Ruff

Why it matters

Before Ruff, "run the linter" was a coffee-break activity on large codebases. Ruff made linting and formatting effectively instantaneous — and in doing so changed behavior: when the check takes 50ms, developers run it on every save instead of in CI. Speed didn't just improve the tool; it improved the habit.

Engineering notes

• 800+ rules, one binary. Implements the rule sets of a dozen legacy tools behind a familiar config surface, so adoption doesn't require relearning.
• Formatter + linter that agree. Black-compatible formatting plus import sorting plus linting in a single pass — no more tools fighting over the same file.
• LSP-native. The language server brings the same speed into the editor, with fix-on-save that developers actually leave enabled.

Limitations

• Not every exotic Flake8 plugin has an equivalent rule; plugin authors are still catching up.
• Auto-fix is powerful but occasionally opinionated — review large ruff check --fix diffs before committing.