Track Python 3.15 prerelease Polars runtime compatibility

[stefan-jansen]

The informational Python 3.15 matrix fails on Linux, macOS, and Windows with CPython 3.15.0rc1.

Evidence: https://github.com/ml4t/data/actions/runs/31316629578

• Installation, package import, and ty pass.
• The prerelease environment resolves polars==1.43.2 and polars-runtime-32==1.43.2.
• Basic Polars DataFrame and Series construction fails inside polars._utils.construction.series.sequence_to_pyseries because the constructed series is None (AttributeError: NoneType object has no attribute _s).
• The same failure occurs on all three operating systems and causes broad downstream test failures.
• All supported Python 3.12 through 3.14 cells pass on all three operating systems.

Python 3.15 is not currently advertised as supported, so this remains non-blocking. Recheck against newer Polars/runtime releases, update the prerelease dependency set when upstream support is available, and make the 3.15 matrix blocking when Python 3.15 becomes supported.

[stefan-jansen]

Maintainer update for 2026-08-11:

• The failure remains upstream-owned and reproducible across Linux, macOS, and Windows.
• fix(python): Support Python 3.15 expression dispatch pola-rs/polars#28750 is still open and blocked from merge as of this review.
• PyPI still reports Polars 1.43.2 as the latest release, so no released artifact contains the accepted Python 3.15 expression-dispatch fix.
• Data main passes the required Python 3.12-3.14 matrix and enforces the approved python-315-polars exception with Requires-Python >=3.12,<3.15.
• The exception remains version-bounded and expires on 2026-09-30. It has not been broadened or renewed.

Next review: 2026-08-18, or immediately after a Polars release containing the upstream fix. Acceptance still requires the complete Python 3.15 matrix to pass on all three operating systems before removing the exception.