---

name: ruff description: >- Lint and format Python with Ruff. Use when a user asks to set up Python linting, replace flake8/black/isort, configure code quality rules, or speed up Python code formatting. license: Apache-2.0 compatibility: "Python 3.7+ for pip install; prebuilt binary, no Rust needed" metadata: author: terminal-skills version: "1.1.0" repository: https://github.com/astral-sh/ruff category: development tags: - ruff - linting - formatting - python - code-quality

Ruff

Overview

Ruff is an extremely fast Python linter and formatter written in Rust. It replaces flake8, black, isort, pyupgrade, pydocstyle, and dozens more tools — running 10-100x faster. One tool for all Python code quality.

Instructions

Step 1: Setup

pip install ruff          # or: uv tool install ruff / pipx install ruff / brew install ruff
ruff --version            # checked against 0.16.10

For a one-off run without installing: uvx ruff check .. Pin the version in your project dependencies so local runs, pre-commit and CI agree.

Step 2: Configuration

# pyproject.toml — Ruff configuration
[tool.ruff]
target-version = "py311"
line-length = 100
src = ["src", "tests"]

[tool.ruff.lint]
select = [
    "E",    # pycodestyle errors
    "W",    # pycodestyle warnings
    "F",    # pyflakes
    "I",    # isort (import sorting)
    "B",    # flake8-bugbear
    "C4",   # flake8-comprehensions
    "UP",   # pyupgrade
    "SIM",  # flake8-simplify
    "TC",   # flake8-type-checking (named TCH in older Ruff versions)
    "RUF",  # ruff-specific rules
]
ignore = [
    "E501",   # line too long (handled by formatter)
]
fixable = ["ALL"]

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

[tool.ruff.lint.per-file-ignores]
"tests/**" = ["B011"]      # allow `assert False` in tests
"__init__.py" = ["F401"]   # re-exports

[tool.ruff.format]
quote-style = "double"
indent-style = "space"
docstring-code-format = true

Step 3: Use

# Lint
ruff check .                   # check all files
ruff check . --fix             # auto-fix issues
ruff check . --fix --unsafe-fixes  # include risky auto-fixes

# Format
ruff format .                  # format all files
ruff format --check .          # check without modifying (CI)

# Watch mode
ruff check --watch .

# Inspect
ruff rule F401                 # explain one rule
ruff format --diff .           # show what the formatter would change
ruff check --select I --fix .  # only sort imports
ruff check --add-noqa .        # silence existing violations when adopting Ruff on old code

Without a [tool.ruff] section Ruff uses its own defaults (line length 88 and a small rule set). The same settings can live in ruff.toml (no tool.ruff prefix) or .ruff.toml. --fix applies only safe fixes; fixes that could change behavior are shown as hidden and need --unsafe-fixes, so review that diff.

Step 4: Pre-commit Hook

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.16.10
    hooks:
      - id: ruff-check
        args: [--fix]
      - id: ruff-format

Step 5: CI Integration

# .github/workflows/lint.yml
- name: Lint with Ruff
  run: |
    ruff check . --output-format=github
    ruff format --check .

Examples

Replace flake8, isort and black in an existing project

User: "We run flake8, isort and black on our FastAPI service. Move it all to Ruff."

pip uninstall -y flake8 isort black
pip install ruff
# add the [tool.ruff] block above to pyproject.toml, delete .flake8 and setup.cfg lint sections
ruff check . --fix
ruff format .

The first run reports remaining violations (for example F401 imported but unused or UP006 Use list instead of List) and rewrites safe ones in place; ruff format . prints N files reformatted, M files left unchanged.

Adopt Ruff on a legacy codebase without a giant diff

User: "Turn on Ruff in CI but don't make me fix 900 warnings today."

ruff check . --select E,F,I,B,UP --add-noqa   # adds "# noqa: F401" style comments to existing hits
ruff check .                                   # now passes, new violations still fail

Remove the noqa comments gradually as files are touched.

Guidelines

• Ruff replaces flake8, black, isort, pyupgrade in a single tool — remove the old ones.
• Start with a broad rule set (select = ["E", "W", "F", "I", "B", "UP"]) and expand.
• ruff check --fix auto-fixes most issues — safe to run in pre-commit hooks.
• The ruff hook id was renamed ruff-check in ruff-pre-commit; the old id still works as a legacy alias.
• ruff format is designed to match Black closely but is not byte-identical in every case; expect a few small diffs on first run.
• Ruff does not do type checking; keep mypy or pyright for that.
• Use ruff format as a near drop-in replacement for black.