fix python constraint derived from markers to better support pre-release versions of Python

[radoering]

https://github.com/python-poetry/poetry/actions/runs/17171517004/job/48721759731?pr=10514 (failure in tests/utils/test_isolated_build.py::test_isolated_env_install_discards_requirements_not_needed_by_env with Python 3.14rc2) shows that our installer does not handle environments of Python pre-release versions well. A dependency with the marker python_version == "3.14" is not installed because it is translated into a Python constraint >=3.14,<3.15, which does not allow Python 3.14rc2. With this PR the Python constraint is >=3.14.dev0,<3.15.

Downstream tests are fixed in python-poetry/poetry#10515

• Added tests for changed code.
• Updated documentation for changed code.

Summary by Sourcery

Improve marker-to-constraint translation to properly support Python pre-release versions by introducing .dev0 bounds and wildcard expansions for partial-precision markers

New Features:

• Support pre-release Python versions in marker-derived constraints by using .dev0 for lower bounds

Enhancements:

• Normalize Python version markers to expand ==x and !=x to x.* and append .dev0 to >=x bounds when precision is less than two segments
• Treat marker-based >= constraints as stable versions in the version parser for correct handling of dev releases

Tests:

• Add tests for parsing and formatting of .dev0-based version constraints and update existing expectations to match new wildcard and dev0 conventions

[sourcery-ai]

Reviewer's Guide

Refactor normalization and parsing of Python version markers to handle pre-release versions by appending '.dev0' for '>=' constraints lacking full precision, replacing tilde-based wildcards with '.*', and ensuring marker-based '>=' operations use the stable version. The test suite is updated throughout to align expectations with these behaviors.

Class diagram for updated Python version marker normalization

classDiagram
    class Utils {
        +normalize_python_version_markers()
    }
    class Parser {
        +parse_single_constraint(op, version, is_marker_constraint)
    }
    Utils --> Parser: uses

    %% Highlight changes
    Utils : + Handles '>=X.Y' by appending '.dev0'
    Utils : + Replaces '~X.Y' with 'X.Y.*' for '=='
    Parser : + For marker-based '>=', uses stable version

Loading

File-Level Changes

Change	Details	Files
Refactor python version marker normalization to better support pre-release constraints	Switch wildcard expansion for '==' markers from '~X.Y' to 'X.Y.*' Append '.dev0' to versions for '>=' markers with only major.minor precision	src/poetry/core/packages/utils/utils.py
Ensure marker-based '>=' constraints use the stable version in parsing	Detect marker context in parse_single_constraint and set version to its stable form before building the VersionRange	src/poetry/core/constraints/version/parser.py
Update test suite to expect new wildcard and '.dev0' behaviors	Add parameterized tests for parsing '.dev0' constraints and special marker cases Replace all '~X.Y' expectations with 'X.Y.*' in test outputs Update expected python_versions to include '.dev0' for minimal '>=' constraints	tests/constraints/version/test_parse_constraint.py tests/packages/utils/test_utils.py tests/test_factory.py tests/packages/test_main.py tests/packages/test_dependency.py

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• The new .stable conversion in parse_single_constraint only applies to '>=' — consider applying a similar stable adjustment for '<=' or '<' marker constraints to correctly handle upper bounds with pre-releases.
• It could be useful to add a test for explicit prerelease markers (e.g. '==3.14rc1') to verify whether parse_marker_version_constraint accepts or rejects them as intended.
• Consider extracting the '.dev0' bump logic into a shared helper to reduce duplication between normalize_python_version_markers and the parser.

Prompt for AI Agents

Please address the comments from this code review:
## Overall Comments
- The new .stable conversion in parse_single_constraint only applies to '>=' — consider applying a similar stable adjustment for '<=' or '<' marker constraints to correctly handle upper bounds with pre-releases.
- It could be useful to add a test for explicit prerelease markers (e.g. '==3.14rc1') to verify whether parse_marker_version_constraint accepts or rejects them as intended.
- Consider extracting the '.dev0' bump logic into a shared helper to reduce duplication between normalize_python_version_markers and the parser.

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[dimbleby]

Discussion here

Tldr the spec is unhelpful, insofar as there is a consensus it probably is that poetry's current behaviour is correct - but the change you are making is desirable and the spec ought to be changed

However I see no such change has happened.

[radoering]

Thanks for the link and the summary.

I intended a more subtle change to only derive correct constraints from markers but the poetry-plugin-export failures in python-poetry/poetry#10515 seem to show that this is not possible because it results in inconsistencies. I may try another approach to fix the original issue.