## Testing Principles

### Testing Philosophy

- **Test-Driven Development (TDD)**: Write tests before implementation when feasible
- **Test behavior, not implementation**: Tests should validate outcomes, not internal details
- **Arrange-Act-Assert (AAA)**: Structure tests clearly: setup, execute, verify
- **Independent tests**: Each test should run in isolation without dependencies
- **Fast tests**: Unit tests should run in milliseconds, integration tests in seconds

### Test Coverage

- **Minimum coverage target**: 80-85% code coverage
- **Critical paths**: 100% coverage for core business logic and security features
- **Edge cases**: Test boundary conditions, null values, empty collections
- **Error paths**: Test failure scenarios and error handling

### Test Types

**Unit Tests:**
- Test individual functions/methods in isolation
- Use mocks/stubs for dependencies
- Fast execution (< 1ms per test)
- High volume (majority of tests)

**Integration Tests:**
- Test interaction between components
- May use real databases/services (or realistic mocks)
- Medium execution time
- Moderate volume

**End-to-End Tests:**
- Test complete user workflows
- Simulate real user behavior
- Slower execution
- Fewer tests, focus on critical paths

### Test Naming

Use descriptive names that explain what is being tested:

**Good:**
```
test_create_user_with_valid_email_succeeds()
test_create_user_with_duplicate_email_raises_error()
test_create_user_with_invalid_email_format_raises_validation_error()
```

**Bad:**
```
test_user_1()
test_user_creation()
test_error()
```

### Testing Best Practices

- **One assertion per test**: Tests should verify one specific behavior
- **No logic in tests**: Tests should be simple and straightforward
- **Use fixtures/factories**: Create reusable test data setup
- **Mock external services**: Don't depend on network, APIs, or external systems in unit tests
- **Run tests automatically**: CI/CD should run full test suite on every commit
- **Keep tests maintainable**: Refactor tests when refactoring code
