f17581046b3a2227d5c611fb25ec70ca1ba8554b24b0e69331a484/packaging-25.0-py3-none-any.whl/packaging/specifiers.py", line 552, in contains
    normalized_item = _coerce_version(item)
  File "/home/jsirois/.cache/pex/installed_wheels/0/29572ef2b1f17581046b3a2227d5c611fb25ec70ca1ba8554b24b0e69331a484/packaging-25.0-py3-none-any.whl/packaging/specifiers.py", line 28, in _coerce_version
    version = Version(version)
  File "/home/jsirois/.cache/pex/installed_wheels/0/29572ef2b1f17581046b3a2227d5c611fb25ec70ca1ba8554b24b0e69331a484/packaging-25.0-py3-none-any.whl/packaging/version.py", line 202, in __init__
    raise InvalidVersion(f"Invalid version: {version!r}")
packaging.version.InvalidVersion: Invalid version: '3.13.*'
>>>

Unless I've misread the spec, this seems like a bug; otherwise, the spec has a bug. I personally think the spec has a bug, allowing more flexibility than needed; it should just mandate marker variables can only appear on the LHS of terms.

Footnotes

1. https://packaging.python.org/en/latest/specifications/dependency-specifiers/#environment-markers ↩

2. https://packaging.python.org/en/latest/specifications/dependency-specifiers/#grammar ↩

3. https://packaging.python.org/en/latest/specifications/dependency-specifiers/#complete-grammar ↩