Testing with Pytest

Quick Start

# 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

# 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

Feature	Description	Reference
Fixtures	Reusable test setup with dependency injection	Fixtures Guide
Parametrization	Test multiple inputs with @pytest.mark.parametrize	Parametrize
Mocking	unittest.mock integration for patching	pytest-mock
Async Testing	Native async/await support with pytest-asyncio	pytest-asyncio
Markers	Categorize and filter tests	Markers
Coverage	Code coverage with pytest-cov	pytest-cov

Common Patterns

Parametrized Validation Tests

import 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

@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

from 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

import 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

Do	Avoid
Use descriptive test names explaining the scenario	Sharing state between tests
Create reusable fixtures for common setup	Using sleep for timing issues
Use parametrize for testing multiple inputs	Testing implementation details
Mock external dependencies	Writing tests that depend on order
Group related tests in classes	Using hardcoded file paths
Use factories for test data creation	Leaving commented-out test code

References

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