Designing Tests

Strategies and patterns for designing effective, maintainable test suites.

When to Use This Skill

• Planning test coverage for new features
• Choosing between testing approaches (TDD, BDD)
• Designing integration or E2E tests
• Improving existing test suites
• Setting up testing infrastructure
• Debugging flaky tests

---

The Testing Pyramid

                 ┌─────────┐
                 │   E2E   │  ← Few, slow, expensive
                 │  Tests  │     (Selenium, Playwright)
                 ├─────────┤
                 │         │
              ┌──┤ Integr- │  ← Some, medium speed
              │  │  ation  │     (API tests, DB tests)
              │  │  Tests  │
              │  ├─────────┤
              │  │         │
              │  │  Unit   │  ← Many, fast, cheap
              │  │  Tests  │     (Pure functions, isolated)
              └──┴─────────┘

Level	Speed	Scope	Quantity	Purpose
Unit	~ms	Single function/class	Many (70-80%)	Logic correctness
Integration	~s	Multiple components	Some (15-20%)	Component interaction
E2E	~10s+	Full system	Few (5-10%)	User flows work

---

Test-Driven Development (TDD)

The Red-Green-Refactor Cycle

     ┌─────────────────────────────────┐
     │                                 │
     ▼                                 │
┌─────────┐    ┌─────────┐    ┌────────┴──┐
│   RED   │───▶│  GREEN  │───▶│ REFACTOR  │
│  Write  │    │  Make   │    │  Clean    │
│ failing │    │   it    │    │   up      │
│  test   │    │  pass   │    │  code     │
└─────────┘    └─────────┘    └───────────┘

TDD Best Practices

1. Write the test first - Don't write production code without a failing test
2. Write the minimal test - One behavior per test
3. Write the minimal code - Just enough to pass
4. Refactor ruthlessly - Clean up after green
5. Run tests frequently - After every small change

TDD Example Flow

# Step 1: RED - Write failing test
def test_calculate_total_with_discount():
    order = Order(items=[Item(price=100)])
    order.apply_discount(10)  # 10%
    assert order.total() == 90

# Step 2: GREEN - Minimal implementation
class Order:
    def __init__(self, items):
        self.items = items
        self.discount = 0

    def apply_discount(self, percent):
        self.discount = percent

    def total(self):
        subtotal = sum(i.price for i in self.items)
        return subtotal * (100 - self.discount) / 100

# Step 3: REFACTOR - Clean up (if needed)

---

Behavior-Driven Development (BDD)

Gherkin Syntax

Feature: Shopping Cart
  As a customer
  I want to add items to my cart
  So that I can purchase them later

  Scenario: Add item to empty cart
    Given I have an empty cart
    When I add a product "Widget" priced at $10
    Then my cart should contain 1 item
    And my cart total should be $10

  Scenario: Apply discount code
    Given I have a cart with total $100
    When I apply discount code "SAVE10"
    Then my cart total should be $90

BDD Benefits

• Tests as documentation
• Shared language with stakeholders
• Focus on behavior, not implementation
• Easy to understand test intent

---

Test Design Patterns

Arrange-Act-Assert (AAA)

def test_user_registration():
    # Arrange - Set up preconditions
    user_data = {"email": "test@example.com", "password": "secure123"}
    user_service = UserService(mock_repository)

    # Act - Perform the action
    result = user_service.register(user_data)

    # Assert - Verify the outcome
    assert result.success is True
    assert result.user.email == "test@example.com"

Given-When-Then (BDD style)

def test_order_cancellation():
    # Given - a confirmed order
    order = create_confirmed_order()

    # When - the customer cancels it
    order.cancel()

    # Then - the order is cancelled and refund initiated
    assert order.status == "cancelled"
    assert order.refund_initiated is True

Test Data Builders

class UserBuilder:
    def __init__(self):
        self.email = "default@test.com"
        self.name = "Test User"
        self.role = "user"

    def with_email(self, email):
        self.email = email
        return self

    def with_role(self, role):
        self.role = role
        return self

    def build(self):
        return User(email=self.email, name=self.name, role=self.role)

# Usage
admin = UserBuilder().with_role("admin").build()

Object Mother Pattern

class TestUsers:
    @staticmethod
    def admin():
        return User(email="admin@test.com", role="admin")

    @staticmethod
    def customer():
        return User(email="customer@test.com", role="customer")

    @staticmethod
    def guest():
        return User(email=None, role="guest")

---

Mocking Strategies

When to Mock

Mock	Don't Mock
External APIs	Pure business logic
Database (for unit tests)	Simple value objects
File system	Deterministic functions
Time/random	Core domain entities
Third-party services	Internal collaborators (usually)

Mock Types

Type	Purpose	Example
Stub	Return canned responses	mock.return_value = 42
Mock	Verify interactions	mock.assert_called_with(...)
Spy	Track real calls	Wraps real object, records calls
Fake	Simplified implementation	In-memory database

Mocking Example

# Using unittest.mock
from unittest.mock import Mock, patch

def test_send_email_on_registration():
    # Arrange
    mock_email_service = Mock()
    user_service = UserService(email_service=mock_email_service)

    # Act
    user_service.register({"email": "test@example.com"})

    # Assert
    mock_email_service.send_welcome_email.assert_called_once_with("test@example.com")

# Using patch decorator
@patch("app.services.EmailService")
def test_with_patch(mock_email_class):
    mock_email_class.return_value.send.return_value = True
    # Test code...

