Python Testing

Testing with pytest

Stub-Driven TDD and layer boundary testing patterns for Python applications.

Core Principle: Stub-Driven TDD

Test at component boundaries, not internal implementation:

Router → Service → Repository → Entity → Database
   ↓        ↓           ↓          ↓
 Test    Test        Test       Test

Follow the Stub → Test → Implement → Refactor workflow:

1. Stub - Create function signature with pass
2. Test - Write test for expected behavior
3. Implement - Make test pass
4. Refactor - Clean up code

# 1. Stub
def calculate_discount(total: Decimal) -> Decimal:
    pass

# 2. Test
def test_discount_for_large_order():
    result = calculate_discount(Decimal("150"))
    assert result == Decimal("15")

# 3. Implement
def calculate_discount(total: Decimal) -> Decimal:
    if total > 100:
        return total * Decimal("0.1")
    return Decimal("0")

Layer Boundary Testing Overview

Test what crosses layer boundaries, not internal implementation:

• Entity Layer: Domain logic, validation, transformations (fromrequest, toresponse, to_record)
• Service Layer: Business workflows, error handling, dependency orchestration
• Repository Layer: CRUD operations, query logic, entity ↔ record transformations
• Router Layer: Request validation, response serialization, status codes

See references/boundaries.md for comprehensive layer-specific examples.

Entity Testing Example

Test transformations and business logic:

def test_product_from_request():
    """Test creation from request"""
    request = CreateProductRequest(name="Widget", price=Decimal("9.99"))
    product = Product.from_request(request)

    assert product.name == "Widget"
    assert product.price == Decimal("9.99")
    assert isinstance(product.id, UUID)

def test_product_apply_discount():
    """Test business logic"""
    product = Product(id=uuid4(), name="Widget", price=Decimal("100"))
    discounted = product.apply_discount(Decimal("0.1"))

    assert discounted.price == Decimal("90")

Service Testing Example

Test orchestration with stubbed dependencies:

from unittest.mock import Mock

def test_create_product_service():
    """Test with mocked repository"""
    mock_repo = Mock()
    mock_repo.save.return_value = Product(id=uuid4(), name="Widget")

    service = ProductService(repo=mock_repo)
    result = service.create(CreateProductRequest(name="Widget", price=Decimal("9.99")))

    mock_repo.save.assert_called_once()
    assert result.name == "Widget"

Repository Testing Example

Test data access with real test database:

@pytest.fixture
def test_db():
    """In-memory test database"""
    engine = create_engine("sqlite:///:memory:")
    Base.metadata.create_all(engine)
    return sessionmaker(bind=engine)()

def test_repository_save(test_db):
    """Test database operations"""
    repo = ProductRepository(test_db)
    product = Product(id=uuid4(), name="Widget", price=Decimal("9.99"))

    saved = repo.save(product)

    assert saved.id == product.id
    assert test_db.query(ProductRecord).count() == 1

Router Testing Example

Test HTTP layer with TestClient:

from fastapi.testclient import TestClient

def test_create_product_endpoint():
    """Test POST endpoint"""
    client = TestClient(app)

    response = client.post(
        "/products",
        json={"name": "Widget", "price": 9.99},
    )

    assert response.status_code == 201
    assert response.json()["name"] == "Widget"

Test Organization Basics

tests/
├── unit/
│   ├── test_entities.py      # Entity + Value object tests
│   └── test_services.py      # Service tests (with mocks)
├── integration/
│   ├── test_repositories.py  # Repository tests (with DB)
│   └── test_endpoints.py     # Router tests (with client)
└── conftest.py               # Shared fixtures

Reference Documentation

For comprehensive patterns and examples, see:

• references/boundaries.md - Layer boundary testing patterns with complete examples for each layer
• references/mocking.md - Mock strategies, verification methods, and anti-patterns
• references/pytest.md - Configuration, fixtures, markers, parametrization, and debugging

Progressive disclosure: SKILL.md provides quick reference, references/ contain full details.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: martinffx
• Source: martinffx/atelier
• License: MIT
• Homepage: https://github.com/martinffx/atelier

Install and usage instructions live in the source repository linked above.