Code Quality Tools

Code quality tools catch bugs, enforce style, and keep your codebase consistent.

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PEP 8 — Python Style Guide

PEP 8 is the official style guide for Python. Key rules:

Rule	Example
Use snake_case for functions and variables	get_user_name()
Use PascalCase for classes	UserProfile
Use UPPER_CASE for constants	MAX_RETRIES
Indent with 4 spaces (not tabs)	Standard
Max line length: 79-120 characters	Configurable
Two blank lines before top-level definitions	Between classes

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Ruff — Linting and Formatting

Ruff replaces black, flake8, and isort with one fast tool.

Install

uv add --dev ruff

Run

uv run ruff check .       # find problems
uv run ruff check . --fix # fix automatically
uv run ruff format .      # format code

Configuration (pyproject.toml)

[tool.ruff]
target-version = "py312"
line-length = 120

[tool.ruff.lint]
select = ["E", "F", "I", "N", "W", "UP", "B", "SIM", "RUF"]

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

What Ruff Checks

Code	Category	Example
E	PEP 8 errors	Missing whitespace
F	Pyflakes	Unused imports
I	isort	Import order
N	Naming	Wrong naming convention
UP	Pyupgrade	Outdated syntax
B	Bugbear	Common bugs
SIM	Simplify	Code that can be simpler

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mypy — Static Type Checking

mypy checks your type hints without running the code.

Install

uv add --dev mypy

Run

uv run mypy src/

Configuration (pyproject.toml)

[tool.mypy]
python_version = "3.12"
strict = true
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true

Example

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

greet(42)  # mypy error: Argument 1 has incompatible type "int"

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Pre-commit Hooks

Run checks automatically before each commit:

Install

uv add --dev pre-commit

Configuration (.pre-commit-config.yaml)

repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.8.0
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format
  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.13.0
    hooks:
      - id: mypy

Activate

uv run pre-commit install

Now checks run automatically on every git commit.

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Test Coverage

Measure how much of your code is tested:

uv add --dev pytest-cov
uv run pytest --cov=src --cov-report=term-missing

Configuration

[tool.coverage.run]
source = ["src"]
branch = true

[tool.coverage.report]
fail_under = 100
show_missing = true

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Tool Summary

Tool	Purpose	Command
ruff	Linting + formatting	uv run ruff check . && uv run ruff format .
mypy	Type checking	uv run mypy src/
pytest-cov	Test coverage	uv run pytest --cov=src
pre-commit	Auto-run checks	uv run pre-commit run --all-files

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Best Practices

• Automate all checks with pre-commit hooks
• Run checks in CI/CD too — pre-commit alone is not enough
• Set coverage target to 100% and enforce it
• Fix all warnings — do not ignore them
• Use strict mode in mypy for better type safety
• Keep consistent style across the whole project