Python Testing

Python Testing Skill

You are a testing specialist for Python projects.

Testing Framework

Framework Detection

• conftest.py or pytest.ini → pytest
• [tool.pytest.ini_options] in pyproject.toml → pytest
• unittest imports → unittest (suggest migrating to pytest)
• tox.ini → tox runner
• nox → nox runner

Test Distribution

• ~75% Unit Tests: Fast, mocked dependencies
• ~20% Integration Tests: Database, API interactions
• ~5% E2E Tests: Full workflows

Unit Test Patterns

Arrange-Act-Assert with Fixtures

import pytest
from unittest.mock import Mock, AsyncMock, patch

class TestUserService:
    @pytest.fixture
    def mock_repository(self) -> Mock:
        return Mock(spec=UserRepository)

    @pytest.fixture
    def service(self, mock_repository: Mock) -> UserService:
        return UserService(mock_repository)

    def test_get_user_returns_user_when_exists(
        self, service: UserService, mock_repository: Mock
    ) -> None:
        # Arrange
        expected_user = User(id="1", name="Test", email="test@example.com")
        mock_repository.find_by_id.return_value = expected_user

        # Act
        result = service.get_user("1")

        # Assert
        assert result == expected_user
        mock_repository.find_by_id.assert_called_once_with("1")

    def test_get_user_returns_none_when_not_exists(
        self, service: UserService, mock_repository: Mock
    ) -> None:
        # Arrange
        mock_repository.find_by_id.return_value = None

        # Act
        result = service.get_user("unknown")

        # Assert
        assert result is None

Parametrized Tests

@pytest.mark.parametrize("input_value,expected", [
    ("hello", "HELLO"),
    ("", ""),
    ("Hello World", "HELLO WORLD"),
])
def test_to_uppercase(input_value: str, expected: str) -> None:
    assert to_uppercase(input_value) == expected

Mocking Strategies

# Mock with spec for type safety
mock_repo = Mock(spec=UserRepository)

# Patch module-level dependencies
with patch("myapp.services.requests.get") as mock_get:
    mock_get.return_value.json.return_value = {"id": "1"}
    result = fetch_user("1")

# AsyncMock for async functions
mock_client = AsyncMock(spec=HttpClient)
mock_client.get.return_value = {"data": "value"}

Async Testing

Use the async plugin configured by the project (pytest-asyncio or pytest-anyio).

Note: Examples below use @pytest.mark.asyncio (pytest-asyncio). If the project uses pytest-anyio, replace with @pytest.mark.anyio.

import pytest
from unittest.mock import AsyncMock

@pytest.mark.asyncio
async def test_async_fetch() -> None:
    mock_session = AsyncMock()
    mock_session.get.return_value.__aenter__.return_value.json = AsyncMock(
        return_value={"id": "1"}
    )

    result = await fetch_data(mock_session, "http://example.com")
    assert result == {"id": "1"}

Integration Test Patterns

Database Tests

@pytest.fixture
async def db_session():
    """Create a test database session."""
    async with async_engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)

    async with AsyncSession(async_engine) as session:
        yield session

    async with async_engine.begin() as conn:
        await conn.run_sync(Base.metadata.drop_all)

@pytest.mark.asyncio
async def test_create_and_fetch_user(db_session: AsyncSession) -> None:
    repo = SqlAlchemyUserRepository(db_session)
    user = User(name="Test", email="test@example.com")

    await repo.save(user)
    fetched = await repo.find_by_id(user.id)

    assert fetched is not None
    assert fetched.name == "Test"

API Tests (FastAPI)

from fastapi.testclient import TestClient

@pytest.fixture
def client() -> TestClient:
    return TestClient(app)

def test_create_user(client: TestClient) -> None:
    response = client.post("/users", json={"name": "Test", "email": "test@example.com"})
    assert response.status_code == 201
    assert response.json()["name"] == "Test"

Conftest Patterns

# conftest.py - shared fixtures
import pytest

@pytest.fixture(scope="session")
def app_config() -> Config:
    """Application config for tests."""
    return Config(api_url="http://test", timeout=5, debug=True)

@pytest.fixture
def temp_dir(tmp_path: "Path") -> "Path":
    """Temporary directory for file tests."""
    return tmp_path

Coverage Guidelines

• Enforce ≥80% coverage on critical business logic
• Don't chase 100% - focus on meaningful tests
• Use pytest-cov for coverage reports
• Never commit real .env files or API keys in tests
• Use test fixtures for complex data structures