Ruff

Ruff Skill

This skill provides expertise in using Ruff, an extremely fast Python linter and code formatter written in Rust. Ruff combines the functionality of multiple tools (Flake8, isort, Black, and more) into a single, high-performance package.

IMPORTANT: Proactive Usage

You MUST use this skill proactively in the following scenarios:

1. After writing or modifying Python code - Always run ruff format and ruff check --fix after creating or editing Python files
2. Before committing code - Ensure all Python code passes linting and formatting checks
3. When encountering linting errors - Immediately fix them using this skill's guidance
4. During code reviews - Check code quality before finalizing changes

Default workflow when writing Python code:

# Step 1: Format the code
uv run ruff format .

# Step 2: Fix auto-fixable linting issues
uv run ruff check --fix .

# Step 3: Report any remaining issues
uv run ruff check .

Use the project's just commands when available:

# Format Python code (uses uv run ruff format)
just fmt-python

# Lint Python code (uses uv run ruff check)
just lint-python

About Ruff

Ruff is designed to replace multiple Python development tools with a single, fast, and comprehensive solution. It's 10-100x faster than traditional Python linters while providing comparable or better functionality.

Key Capabilities

• Linting: Check Python code against 800+ lint rules from popular tools
• Formatting: Format Python code with Black-compatible output
• Auto-fixing: Automatically fix many linting issues
• Import Sorting: Organize imports (isort-compatible)
• Configuration: Highly customizable via pyproject.toml or ruff.toml
• Fast: Written in Rust for exceptional performance
• All-in-one: Replaces Flake8, Black, isort, pydocstyle, pyupgrade, and more

When to Use This Skill

Use this skill when users:

• Need to lint or format Python code
• Want to enforce Python code quality standards (PEP 8, etc.)
• Need to identify bugs, security issues, or code smells
• Want to automatically fix common Python issues
• Need to format and organize imports
• Want to modernize Python code (e.g., upgrade syntax)
• Need to integrate linting/formatting into CI/CD pipelines
• Ask about Python code quality best practices
• Want to configure custom linting or formatting rules
• Need faster alternatives to Flake8, Black, or isort

How to Use This Skill

Basic Ruff Workflow

Ruff has two main commands:

1. ruff check: Lint Python code (finds issues)
2. ruff format: Format Python code (fixes style)

Linting with ruff check

Basic Linting

Check a single file:

uv run ruff check script.py

Check a directory:

uv run ruff check src/
uv run ruff check .

Check multiple paths:

uv run ruff check src/ tests/ script.py

Auto-fixing Issues

Fix issues automatically where possible:

uv run ruff check --fix script.py
uv run ruff check --fix src/

The --fix option will modify files in place to resolve fixable issues.

Unsafe Fixes

Some fixes are considered "unsafe" because they may change code behavior. To apply these:

uv run ruff check --fix --unsafe-fixes script.py

Always review unsafe fixes carefully before committing.

Show Fixes Without Applying

Preview what fixes would be applied:

uv run ruff check --diff script.py

Statistics and Reporting

Show statistics about issues found:

uv run ruff check --statistics src/

Output in different formats:

uv run ruff check --output-format=json src/
uv run ruff check --output-format=github src/  # For GitHub Actions
uv run ruff check --output-format=gitlab src/  # For GitLab CI
uv run ruff check --output-format=junit src/   # For CI/CD tools

Rule Selection

Select specific rules or categories:

# Check only unused imports
uv run ruff check --select F401 src/

# Check multiple rule categories
uv run ruff check --select E,F,W src/

# Ignore specific rules
uv run ruff check --ignore E501 src/

# Extend existing configuration
uv run ruff check --extend-select B src/

Common rule prefixes:

• F: Pyflakes (errors, undefined names, unused imports)
• E/W: pycodestyle errors and warnings (PEP 8)
• C90: mccabe (complexity)
• I: isort (import sorting)
• N: pep8-naming
• UP: pyupgrade (modernize syntax)
• B: flake8-bugbear (likely bugs)
• S: flake8-bandit (security)
• A: flake8-builtins
• Q: flake8-quotes
• SIM: flake8-simplify

Watch Mode

Continuously watch for changes and re-lint:

uv run ruff check --watch src/

Formatting with ruff format

Basic Formatting

Format a single file:

uv run ruff format script.py

Format a directory:

uv run ruff format src/
uv run ruff format .

Check Format Without Modifying

Check if files need formatting:

uv run ruff format --check src/

This is useful in CI/CD to verify code is formatted without modifying files.

