Attempt to clarify environment marker evaluation #1988
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Preparation for the release of packaging 25.1 revealed multiple deficiencies in the specification of environment marker evaluation. Review of the proposed amendments to resolve those deficiencies highlighted multiple other problems, including some dating from the original PEP 508 specification: * other pages still referencing PEP 508 instead of the living spec * direct reference to PEP 685 instead of the core metadata spec * the "extra" special case not being properly defined * lacking guidance to tool developers regarding what should be considered errors to disallow entirely vs issues to work around Inspired by the initial PR at pypa#1971
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ncoghlan
commented
Jan 10, 2026
| emit an error if the user supplied constant cannot be parsed as a valid version | ||
| specifier, while locking and installation tools MAY either emit an error or else | ||
| fall back to ``String`` field comparison logic if either the marker field value | ||
| or the user supplied constant cannot be parsed as a valid version specifier. |
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Need to note that in and not in aren't valid for version fields.
ncoghlan
commented
Jan 10, 2026
| precedence (with all comparison operations having a higher precedence). | ||
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| Python's comparison chaining (such as ``3.4 < python_version < 3.9``) is NOT | ||
| supported in environment markers. |
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Add a couple of examples here, such as checking for SMP in the platform description after constricting the platform to Linux, or checking for a minimum macOS version.
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Preparation for the release of packaging 25.1 revealed multiple deficiencies in the specification of environment marker evaluation. Review of the proposed amendments to resolve those deficiencies highlighted multiple other problems, including some dating from the original PEP 508 specification:
Inspired by the initial PR at #1971
📚 Documentation preview 📚: https://python-packaging-user-guide--1988.org.readthedocs.build/en/1988/