Python Ruff

Python Ruff — Linting and Formatting

Ruff is an extremely fast Python linter and code formatter written in Rust. It replaces Flake8, Black, isort, pydocstyle, pyupgrade, and autoflake with a single unified tool that runs 10–100x faster than any of them individually. Ruff supports over 800 built-in rules and provides automatic fix capabilities for many violations.

Installation

Install Ruff via pip, uv, or as a development dependency:

pip install ruff
uv add --dev ruff

Core Commands

ruff check                    # Lint all files in current directory
ruff check --fix              # Lint and auto-fix safe violations
ruff check --unsafe-fixes     # Show unsafe fixes (review before applying)
ruff check --fix --unsafe-fixes  # Apply all fixes including unsafe ones
ruff check --watch            # Re-lint on file changes
ruff format                   # Format all files in current directory
ruff format --check           # Check formatting without writing changes
ruff format --diff            # Show formatting diff without writing

Run both linting (with import sorting) and formatting in sequence:

ruff check --select I --fix   # Sort imports first
ruff format                   # Then format

Configuration

Ruff reads from pyproject.toml, ruff.toml, or .ruff.toml. All three support the same schema; ruff.toml and .ruff.toml omit the [tool.ruff] prefix.

Recommended Starter Configuration

[tool.ruff]
target-version = "py312"
line-length = 88

[tool.ruff.lint]
select = [
    "E",    # pycodestyle errors
    "F",    # Pyflakes
    "UP",   # pyupgrade
    "B",    # flake8-bugbear
    "SIM",  # flake8-simplify
    "I",    # isort
]
ignore = ["E501"]  # line-too-long (handled by formatter)

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

See references/configuration-guide.md for the full configuration reference.

Rule Selection Best Practices

Rule codes follow the pattern PREFIX + digits (e.g., F401 = Pyflakes unused import, E711 = pycodestyle comparison to None).

Start small, then expand:

# Step 1: Minimal (safe baseline)
select = ["E4", "E7", "E9", "F"]

# Step 2: Recommended expansion
select = ["E", "F", "UP", "B", "SIM", "I"]

# Step 3: Strict (with targeted ignores)
select = ["ALL"]
ignore = ["D", "ANN", "COM812", "ISC001"]

Key rule prefixes:

Use lint.select (not lint.extend-select) to make rule sets explicit. Avoid enabling ALL without carefully curating an ignore list, as it enables new rules on every Ruff upgrade.

See references/rule-categories.md for detailed rule guidance and common ignores.

Automatic Fixes

Ruff distinguishes between safe and unsafe fixes:

• Safe fixes preserve code behavior exactly — applied by default with --fix
• Unsafe fixes may change runtime behavior (e.g., exception types, removed comments) — opt-in with --unsafe-fixes

Promote or demote fix safety per rule:

[tool.ruff.lint]
extend-safe-fixes = ["UP"]    # Treat pyupgrade fixes as safe
extend-unsafe-fixes = ["B"]   # Require explicit opt-in for bugbear fixes

Disable auto-fix for specific rules while keeping them as violations:

[tool.ruff.lint]
fixable = ["ALL"]
unfixable = ["F401"]  # Flag unused imports but don't auto-remove them

Error Suppression

Line-level suppression

import os  # noqa: F401
x = 1  # noqa: E741, F841   # suppress multiple rules
y = 1  # noqa                # suppress all (avoid — too broad)

Block-level suppression (preferred over blanket noqa)

# ruff: disable[E501]
LONG_CONSTANT_1 = "Lorem ipsum dolor sit amet, consectetur adipiscing..."
LONG_CONSTANT_2 = "Lorem ipsum dolor sit amet, consectetur adipiscing..."
# ruff: enable[E501]

File-level suppression

# ruff: noqa: F841      # suppress specific rule for entire file
# ruff: noqa            # suppress all (avoid — use per-file-ignores instead)

Per-file ignores in config (preferred)

[tool.ruff.lint.per-file-ignores]
"__init__.py" = ["F401"]          # re-exports are intentional
"tests/**/*.py" = ["S101", "ANN"] # asserts and no annotations in tests
"scripts/**/*.py" = ["T20"]       # print() allowed in scripts

Detecting unused suppressions

ruff check --extend-select RUF100        # flag stale noqa comments
ruff check --extend-select RUF100 --fix  # auto-remove stale noqa

Formatter Configuration

The Ruff formatter is a drop-in replacement for Black. Key options:

[tool.ruff.format]
quote-style = "double"            # "double" | "single" | "preserve"
indent-style = "space"            # "space" | "tab"
line-ending = "auto"              # "auto" | "lf" | "crlf" | "native"
skip-magic-trailing-comma = false # respect trailing commas (like Black)
docstring-code-format = true      # format code examples in docstrings

Suppressing formatter

# fmt: off
matrix = [1,0,0,
          0,1,0,
          0,0,1]
# fmt: on

result = some_call()  # fmt: skip

Rules incompatible with the formatter

Disable these rules to avoid conflicts when using ruff format:

[tool.ruff.lint]
ignore = [
    "W191",   # tab-indentation
    "E111",   # indentation-with-invalid-multiple
    "COM812", # missing-trailing-comma
    "ISC002", # multi-line-implicit-string-concatenation
    "Q000",   # bad-quotes-inline-string
    "Q001",   # bad-quotes-multiline-string
    "Q002",   # bad-quotes-docstring
]

CI and Pre-commit Integration

GitHub Actions

- name: Lint and format check
  run: |
    ruff check .
    ruff format --check .

pre-commit hooks

repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.9.0
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

Migrating an existing codebase

Auto-add noqa directives to all current violations, then clean up incrementally:

ruff check --add-noqa .    # adds noqa to all failing lines
ruff check --extend-select RUF100 --fix .  # remove stale noqa over time

Quick Reference

Additional Resources

Reference Files

• references/rule-categories.md — Detailed rule prefixes, common ignores, and per-category guidance
• references/configuration-guide.md — Full configuration options for linter and formatter

Example Files

• examples/pyproject.toml — Production-ready pyproject.toml configuration
• examples/ruff.toml — Standalone ruff.toml configuration (monorepo root)
• examples/pre-commit-config.yaml — pre-commit hooks configuration