---

Integration Test Patterns

Database Tests

import pytest
from testcontainers.postgres import PostgresContainer

@pytest.fixture(scope="session")
def database():
    with PostgresContainer("postgres:15") as postgres:
        yield postgres.get_connection_url()

def test_user_persistence(database):
    repo = UserRepository(database)
    user = User(email="test@example.com")

    repo.save(user)
    retrieved = repo.find_by_email("test@example.com")

    assert retrieved.email == user.email

API Tests

def test_create_user_endpoint(client):
    response = client.post("/api/users", json={
        "email": "new@example.com",
        "password": "secure123"
    })

    assert response.status_code == 201
    assert response.json["email"] == "new@example.com"
    assert "id" in response.json

---

E2E Test Patterns

Page Object Model

class LoginPage:
    def __init__(self, page):
        self.page = page
        self.email_input = page.locator("#email")
        self.password_input = page.locator("#password")
        self.submit_button = page.locator("button[type=submit]")

    def login(self, email, password):
        self.email_input.fill(email)
        self.password_input.fill(password)
        self.submit_button.click()
        return DashboardPage(self.page)

# Usage
def test_successful_login(page):
    login_page = LoginPage(page)
    dashboard = login_page.login("user@example.com", "password")
    assert dashboard.welcome_message.is_visible()

E2E Best Practices

1. Use stable selectors - data-testid, not CSS classes
2. Wait for conditions - Not arbitrary sleeps
3. Isolate test data - Each test gets fresh data
4. Test critical paths - Happy paths, key user journeys
5. Keep them fast - Parallelize, minimize scope

---

Test Coverage Strategy

What to Cover

Priority	What	Why
High	Business logic	Core value
High	Edge cases	Where bugs hide
High	Error paths	Graceful failures
Medium	Integration points	Contract validation
Low	UI layout	Brittle, low value
Low	Third-party code	Not your responsibility

Coverage Metrics

Metric	Target	Notes
Line coverage	70-80%	Basic minimum
Branch coverage	60-70%	Catches conditionals
Mutation score	50-70%	Measures test quality

Meaningful Coverage

HIGH VALUE:
  ✓ Core business logic
  ✓ Data transformations
  ✓ Error handling
  ✓ Security-sensitive code

LOW VALUE:
  ✗ Getters/setters
  ✗ Constructor-only classes
  ✗ Framework boilerplate
  ✗ Configuration files

---

Handling Flaky Tests

Common Causes

Cause	Solution
Timing issues	Use explicit waits, not sleep
Shared state	Isolate test data
External dependencies	Mock or use containers
Race conditions	Add synchronization
Date/time	Mock time providers
Random data	Seed random generators

Flaky Test Checklist

• Is the test relying on timing?
• Is there shared state between tests?
• Is there an external dependency?
• Is the order of execution assumed?
• Is there non-deterministic data?

---

Test Organization

File Structure

tests/
├── unit/                    # Unit tests
│   ├── services/
│   │   └── test_user_service.py
│   └── models/
│       └── test_order.py
├── integration/             # Integration tests
│   ├── api/
│   │   └── test_user_endpoints.py
│   └── repositories/
│       └── test_user_repository.py
├── e2e/                     # End-to-end tests
│   └── test_checkout_flow.py
├── fixtures/                # Shared fixtures
│   └── factories.py
└── conftest.py              # Pytest configuration

Naming Conventions

# Pattern: test_[what]_[condition]_[expected]

def test_calculate_total_with_discount_returns_reduced_price():
    pass

def test_login_with_invalid_password_raises_auth_error():
    pass

def test_order_when_cancelled_sends_refund_notification():
    pass

---

Anti-Patterns to Avoid

1. Testing implementation, not behavior - Tests break on refactor
2. Large test methods - Hard to debug, unclear intent
3. Excessive mocking - Tests don't reflect reality
4. Shared mutable state - Tests affect each other
5. Ignoring test failures - Broken windows effect
6. Testing private methods - Coupling to implementation
7. No assertion - Tests that can't fail
8. Copy-paste tests - Maintenance nightmare

---

Quick Reference

PYRAMID:
  Unit (70%) → Integration (20%) → E2E (10%)

TDD CYCLE:
  Red → Green → Refactor

PATTERNS:
  AAA: Arrange-Act-Assert
  Builder: Fluent test data creation
  Page Object: E2E abstraction

MOCK WHEN:
  External APIs, Database (unit), Time, Random

COVERAGE:
  70-80% line, focus on business logic

NAMING:
  test_[what]_[condition]_[expected]