Code Quality

Python Code Quality Skill

This skill sets up and enforces comprehensive Python code quality using a battle-tested toolchain. Based on real production Makefiles using uv for fast dependency management.

Philosophy

• Fast feedback: Use uv run and uvx for instant tool execution — no global installs
• Layered checks: Lint → Format → Typecheck → Complexity → Dead Code → File Length
• CI-ready: Every check is a Makefile target that returns non-zero on failure
• Opinionated defaults: Start strict, relax only with justification

Tool Stack

Setup: pyproject.toml Configuration

When setting up code quality for a Python project, add these sections to pyproject.toml:

# =============================================================================
# Ruff — Linting & Formatting
# =============================================================================
[tool.ruff]
target-version = "py312"           # Adjust to project's minimum Python version
line-length = 120
src = ["src", "tests"]

[tool.ruff.lint]
select = [
    "E",      # pycodestyle errors
    "W",      # pycodestyle warnings
    "F",      # pyflakes
    "I",      # isort (import sorting)
    "N",      # pep8-naming
    "UP",     # pyupgrade
    "B",      # flake8-bugbear
    "SIM",    # flake8-simplify
    "S",      # flake8-bandit (security)
    "A",      # flake8-builtins
    "C4",     # flake8-comprehensions
    "DTZ",    # flake8-datetimez
    "T20",    # flake8-print
    "PT",     # flake8-pytest-style
    "RET",    # flake8-return
    "PTH",    # flake8-use-pathlib
    "ERA",    # eradicate (commented-out code)
    "PL",     # pylint subset
    "RUF",    # ruff-specific rules
]
ignore = [
    "S101",   # assert usage (fine in tests)
    "PLR0913", # too many arguments (relax for data-heavy functions)
]

[tool.ruff.lint.per-file-ignores]
"tests/**/*.py" = ["S101", "PLR2004", "T20"]  # Allow asserts, magic values, prints in tests

[tool.ruff.lint.isort]
known-first-party = ["PROJECT_NAME"]   # Replace with actual package name

[tool.ruff.format]
quote-style = "double"
indent-style = "space"
line-ending = "lf"

# =============================================================================
# Mypy — Type Checking
# =============================================================================
[tool.mypy]
python_version = "3.12"               # Adjust to project's minimum Python version
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true
disallow_incomplete_defs = true
check_untyped_defs = true
strict_equality = true
warn_redundant_casts = true
warn_unused_ignores = true
no_implicit_reexport = true

[[tool.mypy.overrides]]
module = "tests.*"
disallow_untyped_defs = false          # Relax for test functions

# =============================================================================
# Pytest
# =============================================================================
[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-v --strict-markers"
markers = [
    "slow: marks tests as slow (deselect with '-m \"not slow\"')",
]

Key Decisions

• Line length 120: 80 is too restrictive for modern screens; 120 balances readability and density
• select not extend-select: Explicit about exactly which rules are active
• S rules included: Ruff's built-in bandit subset catches common security issues during linting
• Strict mypy: disallow_untyped_defs forces type annotations — relax per-module if needed
• Tests relaxed: Asserts, magic values, and print statements are fine in test code

Setup: Dev Dependencies

Add to pyproject.toml under dev dependencies:

[project.optional-dependencies]
dev = [
    "ruff>=0.8",
    "mypy>=1.13",
    "pytest>=8.0",
    "pytest-cov>=6.0",
    "pre-commit>=4.0",
]

Or with uv groups:

[dependency-groups]
dev = [
    "ruff>=0.8",
    "mypy>=1.13",
    "pytest>=8.0",
    "pytest-cov>=6.0",
    "pre-commit>=4.0",
]

Tools that run via uvx (no install needed): xenon, vulture.

