---

name: Testing with Pytest description: Claude writes comprehensive Python tests using pytest. Use when writing unit tests, creating fixtures, parametrizing tests, mocking dependencies, testing async code, or setting up CI test pipelines.

Testing with Pytest

Quick Start

Copy# pytest.ini
[pytest]
testpaths = tests
python_files = test_*.py
addopts = -v --strict-markers --cov=src --cov-report=term-missing --cov-fail-under=80
markers =
    slow: marks tests as slow
    integration: marks tests as integration tests
asyncio_mode = auto

Copy# tests/conftest.py
import pytest
from sqlalchemy.orm import Session

@pytest.fixture
def db_session(engine) -> Session:
    """Create database session with automatic rollback."""
    connection = engine.connect()
    transaction = connection.begin()
    session = sessionmaker(bind=connection)()
    yield session
    session.close()
    transaction.rollback()
    connection.close()

Features

Common Patterns

Parametrized Validation Tests

Copyimport pytest
from src.validators import validate_email, ValidationError

class TestEmailValidation:
    @pytest.mark.parametrize("email", [
        "user@example.com",
        "user.name@example.com",
        "user+tag@example.co.uk",
    ])
    def test_valid_emails(self, email: str):
        assert validate_email(email) is True

    @pytest.mark.parametrize("email,error", [
        ("", "Email is required"),
        ("invalid", "Invalid email format"),
        ("@example.com", "Invalid email format"),
    ])
    def test_invalid_emails(self, email: str, error: str):
        with pytest.raises(ValidationError, match=error):
            validate_email(email)

Factory Fixtures

Copy@pytest.fixture
def user_factory(db_session):
    """Factory for creating test users."""
    def _create_user(email="test@example.com", name="Test User", **kwargs):
        user = User(email=email, name=name, **kwargs)
        db_session.add(user)
        db_session.commit()
        return user
    return _create_user

@pytest.fixture
def test_user(user_factory):
    return user_factory(email="testuser@example.com")

Mocking External Services

Copyfrom unittest.mock import patch, MagicMock

class TestUserService:
    def test_create_user_sends_email(self, user_service, mock_email_service):
        user = user_service.create_user(email="new@example.com", name="New")

        mock_email_service.send_email.assert_called_once_with(
            to="new@example.com",
            template="welcome",
            context={"name": "New"},
        )

    @patch("src.services.user_service.datetime")
    def test_last_login_updated(self, mock_datetime, user_service, test_user):
        from datetime import datetime
        mock_datetime.utcnow.return_value = datetime(2024, 1, 15, 10, 30)

        user_service.authenticate(test_user.email, "password")
        assert test_user.last_login == datetime(2024, 1, 15, 10, 30)

Async API Testing

Copyimport pytest
from httpx import AsyncClient

@pytest.mark.asyncio
class TestAPIEndpoints:
    async def test_get_users(self, async_client: AsyncClient, test_user):
        response = await async_client.get("/api/users")
        assert response.status_code == 200
        assert len(response.json()["users"]) >= 1

    async def test_create_user(self, async_client: AsyncClient):
        response = await async_client.post("/api/users", json={
            "email": "new@example.com",
            "name": "New User",
            "password": "SecurePass123!",
        })
        assert response.status_code == 201

Best Practices

References

• Pytest Documentation
• pytest-asyncio
• pytest-mock
• pytest-cov