ruff: Python Linter & Formatter

ruff is three tools in one Rust binary: linter (ruff check), formatter (ruff format), and dependency analyzer (ruff analyze graph). It replaces Flake8, Black, isort, pyupgrade, and dozens more.

Snapshot: v0.15.12, Apr 2026. The installed binary outranks this file. Check ruff --version and ruff check --help when a flag or rule code here doesn't land.

The built-in language server (ruff server) replaces the deprecated ruff-lsp package (archived Dec 2025).

Invocation

uv run ruff ...     # Project dependency (pinned version)
uvx ruff ...        # One-off (latest)
ruff ...            # Global install

Rule Selection: The Critical Decision

Default rules are minimal: only ["E4", "E7", "E9", "F"], catches syntax errors and undefined names but misses most quality rules. You almost certainly need to extend this.