Show Formatting Differences

Show what changes would be made:

uv run ruff format --diff src/

Format stdin

Format code from standard input:

echo "x=1" | uv run ruff format -
cat script.py | uv run ruff format -

Configuration

Configuration Files

Ruff looks for configuration in the following order:

1. ruff.toml
2. .ruff.toml
3. pyproject.toml (under [tool.ruff])

Basic Configuration (pyproject.toml)

[tool.ruff]
# Set the maximum line length to 100
line-length = 100

# Assume Python 3.11
target-version = "py311"

# Enable additional rule sets
select = [
    "E",    # pycodestyle errors
    "W",    # pycodestyle warnings
    "F",    # Pyflakes
    "I",    # isort
    "B",    # flake8-bugbear
    "UP",   # pyupgrade
    "S",    # flake8-bandit
    "SIM",  # flake8-simplify
]

# Ignore specific rules
ignore = [
    "E501",  # Line too long (handled by formatter)
]

# Exclude directories
exclude = [
    ".git",
    ".venv",
    "__pycache__",
    "build",
    "dist",
]

# Allow autofix for all enabled rules
fixable = ["ALL"]
unfixable = []

[tool.ruff.per-file-ignores]
# Ignore specific rules in specific files
"tests/**/*.py" = ["S101"]  # Allow assert in tests
"__init__.py" = ["F401"]    # Allow unused imports

[tool.ruff.format]
# Use single quotes for strings
quote-style = "single"

# Indent with spaces (not tabs)
indent-style = "space"

# End files with a newline
skip-magic-trailing-comma = false
line-ending = "auto"

[tool.ruff.isort]
# Configure import sorting
known-first-party = ["myapp"]

Standalone ruff.toml

line-length = 100
target-version = "py311"

select = ["E", "F", "I", "B", "UP"]
ignore = ["E501"]

[format]
quote-style = "double"
indent-style = "space"

Command-Line Configuration

Override configuration from command line:

# Set line length
uv run ruff check --line-length=120 src/

# Select specific rules
uv run ruff check --select=E,F,I src/

# Set target Python version
uv run ruff check --target-version=py39 src/

Common Workflows

Full Code Quality Check

Run both linting and formatting checks:

# Check formatting
uv run ruff format --check .

# Run linter
uv run ruff check .

Auto-fix Everything

Fix and format all code:

# Fix linting issues
uv run ruff check --fix .

# Format code
uv run ruff format .

Pre-commit Integration

Add to .pre-commit-config.yaml:

repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.7.0
    hooks:
      # Run the linter
      - id: ruff
        args: [--fix]
      # Run the formatter
      - id: ruff-format

CI/CD Integration

GitHub Actions example:

- name: Ruff Linter
  run: uv run ruff check --output-format=github .

- name: Ruff Formatter
  run: uv run ruff format --check .

Migration from Other Tools

Replace existing tools with Ruff:

From Black + isort + Flake8:

# Old workflow
black .
isort .
flake8 .

# New workflow
uv run ruff format .
uv run ruff check --select I --fix .  # isort
uv run ruff check .                    # flake8

Configuration Migration:

# Convert Black settings
[tool.ruff]
line-length = 88  # Black default

[tool.ruff.format]
quote-style = "double"  # Black default

# Convert isort settings
[tool.ruff.isort]
known-first-party = ["myapp"]
force-single-line = false

# Convert Flake8 settings
[tool.ruff]
select = ["E", "F", "W", "C90"]  # Flake8 defaults
ignore = ["E203", "E501", "W503"]  # Common ignores

Gradual Adoption

Introduce Ruff gradually to existing projects:

# Start with just errors and critical issues
uv run ruff check --select=E,F .

# Add more rules progressively
uv run ruff check --select=E,F,B .
uv run ruff check --select=E,F,B,UP .

# Finally add formatting
uv run ruff format .

Use --add-noqa to add # noqa comments for existing violations:

uv run ruff check --add-noqa src/

This allows you to enforce rules for new code while temporarily allowing existing violations.

