ruff
Ruff is an extremely fast Python linter and code formatter. It replaces Flake8,
isort, Black, pyupgrade, autoflake, and dozens of other tools.
When to use ruff
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
However, 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.
How to invoke ruff
uv run ruff ... - Use when ruff is in the project's dependencies to ensure
you use the pinned versionuvx ruff ... - Use when ruff is not a project dependency, or for quick
one-off checksruff ... - Use if ruff is installed globally
Commands
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
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
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"]
Migrating from other tools
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 .
Common patterns
Apply lint fixes before formatting
Run ruff check --fix before ruff format. Lint fixes can change code
structure (e.g., reordering imports), which formatting then cleans up.
ruff check --fix .
ruff format .
Applying and reviewing unsafe fixes
Ruff categorizes some auto-fixes as "unsafe" because they may change code
behavior, not just style. For example, removing unused imports could break code
that relies on side effects.
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
For detailed information, read the official documentation: