Track Python 3.15 prerelease Polars runtime compatibility

[stefan-jansen]

Data declares Requires-Python: >=3.12,<3.15 under the ecosystem Python 3.15 waiting policy.
Python 3.12 through 3.14 remain the supported release range.

A fresh CPython 3.15.0rc1 probe using the current locked core dependency set reproduces the
required Polars failure: DataFrame construction raises AttributeError: NoneType object has no attribute _s before Data code can process the result. A stable-resolution probe can also stop
before the suite when an indirect dependency has no CPython 3.15 wheel. Neither condition is a
repository-owned defect that can be removed without reducing Data's supported behavior.

Run the visible, non-blocking Python 3.15 canary at least monthly. Recheck immediately when Python
3.15 final ships or when Polars publishes a stable release after 1.44.2. Removing the upper bound
requires the complete suite, package build, and installed-wheel import to pass on Linux, macOS, and
Windows.

This is an external-dependency waiting record, not a blocker for an otherwise qualified 0.1 patch
release. Tracked by ml4t/ecosystem#2 and Decision 0013.

[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.

[stefan-jansen]

Maintainer recheck on 2026-09-19:

• PyPI now reports Polars 1.44.2 as the latest stable release.
• A fresh CPython 3.15.0rc1 run still fails in the Polars construction path: 83 tests passed, 3 failed, and 17 errored, with the same AttributeError: 'NoneType' object has no attribute '_s' signature.
• The stable Pydantic resolver path also does not yet provide the required CPython 3.15 support.
• Python 3.12 through 3.14 remain the supported range, so the existing bounded exception remains appropriate and must not be widened.

This stays open. Recheck immediately after a Polars release containing pola-rs/polars#28750, or by 2026-09-26 ahead of the 2026-09-30 exception expiry. Closure still requires the complete Python 3.15 matrix on Linux, macOS, and Windows.

[stefan-jansen]

Decision 0013 in ecosystem pull request ml4t/ecosystem#41 replaces the September 30 expiry with a
version-scoped prerelease wait for the Data 0.1 release line.

Data owns the runtime evidence. Recheck when Python 3.15 final ships or when Polars publishes a
release after 1.44.2. Until the complete suite passes on Linux, macOS, and Windows, Data keeps its
truthful <3.15 upper bound and runs a visible non-blocking canary at least monthly. Python 3.12
through 3.14 qualification remains blocking; Python 3.15 readiness does not block an otherwise
qualified patch release.

[stefan-jansen]

Fresh closure-audit evidence on 2026-09-20:

• Supported Python 3.12-3.14 remains green locally: 3,649 tests pass in both the standard and ResourceWarning-as-error lanes.
• A CPython 3.15.0rc1 source probe using the locked core dependency set reproduces the accepted Polars failure: 4 failures and 6 setup errors in 66 focused tests, all with AttributeError: NoneType object has no attribute _s during DataFrame construction.
• Resolving current stable core constraints dynamically stops earlier because stable Pydantic still selects pydantic-core==2.46.5, which has no CPython 3.15 wheel.
• Pull request ci: add monthly Python 3.15 dependency canary #59 adds the required monthly, Ubuntu-only, non-blocking canary. It is deliberately absent from pull-request triggers and supported-version release gates.

The issue remains open. Recheck on Python 3.15 final or after a stable Polars release newer than 1.44.2, then run the full Linux, macOS, and Windows qualification before changing Requires-Python.