Code Quality Skill

When to Activate

Activate this skill when:

• Formatting Python code
• Running linters
• Adding type annotations
• Fixing code style issues
• Setting up quality tools

Quick Commands

# Format with Black
uv run black .
uv run black --check .  # Dry run

# Lint with Ruff
uv run ruff check .
uv run ruff check --fix .  # Auto-fix

# Type check with mypy
uv run mypy .

# All checks
uv run black . && uv run ruff check . && uv run mypy .

Tool Overview

Tool	Purpose	Speed	Auto-fix
Black	Code formatting	Fast	Yes
Ruff	Linting & imports	Very Fast	Most rules
mypy	Type checking	Moderate	No

Installation

uv add --dev black ruff mypy

Black - Code Formatting

Zero-config formatting with consistent style.

uv run black .                # Format all
uv run black --check .        # Dry run
uv run black --diff .         # Show changes

Configuration

[tool.black]
line-length = 88
target-version = ['py311']

Ruff - Fast Linting

Rust-based linter replacing flake8, isort, pylint, and 50+ tools.

uv run ruff check .               # Check
uv run ruff check --fix .         # Auto-fix
uv run ruff check --show-source . # Show context

Configuration

[tool.ruff]
line-length = 88
target-version = "py311"
select = ["E", "W", "F", "I", "B", "SIM"]
ignore = ["E501"]  # Black handles line length

[tool.ruff.per-file-ignores]
"__init__.py" = ["F401"]
"tests/*" = ["S101"]

Common Issues

• F401: Unused import (auto-fixable)
• F841: Unused variable
• I001: Import sorting (auto-fixable)
• B008: Mutable default argument

mypy - Type Checking

Static type checker for Python.

def greet(name: str) -> str:
    return f"Hello, {name}"

greet(123)  # mypy error: incompatible type "int"

Configuration

[tool.mypy]
python_version = "3.11"
warn_return_any = true
disallow_untyped_defs = false  # Enable gradually
check_untyped_defs = true

Common Type Hints

from typing import Optional, List, Dict

def find_user(user_id: int) -> Optional[Dict[str, str]]:
    return database.get(user_id)

def process_items(items: List[str]) -> int:
    return len(items)

Unified Configuration

# pyproject.toml
[tool.black]
line-length = 88
target-version = ['py311']

[tool.ruff]
line-length = 88
target-version = "py311"
select = ["E", "W", "F", "I", "B", "SIM"]

[tool.mypy]
python_version = "3.11"
warn_return_any = true

Skipping Rules

import os  # noqa: F401       # Ignore Ruff rule

# fmt: off                     # Disable Black
matrix = [[1, 2, 3], [4, 5, 6]]
# fmt: on

Workflow Best Practices

Development (before commit)

uv run black . && uv run ruff check --fix . && uv run mypy .

CI/CD (strict, no auto-fix)

uv run black --check . && uv run ruff check . && uv run mypy --strict .

Gradual Adoption

1. Start with Black - Zero config, immediate benefits
2. Add Ruff - Basic rules first, expand gradually
3. Introduce mypy - Lenient initially, increase strictness

Code Style Principles

Clarity Over Cleverness

# ❌ Clever but unclear
result = [x for x in range(10) if x % 2 == 0 if x > 5]

# ✅ Clear and readable
even_numbers = [x for x in range(10) if x % 2 == 0]
result = [x for x in even_numbers if x > 5]

Meaningful Names

# ❌ Unclear
def proc(d, x):
    return d[x] if x in d else None

# ✅ Clear
def get_user_by_id(users_dict, user_id):
    return users_dict.get(user_id)

Early Returns

# ❌ Nested
def process(amount, user):
    if amount > 0:
        if user.has_payment():
            return charge(user, amount)

# ✅ Early returns
def process(amount, user):
    if amount <= 0:
        return "Invalid amount"
    if not user.has_payment():
        return "No payment method"
    return charge(user, amount)

Related Resources

See AgentUsage/code_quality.md and AgentUsage/code_style_guide.md for:

• IDE integration
• Pre-commit hook setup
• Comprehensive style guidelines
• Error handling patterns