---

name: improving-python-code-quality description: Improves Python library code quality through ruff linting, mypy type checking, Pythonic idioms, and refactoring. Use when reviewing code for quality issues, adding type hints, configuring static analysis tools, or refactoring Python library code.

Python Code Quality

Quick Reference

Ruff Configuration

Minimal config in pyproject.toml:

Copy[tool.ruff]
line-length = 88
target-version = "py310"

[tool.ruff.lint]
select = ["E", "W", "F", "I", "B", "C4", "UP"]

For full configuration options, see RUFF_CONFIG.md.

MyPy Configuration

Copy[tool.mypy]
python_version = "3.10"
disallow_untyped_defs = true
warn_return_any = true

For strict settings and overrides, see MYPY_CONFIG.md.

Type Hints Patterns

Copy# Basic
def process(items: list[str]) -> dict[str, int]: ...

# Optional
def fetch(url: str, timeout: int | None = None) -> bytes: ...

# Callable
def apply(func: Callable[[int], str], value: int) -> str: ...

# Generic
T = TypeVar("T")
def first(items: Sequence[T]) -> T | None: ...

For protocols and advanced patterns, see TYPE_PATTERNS.md.

Common Anti-Patterns

Copy# Bad: Mutable default
def process(items: list = []):  # Bug!
    ...

# Good: None default
def process(items: list | None = None):
    items = items or []
    ...

Copy# Bad: Bare except
try:
    ...
except:
    pass

# Good: Specific exception
try:
    ...
except ValueError as e:
    logger.error(e)

Pythonic Idioms

Copy# Iteration
for item in items:           # Not: for i in range(len(items))
for i, item in enumerate(items):  # When index needed

# Dictionary access
value = d.get(key, default)  # Not: if key in d: value = d[key]

# Context managers
with open(path) as f:        # Not: f = open(path); try: finally: f.close()

# Comprehensions (simple only)
squares = [x**2 for x in numbers]

Module Organization

Copysrc/my_library/
├── __init__.py      # Public API exports
├── _internal.py     # Private (underscore prefix)
├── exceptions.py    # Custom exceptions
├── types.py         # Type definitions
└── py.typed         # Type hint marker

Checklist

CopyCode Quality:
- [ ] ruff check passes
- [ ] mypy passes (strict mode)
- [ ] Public API has type hints
- [ ] Public API has docstrings
- [ ] No mutable default arguments
- [ ] Specific exception handling
- [ ] py.typed marker present