Setup: Makefile Targets

Add these targets to the project Makefile:

# =============================================================================
# Code Quality
# =============================================================================

lint:
	uv run ruff check src tests

lint-fix:
	uv run ruff check --fix src tests

format:
	uv run ruff format src tests

format-check:
	uv run ruff format --check src tests

typecheck:
	uv run mypy src/PROJECT_NAME

# File length gate (max 500 lines per .py file)
MAX_LINES := 500
file-length:
	@FAILED=0; \
	for f in $$(find src/ -name '*.py'); do \
		count=$$(wc -l < "$$f"); \
		if [ "$$count" -gt $(MAX_LINES) ]; then \
			echo "ERROR: $$f has $$count lines (max $(MAX_LINES))"; \
			FAILED=1; \
		fi; \
	done; \
	if [ "$$FAILED" -eq 1 ]; then \
		echo ""; \
		echo "Files exceeding $(MAX_LINES) lines must be split into smaller modules."; \
		exit 1; \
	fi; \
	echo "All files under $(MAX_LINES) lines."

# Cyclomatic complexity gate (Xenon grade C)
complexity:
	cd /tmp && uvx xenon --max-absolute C --max-modules D --max-average C $(CURDIR)/src/

# Dead code detection
dead-code:
	uvx vulture src/ --min-confidence 90

# Ensure dev dependencies are installed
ensure-dev:
	@uv sync --all-extras --quiet

# Run all checks (same as CI)
check: ensure-dev lint format-check typecheck file-length complexity test
	@echo "All checks passed!"

# Full CI-equivalent pipeline (locally)
ci: ensure-dev lint format-check typecheck file-length complexity dead-code test
	@echo "Full CI pipeline passed!"

Setup: Pre-commit Hooks

Create .pre-commit-config.yaml:

repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.8.6    # Pin to a specific version
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.13.0
    hooks:
      - id: mypy
        additional_dependencies: []  # Add stubs your project needs

  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v5.0.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-added-large-files
        args: ['--maxkb=500']
      - id: check-merge-conflict
      - id: debug-statements

Install hooks:

uv run pre-commit install

Running Checks

Quick Check (During Development)

make lint format-check typecheck

Full Quality Gate (Before Committing)

make check

This runs: lint → format-check → typecheck → file-length → complexity → test

Full CI Pipeline (Before Pushing)

make ci

Adds: dead-code to the check pipeline.

When Reviewing Existing Code

When asked to review or improve code quality in an existing project, follow this order:

1. Assess Current State

# Check if quality tools are configured
cat pyproject.toml | grep -A5 'tool.ruff'
cat pyproject.toml | grep -A5 'tool.mypy'
ls .pre-commit-config.yaml
ls Makefile

2. Run Existing Checks (If Available)

make lint 2>/dev/null || uv run ruff check src/
make typecheck 2>/dev/null || uv run mypy src/

3. Report Findings

Structure findings as:

• Critical: Type errors, undefined names, security issues (ruff S rules)
• Warning: Complexity issues, dead code, long files
• Style: Formatting, import ordering, naming conventions

4. Fix Incrementally

• Fix critical issues first
• Auto-fix what ruff can handle: make lint-fix
• Format: make format
• Address type errors manually
• Split long files if over 500 lines

Complexity Thresholds

Common Ruff Rule Groups Explained

Anti-Patterns This Skill Prevents

• No linting configured: Every Python project must have ruff
• Black + isort + flake8 separately: Use ruff — it replaces all three, 10-100x faster
• Mypy with --ignore-missing-imports everywhere: Fix the imports, add stubs
• No complexity gates: Functions grow unbounded without xenon checks
• 500+ line files: Sign of poor module decomposition — split them
• Dead code accumulation: Vulture catches unused functions/classes
• Manual formatting: Pre-commit hooks automate this on every commit
• Global tool installs: Use uv run and uvx — no system pollution

Adapting to Existing Projects

When a project already has some quality tooling:

1. Don't replace working configs — extend them
2. Migrate incrementally: If using black+isort+flake8, migrate to ruff one tool at a time
3. Start mypy in lenient mode if not yet typed: disallow_untyped_defs = false, then tighten
4. Add # type: ignore[specific-error] for known issues, never blanket ignores
5. Set --min-confidence 90 for vulture to reduce false positives in new projects