[albertvillanova]

This PR documents in the contributing guide when TRL adds a Python version, when it drops one, and which version CI and the Docker images run by default.

Fix #7742.

Motivation

These decisions have been made case by case. The default Python for CI moved to 3.12 in #2298 and to 3.13 in #7648, each time on a different argument, and the timing of a drop depends on an end-of-life date that the CPython schedule only gives as a month. #7742 proposes the criteria; this PR writes them down so any maintainer can apply them the same way each October.

Solution

The criteria follow what TRL already does:

• Python 3.9 and 3.10 were dropped right after their final releases (🐍 Drop Python 3.9 #4183 and Drop Python 3.10 support #7497), so a version is dropped once its final release is out.
• The default is the version most TRL users run. In September 2026, 3.13 accounted for 49% of TRL downloads, 3.12 for 35%, 3.11 for 8% and 3.14 for 2%, which is what made 3.13 the default in Use Python 3.13 by default in CI #7648.
• The minimum-versions job is the exception to the default: it runs on the oldest supported Python, so its pins are exactly the floors (Run tests_min_versions on Python 3.11 and pin every dependency at its floor #7741).

Changes

• Add a "Python versions" section to the contributing guide, next to the dependency version floors, covering adding, dropping and the default version

---

Note

Low Risk
Documentation-only change to CONTRIBUTING.md with no runtime or CI behavior changes.

Overview
Adds a Python versions subsection to CONTRIBUTING.md, next to dependency version floors, so maintainers have written criteria instead of deciding ad hoc each October.

The section ties TRL to the CPython 5-year support cycle and spells out when to add a version (wheels from PyTorch/core deps, tests green, update pyproject.toml classifiers and the nightly matrix in .github/workflows/tests_python_versions.yml), when to drop one (after the final EOL release, bump the minimum-versions job to the new oldest), and how to pick the default for PR CI, docs, releases, and Docker (most-used version per pypistats, reviewed yearly; minimum-versions stays on oldest supported Python).

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[bot-ci-comment]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[qgallouedec]

pypistats

ah nice, I used our own telemetry for #7648 but happy to see they are in line

[qgallouedec]

Thanks! If the min-versions job moves to 3.11 (see #7744), the default-version bullet should say that job runs on the oldest supported Python.

[albertvillanova]

Done: the default-version bullet now says the minimum-versions job runs on the oldest supported Python, and the dropping bullet says to move that job to the new oldest version when a version is dropped.