Python

Modern Python development with type hints, testing, and code quality tools.

Contents

• Project Setup
• Code Quality
• Testing
• Dataclasses
• Type Hints
• Best Practices

Project Setup

Structure

my-project/
├── pyproject.toml
├── src/my_project/
│   ├── __init__.py
│   ├── main.py
│   └── py.typed          # PEP 561 marker
├── tests/
│   ├── conftest.py
│   └── test_main.py
└── README.md

pyproject.toml

[project]
name = "my-project"
version = "0.1.0"
requires-python = ">=3.9"
dependencies = ["requests>=2.31.0"]

[project.optional-dependencies]
ml = ["scikit-learn>=1.0.0"]

[dependency-groups]
dev = ["pytest>=7.0.0", "ruff>=0.1.0", "mypy>=1.0.0"]

[tool.ruff]
line-length = 100
target-version = "py39"

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

[tool.mypy]
python_version = "3.9"
strict = true

[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-v"

Code Quality

Formatting (ruff)

uv run ruff format .              # Format all
uv run ruff format --check .      # Check only

Linting (ruff)

uv run ruff check .               # Check issues
uv run ruff check . --fix         # Auto-fix

Type Checking (mypy)

uv run mypy src/                  # Check types
uv run mypy --strict src/         # Strict mode

Testing

Running Tests

uv run pytest                     # Run all
uv run pytest -v                  # Verbose
uv run pytest tests/test_main.py  # Specific file
uv run pytest -k "test_add"       # Pattern match
uv run pytest -x                  # Stop on first failure
uv run pytest --cov=src tests/    # With coverage

# Watch mode (use tmux for background, requires pytest-watch)
tmux new -d -s pytest 'uv run ptw'

Test Structure

import pytest
from my_project.utils import add, divide


class TestArithmetic:
    def test_add(self) -> None:
        assert add(2, 3) == 5

    def test_divide_by_zero(self) -> None:
        with pytest.raises(ValueError, match="Cannot divide by zero"):
            divide(10, 0)

    @pytest.mark.parametrize("a,b,expected", [
        (1, 1, 2),
        (10, 20, 30),
    ])
    def test_add_parametrized(self, a: int, b: int, expected: int) -> None:
        assert add(a, b) == expected

Fixtures

import pytest

@pytest.fixture
def db():
    db = Database(":memory:")
    db.init()
    yield db
    db.close()

def test_user_insert(db):
    db.insert("users", {"name": "Test"})
    assert db.count("users") == 1

Dataclasses

Prefer dataclasses over regular classes for data containers. Auto-generates __init__, __repr__, __eq__.

from dataclasses import dataclass, field

@dataclass
class User:
    id: int
    name: str
    email: str

@dataclass(frozen=True)  # Immutable, hashable
class Point:
    x: float
    y: float

@dataclass
class Config:
    name: str
    debug: bool = False                           # Simple default
    tags: list[str] = field(default_factory=list) # Mutable default
    area: float = field(init=False)               # Computed field

    def __post_init__(self) -> None:
        self.area = len(self.tags)

Decorator Options

Option	Effect
frozen=True	Immutable, hashable (use for value objects)
slots=True	Memory-efficient (Python 3.10+)
order=True	Enable <, >, <=, >= comparisons

When to Use

✅ Dataclasses	❌ Regular Classes
DTOs, configs, records	Complex behavior/methods
API request/response models	Custom __init__ logic
Immutable value objects	Mutable state with invariants

Type Hints

Functions

from typing import Optional, List, Dict

def greet(name: str, formal: bool = False) -> str:
    return f"Good day, {name}!" if formal else f"Hello, {name}!"

def process(data: List[Dict[str, int]]) -> Optional[int]:
    return sum(d.get("value", 0) for d in data) or None

Classes

from dataclasses import dataclass
from typing import Generic, TypeVar

@dataclass
class User:
    id: int
    name: str
    email: str

T = TypeVar('T')

class Container(Generic[T]):
    def __init__(self, value: T) -> None:
        self.value = value

    def get(self) -> T:
        return self.value

Error Handling

from pathlib import Path
from typing import Optional

def read_file(path: Path) -> Optional[str]:
    try:
        return path.read_text()
    except FileNotFoundError:
        return None
    except PermissionError as e:
        raise PermissionError(f"Cannot read {path}") from e

Best Practices

1. Type hints: All function parameters and return values
2. Docstrings: Document public APIs
3. Test coverage: Comprehensive tests for business logic
4. Fixtures: Organize test setup, not setup methods
5. Parametrize: Use @pytest.mark.parametrize for multiple cases
6. Strict mypy: Enable --strict in pyproject.toml
7. Logging: Use logging module, not print()

Development Loop

# 1. Format
uv run ruff format .

# 2. Lint
uv run ruff check . --fix

# 3. Type check
uv run mypy src/

# 4. Test
uv run pytest -v

Related Skills

• uv: Manage Python dependencies and environments