# tests/test_everything.py - Monolithic test file
import pytest
from unittest.mock import patch, MagicMock

class TestOrders:
    @patch('app.services.order_service.db')
    @patch('app.services.order_service.stripe')
    @patch('app.services.order_service.email_sender')
    @patch('app.services.order_service.inventory')
    def test_create_order(self, mock_inv, mock_email, mock_stripe, mock_db):
        # Testing implementation details
        mock_db.query.return_value.filter.return_value.first.return_value = Customer(id=1)
        mock_inv.check.return_value = True
        mock_stripe.PaymentIntent.create.return_value = MagicMock(id='pi_123')

        from app.services.order_service import create_order
        result = create_order({'customer_id': 1, 'items': [...]})

        # Asserting on internal calls, not behavior
        mock_db.query.assert_called()
        mock_stripe.PaymentIntent.create.assert_called_once()

# Tests coupled to implementation, break with refactoring
# Tests slow because they patch everything

# tests/unit/domain/test_order.py - Fast, stable, test domain rules
class TestOrder:
    def test_calculates_total_from_line_items(self):
        order = Order.create(customer_id="c1")
        order.add_item(Product("p1", Money(100)))
        order.add_item(Product("p2", Money(50)))

        assert order.total == Money(150)

    def test_rejects_negative_quantity(self):
        order = Order.create(customer_id="c1")

        with pytest.raises(InvalidQuantityError):
            order.add_item(Product("p1", Money(100)), quantity=-1)

# tests/integration/application/test_create_order.py - Test use case
class TestCreateOrderUseCase:
    def test_creates_order_and_reserves_inventory(self):
        # Use test doubles, not mocks of internals
        orders = InMemoryOrderRepository()
        inventory = FakeInventoryService(available={"p1": 10})
        use_case = CreateOrderUseCase(orders, inventory)

        result = use_case.execute(CreateOrderCommand(
            customer_id="c1",
            items=[OrderItem("p1", quantity=2)]
        ))

        # Assert on observable behavior
        assert orders.find_by_id(result.order_id) is not None
        assert inventory.reserved["p1"] == 2

# tests/e2e/test_order_flow.py - Full flow, few tests
class TestOrderFlow:
    def test_complete_order_journey(self, api_client, test_db):
        # Create order via API
        response = api_client.post('/orders', json={...})
        order_id = response.json['order_id']

        # Verify order persisted
        order = test_db.query(Order).get(order_id)
        assert order.status == 'pending'

tests/
├── unit/                    # Fast, isolated
│   ├── domain/              # Entity business rules
│   └── application/         # Use case logic
├── integration/             # Component interaction
│   ├── persistence/         # Repository implementations
│   └── external/            # Gateway implementations
└── e2e/                     # Full system
    └── api/                 # HTTP endpoints