testing

Testing Patterns Skill

Instructions for AI

Apply comprehensive testing best practices. Use this skill when designing test strategies, writing tests, or setting up testing infrastructure.

Test Pyramid

         /\
        / E2E \          ~5%  — Critical user journeys
       /--------\
      / Contract  \      ~10% — API contracts
     /--------------\
    /  Integration    \  ~25% — DB, queues, external APIs
   /--------------------\
  /     Unit Tests       \ ~60% — Business logic, pure functions
 /------------------------\

Unit Testing Patterns

Arrange-Act-Assert

class TestPriceCalculator:
    def test_applies_percentage_discount(self):
        # Arrange
        calculator = PriceCalculator()
        product = Product(price=100.0)
        discount = PercentageDiscount(rate=0.2)

        # Act
        result = calculator.calculate(product, discount)

        # Assert
        assert result == 80.0

Parameterized Tests

import pytest

@pytest.mark.parametrize("input_price,discount_rate,expected", [
    (100.0, 0.0, 100.0),
    (100.0, 0.1, 90.0),
    (100.0, 0.5, 50.0),
    (100.0, 1.0, 0.0),
    (0.0, 0.5, 0.0),
])
def test_discount_calculation(input_price, discount_rate, expected):
    result = calculate_discount(input_price, discount_rate)
    assert result == expected

Fakes over Mocks

class FakeEmailSender:
    """In-memory fake — captures sent emails for verification."""

    def __init__(self):
        self.sent_emails: list[Email] = []

    async def send(self, email: Email) -> None:
        self.sent_emails.append(email)

# In tests
async def test_order_confirmation_sends_email(fake_email_sender):
    service = OrderService(email_sender=fake_email_sender)
    await service.place_order(sample_order)
    
    assert len(fake_email_sender.sent_emails) == 1
    assert fake_email_sender.sent_emails[0].subject == "Order Confirmation"

Factory Pattern for Test Data

from polyfactory.factories.pydantic_factory import ModelFactory

class UserFactory(ModelFactory):
    __model__ = User
    
    @classmethod
    def admin(cls) -> User:
        return cls.build(role=UserRole.ADMIN)
    
    @classmethod
    def with_orders(cls, count: int = 3) -> User:
        user = cls.build()
        user.orders = OrderFactory.batch(count)
        return user