python-code-style

name

python-code-style

description

Owns Python code style for this stack: ruff for lint + format, numpydoc for docstrings. Two responsibilities — (1) place the project's `ruff.toml` from the bundled template once the stack and workspace are in place, and (2) run ruff against any Python files Claude has just generated or edited. Stops at "the touched files pass `ruff check`." TRIGGER when (any of these): (1) a Python file was just created or edited via Write / Edit / MultiEdit — invoke this skill before declaring the task done so ruff is run on the touched files; (2) a fresh ML workspace was just scaffolded by `organize-ml-workspace` and the project has no `ruff.toml` at its root yet — drop the bundled template; (3) the user asks about lint, format, docstring style, or reaches for `black` / `isort` / `flake8` / `pydocstyle` (redirect to ruff — the stack's canonical linter, owned by `data-science-python-stack` Tier 1). SKIP when: the project is non-Python; the only edits in this turn are to Markdown / TOML / JSON / YAML; the file lives in a third-party vendored directory the user doesn't own. HOW TO USE: run ruff manually on the files you just touched — do not configure a PostToolUse hook for this. **Read the "Stop conditions" block and emit the Pre-flight checklist as visible text in your response — both are mandatory before running ruff.**

Python Code Style

Single owner of "what does well-styled Python look like in this stack": ruff (lint + format) and numpydoc docstrings. This skill is explicitly manual — Claude runs ruff on the files it has just touched, no hook involved.

Stop conditions — read before anything else

• Do not configure a PostToolUse / PreToolUse hook for ruff. This skill is intentionally manual. A hook tightens the loop in ways that bite (every micro-edit triggers a fix cycle, partial files fail D-rule checks mid-write, retries can stall the turn). If the user explicitly asks for an automated hook later, redirect to update-config — but the default is "Claude runs ruff itself."
• Do not substitute ruff with black / isort / flake8 / pydocstyle / pylint. Ruff is the canonical linter in this stack (data-science-python-stack Tier 1). If import ruff / pixi run ruff --version fails, route through python-env-manager to install — don't silently fall back.
• One fix attempt per file, then surface. If ruff check reports issues after Claude's first fix, address them once. If the same issue persists after the second pass, stop editing that file and surface the remaining diagnostics + diff to the user. This is the anti-infinite-loop guardrail — do not enter a third cycle on the same warning.
• Don't lint files outside the user's code. The hook scope is src/<pkg>/, experiments/, top-level *.py scripts, and any package directory the user owns. Skip vendored paths, generated files, and anything under .pixi/, .venv/, node_modules/, etc.

Pre-flight — emit this checklist as visible text before running ruff

Pre-flight (python-code-style):
- [ ] ruff importable in the project's env (`pixi run ruff --version`
      succeeds, per `data-science-python-stack` Tier 1)
- [ ] `ruff.toml` present at project root — if absent and the stack
      + workspace are already set up, drop `templates/ruff.toml`
      from this skill into the root before running ruff
- [ ] File list ready: <abs paths of .py files touched this turn>
- [ ] Decision recorded: this is the first ruff pass on these files
      (proceed) | second pass (proceed but stop on persistent
      issues) | third pass on same warning (STOP, surface to user)

Scope

• In scope: running ruff format + ruff check --fix + ruff check on Python files Claude has just generated or edited; authoring numpydoc docstrings on public functions and classes; dropping the ruff.toml template into a fresh project.
• Out of scope: type hints (mypy / pyright are not in the stack); naming conventions ruff doesn't enforce; setting up PostToolUse / PreToolUse hooks; linting non-Python files.

