Ruff: One Fast Tool for Linting and Formatting Python

Ruff is a Python linter and formatter written in Rust. It implements rules from Flake8 and many of its plugins, sorts imports like isort, and formats code in a Black-compatible style, typically in a fraction of a second.

Install and run

uv add --dev ruff
uv run ruff check .          # lint
uv run ruff check . --fix    # lint and auto-fix what's safe
uv run ruff format .         # format

Configure in pyproject.toml

[tool.ruff]
line-length = 100
target-version = "py313"

[tool.ruff.lint]
select = [
  "E", "F",     # pycodestyle errors, Pyflakes
  "I",          # import sorting
  "B",          # bugbear: likely bugs
  "UP",         # pyupgrade: modern syntax
  "SIM",        # simplifications
  "S",          # bandit security checks
]
ignore = ["E501"]   # line length is handled by the formatter

[tool.ruff.lint.per-file-ignores]
"tests/**" = ["S101"]   # assert is fine in tests

Examples of what it catches

import os, sys            # F401: 'sys' imported but unused; I001: unsorted imports

def add_item(item, items=[]):   # B006: mutable default argument
    items.append(item)
    return items

if x == None:             # E711: comparison to None should be 'is None'
    pass

Run it automatically

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: vX.Y.Z          # the Ruff version your project uses
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

# CI: fail if anything needs fixing
ruff check . && ruff format --check .

Install the Ruff editor extension too, so issues show up while you type.