tests/
├── conftest.py          # Shared fixtures
├── unit/                # Unit tests (fast, isolated)
│   ├── test_models.py
│   └── test_services.py
├── integration/         # Integration tests (real dependencies)
│   └── test_api.py
└── fixtures/            # Test data files
    └── sample_data.json

import pytest
from myapp.services import UserService

class TestUserService:
    """Tests for UserService."""

    def test_create_user_with_valid_data(self, user_service):
        # Arrange
        user_data = {"email": "test@example.com", "name": "Test User"}

        # Act
        result = user_service.create(user_data)

        # Assert
        assert result.email == "test@example.com"
        assert result.id is not None

    def test_create_user_with_duplicate_email_raises_error(self, user_service, existing_user):
        # Arrange
        user_data = {"email": existing_user.email, "name": "Another User"}

        # Act & Assert
        with pytest.raises(ValueError, match="Email already exists"):
            user_service.create(user_data)

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import pytest

@pytest.mark.parametrize("input_email,expected_valid", [
    ("valid@example.com", True),
    ("also.valid@domain.co.uk", True),
    ("invalid-email", False),
    ("missing@domain", False),
    ("", False),
])
def test_email_validation(input_email, expected_valid):
    from myapp.validators import is_valid_email
    assert is_valid_email(input_email) == expected_valid

@pytest.mark.parametrize("status,expected_message", [
    ("pending", "Order is being processed"),
    ("shipped", "Order has been shipped"),
    ("delivered", "Order has been delivered"),
], ids=["pending-status", "shipped-status", "delivered-status"])
def test_order_status_message(status, expected_message):
    from myapp.orders import get_status_message
    assert get_status_message(status) == expected_message

from unittest.mock import Mock, patch, AsyncMock

def test_send_email_calls_smtp(user_service):
    # Mock external dependency
    with patch("myapp.services.smtp_client") as mock_smtp:
        mock_smtp.send.return_value = True

        user_service.send_welcome_email("test@example.com")

        mock_smtp.send.assert_called_once_with(
            to="test@example.com",
            subject="Welcome!",
        )

def test_payment_processing_handles_failure():
    mock_gateway = Mock()
    mock_gateway.charge.side_effect = PaymentError("Card declined")

    service = PaymentService(gateway=mock_gateway)

    with pytest.raises(PaymentError):
        service.process_payment(amount=100)

import pytest

@pytest.mark.asyncio
async def test_async_fetch_user(user_service):
    # Arrange
    user_id = 1

    # Act
    user = await user_service.get_by_id(user_id)

    # Assert
    assert user.id == user_id

@pytest.fixture
async def async_db():
    """Async database session fixture."""
    from myapp.database import async_session
    async with async_session() as session:
        yield session
        await session.rollback()

    result = await fetch_and_process()

    assert result["status"] == "ok"

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import pytest

def test_divide_by_zero_raises_error():
    with pytest.raises(ZeroDivisionError):
        divide(10, 0)

def test_invalid_input_raises_with_message():
    with pytest.raises(ValueError, match="must be positive"):
        process_amount(-100)

def test_exception_attributes():
    with pytest.raises(CustomError) as exc_info:
        risky_operation()

    assert exc_info.value.code == "E001"
    assert "failed" in str(exc_info.value)

@pytest.fixture(scope="session")
def database_engine():
    """Create engine once for entire test session."""
    engine = create_engine(TEST_DATABASE_URL)
    yield engine
    engine.dispose()

@pytest.fixture(scope="function")
def db_session(database_engine):
    """Create fresh session per test."""
    connection = database_engine.connect()
    transaction = connection.begin()
    session = Session(bind=connection)

    yield session

    session.close()
    transaction.rollback()
    connection.close()

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