Fix Specific Issue Types

Fix only certain categories of issues:

# Fix only import sorting
uv run ruff check --select I --fix .

# Fix only unused imports and variables
uv run ruff check --select F401,F841 --fix .

# Apply pyupgrade modernizations
uv run ruff check --select UP --fix .

Rule Categories

Essential Rules (Start Here)

[tool.ruff]
select = [
    "F",   # Pyflakes - errors and undefined names
    "E",   # pycodestyle errors - PEP 8 violations
    "I",   # isort - import sorting
]

Recommended Rules (Add Next)

[tool.ruff]
select = [
    "F", "E", "I",
    "W",   # pycodestyle warnings
    "UP",  # pyupgrade - modernize syntax
    "B",   # bugbear - likely bugs
    "SIM", # simplify - simplification suggestions
]

Advanced Rules (Optional)

[tool.ruff]
select = [
    "F", "E", "I", "W", "UP", "B", "SIM",
    "S",     # bandit - security issues
    "N",     # pep8-naming - naming conventions
    "C90",   # mccabe - complexity checking
    "A",     # flake8-builtins - shadowing builtins
    "Q",     # flake8-quotes - quote consistency
    "RET",   # flake8-return - return statement issues
    "ARG",   # flake8-unused-arguments
    "PTH",   # flake8-use-pathlib - use pathlib instead of os.path
    "PD",    # pandas-vet - pandas best practices
]

Inline Configuration

Disable Rules for Lines

Use # noqa comments to disable rules:

# Ignore all rules for this line
import os  # noqa

# Ignore specific rule
import os  # noqa: F401

# Ignore multiple rules
x = 1  # noqa: E701, E702

# Ignore for entire file (at top)
# ruff: noqa

# Disable specific rule for entire file
# ruff: noqa: F401

Per-file Ignores in Config

[tool.ruff.per-file-ignores]
"tests/*.py" = ["S101", "PLR2004"]
"__init__.py" = ["F401"]
"scripts/*.py" = ["T201"]  # Allow print statements

Troubleshooting

Too Many Issues

If you're overwhelmed by issues on an existing project:

1. Start with critical issues only:

   uv run ruff check --select=F,E .
   

2. Add noqa comments to existing violations:

   uv run ruff check --add-noqa .
   

3. Fix auto-fixable issues first:

   uv run ruff check --fix .
   

4. Enable rules gradually over time

Configuration Not Loading

If configuration isn't being applied:

• Check file name: ruff.toml, .ruff.toml, or pyproject.toml
• Validate TOML syntax: uv run ruff check --config=ruff.toml .
• Use absolute paths in exclude patterns if relative paths don't work
• Check for conflicting configurations in parent directories

Formatter vs Black Differences

Ruff formatter aims for 99% compatibility with Black. Known differences:

• Magic trailing comma handling in some edge cases
• Line break decisions in complex nested structures

To report differences: use --diff and compare with Black output

False Positives

If Ruff reports false positives:

1. Use # noqa comments for specific exceptions
2. Configure per-file ignores for patterns
3. Report issues to the Ruff project

Performance Issues

Ruff is typically very fast, but if experiencing slowness:

• Exclude large directories (venv, node_modules, build)
• Use .ruffignore file for complex exclusion patterns
• Check for large files or deeply nested directories

Best Practices

1. Start Simple: Begin with basic rules (F, E, I) and expand gradually
2. Format First: Run uv run ruff format before uv run ruff check to avoid style conflicts
3. Use --fix Liberally: Most auto-fixes are safe and save time
4. Review Unsafe Fixes: Always review --unsafe-fixes changes before committing
5. Configure Line Length: Set line-length to match your team's preference
6. Enable in CI/CD: Enforce checks in CI to maintain code quality
7. Per-file Ignores: Use per-file configuration for test files, scripts, etc.
8. Commit Configuration: Keep ruff.toml or pyproject.toml in version control
9. Document Exceptions: Comment why specific rules are disabled
10. Keep Updated: Ruff evolves quickly; update regularly for new rules and fixes

Integration with IDEs

VS Code

Install the official Ruff extension:

{
  "ruff.enable": true,
  "ruff.organizeImports": true,
  "[python]": {
    "editor.formatOnSave": true,
    "editor.codeActionsOnSave": {
      "source.fixAll": true,
      "source.organizeImports": true
    },
    "editor.defaultFormatter": "charliermarsh.ruff"
  }
}

Installation

Ruff can be installed via uv:

Using uv:

uv tool install ruff

Verify installation:

ruff --version
uv run ruff check --help
uv run ruff format --help

Resources

• Official documentation: https://docs.astral.sh/ruff/
• Linter rules: https://docs.astral.sh/ruff/rules/
• Formatter docs: https://docs.astral.sh/ruff/formatter/
• GitHub repository: https://github.com/astral-sh/ruff
• Configuration options: https://docs.astral.sh/ruff/configuration/
• Migration guides: https://docs.astral.sh/ruff/faq/#how-does-ruff-compare-to-flake8