Linting & Formatting — ruff and black

eslint + prettier → ruff

In the TypeScript world you configure two tools: eslint enforces code quality rules and prettier enforces style. They have separate configs, separate CLI commands, and occasionally conflict with each other.

Python developers historically used the same split: flake8 (lint) + black (format). But ruff — released in 2022 and written in Rust — now replaces both. One tool, one config section in pyproject.toml, and it runs in milliseconds on an entire codebase.

Installing and running

Run this in your terminal — no Playground needed for CLI commands.

# Install (add to your project's dev dependencies)
pip install ruff
# or with uv:
uv add --dev ruff

# Lint the whole project
ruff check .

# Auto-fix fixable lint violations
ruff check --fix .

# Format the whole project (replaces black)
ruff format .

# Check formatting without writing (CI mode)
ruff format --check .

Configuring ruff vs eslint

// .eslintrc.js  (or eslint.config.js)
module.exports = {
  parser: '@typescript-eslint/parser',
  plugins: ['@typescript-eslint'],
  rules: {
    'no-unused-vars': 'error',
    '@typescript-eslint/no-explicit-any': 'warn',
  },
};

// .prettierrc
{
  "singleQuote": true,
  "semi": false,
  "printWidth": 88
}

# pyproject.toml  — one file for everything
[tool.ruff]
line-length = 88
indent-width = 4

[tool.ruff.lint]
# E/W = pycodestyle  F = pyflakes  I = isort  UP = pyupgrade
select = ["E", "F", "I", "UP"]
ignore = ["E501"]   # line-too-long (handled by formatter)

[tool.ruff.format]
quote-style = "double"   # matches black defaults

black — the opinionated formatter

Before ruff format existed, black was the de-facto standard. You will see it everywhere in existing Python projects. It is deliberately opinionated — it offers almost no configuration — exactly like prettier with --no-semi locked on. If you inherit a project using black, keep using it; don’t mix.

pip install black

# Format a single file
black src/main.py

# Format entire project
black .

# Check-only mode for CI
black --check .

The migration story

# Node / TypeScript workflow
npx eslint src/ --fix
npx prettier src/ --write

# Or with a combined script in package.json:
# "lint:fix": "eslint src/ --fix && prettier src/ --write"

# Python workflow — ruff does both
ruff check --fix .
ruff format .

# Or in one shot (check then format):
ruff check --fix . && ruff format .

# Makefile target (common pattern):
# lint:
#     ruff check --fix . && ruff format .

Playground note: ruff and black operate on files and require a terminal environment. Run the commands above in your project directory.