Testing Basics

Core testing patterns that apply to any language or framework.

Test Structure

AAA Pattern

Arrange → Act → Assert

# Arrange
user = User(name="Alice")

# Act
result = user.greet()

# Assert
assert result == "Hello, Alice!"

Test Types

Type	Scope	Speed	When to use
Unit	Single function/class	Fast	Core logic
Integration	Multiple components	Medium	APIs, DB
E2E	Full system	Slow	Critical paths

Naming Tests

test_[what]_[condition]_[expected]

Examples:

• test_login_valid_credentials_returns_token
• test_checkout_empty_cart_raises_error
• test_search_no_results_returns_empty_list

What to Test

Do Test

• Business logic
• Edge cases
• Error handling
• Public interfaces

Don't Test

• Framework code
• Simple getters/setters
• Implementation details
• External services directly

Test Isolation

Each test should:

• Run independently
• Not depend on order
• Clean up after itself
• Not share state

Mocking

Mock external dependencies, not internal logic:

# Good: mock external service
@patch('app.services.email.send')
def test_signup_sends_welcome_email(mock_send):
    signup(user)
    mock_send.assert_called_once()

# Bad: mock internal implementation
@patch('app.models.user.User._validate')  # Don't do this

Coverage

• Aim for meaningful coverage, not 100%
• Cover happy paths first
• Then error cases
• Then edge cases

Quick Checklist

Before committing:

• Tests pass locally
• New code has tests
• No flaky tests added
• Test names are descriptive