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
ncoghlan
commented
Jan 10, 2026
ncoghlan
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Jan 25, 2026
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| For ``Set of String`` fields, as there is no marker syntax for set literals, | ||
| the only valid operations are ``in`` and ``not in`` comparisons with a user | ||
| supplied string literal as the left operand. |
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Publishing tools should emit an error for other operations, consuming tools may just treat the operations as false
| defined as they are for ``String`` fields. However, there is no expectation | ||
| that the parsing of the marker field value or the user supplied constant as a | ||
| valid version will succeed, so tools MUST fall back to processing the field as | ||
| a ``String`` field. Alternatively, tools MAY unconditionally treat such fields |
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The established use of platform_release as a version on macOS means that this field really does need to be checked as a version first.
Due to iOS and Android, platform_version potentially needs the same handling.
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| References to unknown marker fields MUST raise an error rather than resulting | ||
| in a comparison that evaluates to True or False. |
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Explain that this so unknown markers result in a clear installation failure rather than an installation with potentially missing dependencies that then fails at runtime. Soften it to a SHOULD.
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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
discuss.python.org thread: https://discuss.python.org/t/spec-change-bugfix-dependency-specifiers-simplification-pep-508
📚 Documentation preview 📚: https://python-packaging-user-guide--1988.org.readthedocs.build/en/1988/