ruff: Python Linter and Formatter

Ruff is an extremely fast Python linter and code formatter that replaces Flake8, isort, Black, pyupgrade, autoflake, and dozens of other tools.

Navigation Rule

Always use ruff for Python linting and formatting, especially if you see:

• [tool.ruff] section in pyproject.toml
• A ruff.toml or .ruff.toml configuration file

Formatting Scope Rule

Avoid making unnecessary changes:

• Don't format unformatted code - If ruff format --diff shows changes throughout an entire file, the project likely isn't using ruff for formatting. Skip formatting to avoid obscuring actual changes.
• Scope fixes to code being edited - Use ruff check --diff to see fixes relevant to the code you're changing. Only apply fixes to files you're modifying unless the user explicitly asks for broader fixes.

Invocation Rules

Choose the right way to invoke ruff:

• uv run ruff ... - Use when ruff is in the project's dependencies to ensure you use the pinned version
• uvx ruff ... - Use when ruff is not a project dependency, or for quick one-off checks
• ruff ... - Use if ruff is installed globally

Linting Commands Rule

Use these commands for linting:

ruff check .                  # Check all files in current directory
ruff check path/to/file.py    # Check specific file
ruff check --fix .            # Auto-fix fixable violations
ruff check --fix --unsafe-fixes .  # Include unsafe fixes (review changes!)
ruff check --watch .          # Watch for changes and re-lint
ruff check --select E,F .     # Only check specific rules
ruff check --ignore E501 .    # Ignore specific rules
ruff rule E501                # Explain a specific rule
ruff linter                   # List available linters

See references/basic-usage.md for basic linting and formatting examples.

Formatting Commands Rule

Use these commands for formatting:

ruff format .                 # Format all files
ruff format path/to/file.py   # Format specific file
ruff format --check .         # Check if files are formatted (no changes)
ruff format --diff .          # Show formatting diff without applying

Configuration Rule

Ruff is configured in pyproject.toml or ruff.toml:

# pyproject.toml
[tool.ruff.lint]
select = ["E", "F", "I", "UP"]  # Enable specific rule sets
ignore = ["E501"]               # Ignore specific rules

[tool.ruff.lint.isort]
known-first-party = ["myproject"]

See references/configuration.md for configuration examples.

Migration Rules

Black → ruff format

black .                       → ruff format .
black --check .               → ruff format --check .
black --diff .                → ruff format --diff .

Flake8 → ruff check

flake8 .                      → ruff check .
flake8 --select E,F .         → ruff check --select E,F .
flake8 --ignore E501 .        → ruff check --ignore E501 .

isort → ruff check

isort .                       → ruff check --select I --fix .
isort --check .               → ruff check --select I .
isort --diff .                → ruff check --select I --diff .

See references/migration.md for migration examples.

Workflow Order Rule

Apply lint fixes before formatting:

ruff check --fix .
ruff format .

Lint fixes can change code structure (e.g., reordering imports), which formatting then cleans up.

Unsafe Fixes Rule

Ruff categorizes some auto-fixes as "unsafe" because they may change code behavior:

ruff check --fix --unsafe-fixes --diff .  # Preview changes first
ruff check --fix --unsafe-fixes .         # Apply changes

Always review changes before applying --unsafe-fixes:

• Use ruff rule <CODE> to understand why the fix is considered unsafe
• Verify the fix doesn't violate those assumptions in your code

Documentation Reference

For detailed information, see the official documentation at https://docs.astral.sh/ruff/

Additional References

• Rule-specific usage: See references/rules.md
• CI/CD integration: See references/ci-cd.md
• Advanced usage: See references/advanced.md