ruff-0-4-10

name

ruff-0-4-10

description

Lint, format, and configure Python code with Ruff (v0.4+). Use when the user mentions ruff, python linting, python formatting, code quality, replacing flake8/black/isort/pyupgrade, pyproject.toml ruff config, ruff check, ruff format, or needs help setting up Python linting and formatting. Also triggers for import sorting (isort replacement), docstring checks, auto-fixing lint violations, pre-commit hooks with ruff, CI/CD linting pipelines, and migrating from flake8 + black + isort to a single tool.

ruff 0.4.10

Ruff is an extremely fast Python linter and code formatter written in Rust. It replaces Flake8 (plus dozens of plugins), Black, isort, pydocstyle, pyupgrade, autoflake, and more — all in one binary executing 10–100× faster.

Overview

Ruff provides two main capabilities:

• Linter (ruff check) — Over 800 rules across 50+ rule categories (Pyflakes, pycodestyle, isort, pyupgrade, flake8-bugbear, etc.). Default rules: E4, E7, E9, F.
• Formatter (ruff format) — Black-compatible code formatter with near-identical output (>99.9% line parity on Black-formatted projects).

Both share the same configuration strategy via pyproject.toml, ruff.toml, or .ruff.toml.

Usage

Installation

pip install ruff          # PyPI (any platform, no Rust needed)
brew install ruff         # macOS/Linux Homebrew
conda install -c conda-forge ruff  # Conda

Linting

ruff check                        # Current directory
ruff check path/to/code/          # Specific directory
ruff check --fix                  # Auto-fix safe violations
ruff check --fix --unsafe-fixes   # Include unsafe fixes
ruff check --select F401,B        # Enable specific rules only
ruff check --add-noqa .           # Add noqa to existing violations (migration)
ruff check --show-files            # See which files will be checked
ruff check --show-settings         # Debug resolved config for a file

Formatting

ruff format                       # Current directory
ruff format path/to/code/         # Specific directory
ruff format --check               # Dry-run (exit 1 if changes needed)
ruff format --diff                # Show diff without writing

Sorting Imports

Ruff's formatter does not sort imports. Use the linter for that:

ruff check --select I --fix   # Sort imports via isort rules
ruff format                   # Then format

Common Workflows

Quick lint and fix:

ruff check --fix . && ruff format .

CI check (fail on violations):

ruff check . && ruff format --check .

Migrate a codebase incrementally:

ruff check --select UP --add-noqa .   # Suppress existing pyupgrade violations
# Then gradually remove noqa comments as you fix them

Configuration Files

Ruff reads config from pyproject.toml, ruff.toml, or .ruff.toml (in that precedence order). Config is hierarchical — the closest file to a given source file wins. Ruff does not merge parent configs; use extend for inheritance:

# ruff.toml
extend = "../ruff.toml"    # Inherit from parent
line-length = 100           # Override specific setting

In pyproject.toml, prefix sections with tool.ruff:

[tool.ruff]
line-length = 88

[tool.ruff.lint]
select = ["E", "F", "I", "UP", "B"]

[tool.ruff.format]
quote-style = "single"

Key Configuration Settings

Setting	Default	Description
line-length	88	Max line length (same as Black)
target-version	py38	Minimum Python version (py37–py313)
src	["."]	Directories for first-party import detection
exclude	.git, .venv, etc.	Paths to skip discovery
lint.select	["E4","E7","E9","F"]	Rule codes to enable
lint.ignore	[]	Rule codes to disable
lint.per-file-ignores	{}	File-pattern → rule ignores map
preview	false	Enable unstable rules/features

CLI Config Overrides

Some settings have dedicated flags; others use --config "key = value":

ruff check --select F401 --line-length=100 .
ruff check --config "lint.dummy-variable-rgx = '__.*'" .
ruff format --line-length=120 .

Error Suppression

# Inline: ignore specific rules on a line
x = 1  # noqa: F841

# Inline: ignore all rules on a line
x = 1  # noqa

# File-level: ignore specific rule everywhere in file
# ruff: noqa: F841

# File-level: ignore all rules
# ruff: noqa

Format Suppression

# fmt: off
not_formatted = 3
also_not_formatted = 4
# fmt: on

# Single-statement skip
a = [1, 2, 3, 4, 5]  # fmt: skip

Gotchas

• select replaces, extend-select adds — Using --select F on CLI replaces the config's select entirely. Use --extend-select F to add on top of existing rules. Same applies in config files: prefer select for explicit rule sets, extend-select to add categories.

• ALL enables everything including future rules — Enabling select = ["ALL"] means new rules automatically trigger on upgrade. Pin your Ruff version or use explicit select lists instead.

• Formatter-linter conflicts — Running both linter and formatter can clash on rules like E501 (line-too-long), W191 (tab-indentation), COM812/COM819 (trailing commas), ISC001/ISC002 (implicit string concatenation). Ruff's defaults avoid these, but if you enable such rules, add them to lint.ignore.

• Unsafe fixes can change semantics — By default only safe fixes apply with --fix. Some fixes (e.g., list(x)[0] → next(iter(x))) change exception types. Use --unsafe-fixes explicitly when needed.

• Config discovery is closest-wins, not merged — Unlike ESLint, Ruff does not cascade settings from parent configs. The nearest config file is used entirely. Use extend = "../ruff.toml" for inheritance.

• .ruff.toml > ruff.toml > pyproject.toml — If multiple config files exist in the same directory, .ruff.toml takes highest precedence.

• Import sorting requires the linter, not formatter — ruff format does not sort imports. Run ruff check --select I --fix then ruff format.

• Preview rules need preview = true AND explicit selection — Even with preview enabled, a preview rule is only active if its code/category is in select/extend-select. Preview mode alone doesn't auto-enable all preview rules.

References

Detailed reference material loaded on demand:

• 01-rule-categories.md — Rule code prefixes (E, F, I, UP, B, etc.) with descriptions and popular rule examples
• 02-configuration.md — Complete configuration reference: settings tables, hierarchical config, per-file ignores, isort options, pydocstyle conventions
• 03-linter-deep-dive.md — Rule selection resolution, fix safety model, noqa system, action comments, output formats, caching
• 04-formatter-deep-dive.md — Black compatibility, known deviations, docstring formatting, format suppression pragmas, conflicting lint rules
• 05-integrations.md — VS Code, pre-commit, LSP, Neovim, Emacs, GitHub Actions, Docker