What to run, in what order

For every Python file touched this turn (call them <files>), run inside the project's environment manager — pixi run for pixi projects, equivalent for uv / poetry / conda (per python-env-manager):

pixi run ruff format <files>
pixi run ruff check --fix <files>
pixi run ruff check <files>

Three steps, in order:

1. ruff format — applies the formatter (line length, quoting, trailing commas, blank lines around defs). Idempotent.
2. ruff check --fix — auto-fixes everything ruff knows how to fix in place: import sorting (I), legacy syntax (UP), detectable bug patterns (B).
3. ruff check (no --fix) — final pass. Anything reported here needs Claude's attention: missing docstrings (D), undefined names (F), code structure issues. Address them, then re-run the trio. Apply the one-fix-per-file rule from Stop conditions.

If a file under experiments/ has a D100 ("missing module docstring") or D103 ("missing function docstring") warning, that's expected for # %% cells; the bundled ruff.toml ignores those for experiments/**. If you're seeing them, the ruff.toml isn't loaded — check that it lives at the project root.

Numpydoc — the docstring convention

Public functions and classes carry numpydoc-format docstrings; ruff's D rules with pydocstyle.convention = "numpy" enforce the shape. Skeleton:

def predict_price(X, model, *, n_jobs=1):
    """Predict option prices from a feature matrix.

    Parameters
    ----------
    X : pandas.DataFrame
        Feature matrix with one row per option.
    model : sklearn.base.BaseEstimator
        Fitted estimator with a ``predict`` method.
    n_jobs : int, default=1
        Number of parallel jobs.

    Returns
    -------
    numpy.ndarray of shape (n_samples,)
        Predicted prices, one per row of ``X``.
    """

Conventions worth surfacing because they're non-obvious:

• One-line summary on the first line, in the imperative mood ("Predict ..." not "Predicts ..."). No trailing period in the summary line if D400 is enabled — but in numpy convention it is, so write the period.
• Blank line between summary and the rest.
• Parameter shapes go in the type slot, not the description, e.g. X : ndarray of shape (n_samples, n_features).
• Returns section lists the return value; if there are multiple returns, list each on its own row. Don't omit the type.
• Private helpers (_leading_underscore) don't need a docstring under the default D rules — ruff allows that.
• Modules (top of file) should start with a one-line summary. Skipped under experiments/** per the bundled ruff.toml.

Initial setup — dropping the ruff.toml template

When this skill is invoked on a fresh project that has no ruff.toml at its root and the stack + workspace have been scaffolded by their respective skills:

1. Read templates/ruff.toml from this skill (path: .claude/skills/python-code-style/templates/ruff.toml).
2. Write it verbatim to <project-root>/ruff.toml.
3. Verify ruff picks it up: pixi run ruff check --show-settings . should report the numpy convention and the select list from the template.

Do not fold ruff config into pyproject.toml automatically — the project may not have one, or the user may prefer a separate file. The standalone ruff.toml is unambiguous.

When ruff finds something Claude didn't write

A common case: Claude edits one function in a file that already had unrelated D-rule violations. Ruff will report those too.

• In scope of this turn: the lines Claude touched. Fix those.
• Out of scope: pre-existing warnings in untouched code. Mention them in the response so the user can choose to address them, but don't drag every warning into the current task.

This keeps PR scope tight and avoids "while I was here" expansion that the user didn't ask for.

Companion skills

• data-science-python-stack — owns the decision that ruff is Tier 1 mandatory; this skill assumes ruff is already installed. If pixi run ruff --version fails, return there for the install.
• python-env-manager — turns "ruff is missing" into the right install command for the project's manager. Don't run pip install ruff in a pixi project.
• organize-ml-workspace — sets up the directory layout that the bundled ruff.toml's per-file ignores (experiments/**) reference. Drop the template after this skill has run, so the paths it ignores actually exist.
• update-config — only relevant if the user explicitly asks for an automated lint hook later. Default is no hook.

Conventions

• Manual, not automatic. Claude calls ruff itself, file by file. No hook.
• One-fix-per-file rule. Hard cap on retries — second pass max, then surface.
• Project-root config only. ruff.toml lives at the project root and is the single source of truth. Don't add per-directory overrides unless the user asks.
• Don't widen scope on touched files. Fix what Claude wrote; surface (don't auto-fix) pre-existing issues elsewhere.