# Testing Strategies

## Overview

A comprehensive testing strategy ensures software quality and reliability. This guide covers different testing levels, strategies, and when to use each approach.

## Testing Pyramid

### Unit Tests

**Foundation Layer:**
- Test individual components
- Fast execution
- High coverage
- Isolated tests

**Characteristics:**
- Test single functions/classes
- Mock external dependencies
- Fast feedback
- Easy to maintain

**Example:**
```python
def test_calculate_total():
    items = [10, 20, 30]
    assert calculate_total(items) == 60
```

### Integration Tests

**Middle Layer:**
- Test component interactions
- Verify integrations work
- Test with real dependencies
- Moderate execution time

**Characteristics:**
- Test multiple components
- Use test databases
- Verify contracts
- Test API integrations

**Example:**
```python
def test_user_registration():
    user = register_user("test@example.com", "password")
    assert user.email == "test@example.com"
    assert user.is_active is True
```

### End-to-End Tests

**Top Layer:**
- Test complete workflows
- User-facing scenarios
- Slow execution
- Lower coverage

**Characteristics:**
- Test full user journeys
- Use real browsers
- Test critical paths
- Expensive to maintain

**Example:**
```python
def test_checkout_flow():
    # Navigate to product page
    # Add to cart
    # Checkout
    # Complete payment
    # Verify order
```

## Test Types

### Functional Testing

**Verify Functionality:**
- Test features work correctly
- Verify requirements
- Test user workflows
- Validate business logic

### Non-Functional Testing

**Quality Attributes:**
- Performance testing
- Security testing
- Usability testing
- Accessibility testing

### Regression Testing

**Prevent Breakage:**
- Test existing features
- Verify no regressions
- Run after changes
- Automated test suite

## Testing Strategies

### Test-Driven Development (TDD)

**Red-Green-Refactor:**
1. Write failing test
2. Write minimal code to pass
3. Refactor code
4. Repeat

**Benefits:**
- Better design
- High test coverage
- Confidence in code
- Documentation

### Behavior-Driven Development (BDD)

**Given-When-Then:**
- Natural language tests
- Collaboration with stakeholders
- Focus on behavior
- Clear scenarios

**Example:**
```gherkin
Feature: User login
  Scenario: Successful login
    Given a registered user
    When they enter valid credentials
    Then they should be logged in
```

### Acceptance Test-Driven Development (ATDD)

**Acceptance Criteria:**
- Write tests from requirements
- Verify acceptance criteria
- Collaboration with stakeholders
- Clear requirements

## Test Coverage

### Coverage Metrics

**Types of Coverage:**
- **Line Coverage**: Percentage of lines executed
- **Branch Coverage**: Percentage of branches executed
- **Function Coverage**: Percentage of functions called
- **Statement Coverage**: Percentage of statements executed

### Coverage Goals

**Target Coverage:**
- Unit tests: 80-90% coverage
- Critical paths: 100% coverage
- Integration tests: 60-70% coverage
- E2E tests: Cover critical paths

### Coverage Tools

**Common Tools:**
- Coverage.py (Python)
- JaCoCo (Java)
- Istanbul (JavaScript)
- Codecov, Coveralls

## Test Organization

### Test Structure

**Organize Tests:**
- Mirror source structure
- Group by feature
- Use descriptive names
- Separate test data

**Example:**
```
tests/
├── unit/
│   ├── test_user.py
│   └── test_order.py
├── integration/
│   ├── test_api.py
│   └── test_database.py
└── e2e/
    └── test_checkout.py
```

### Test Naming

**Descriptive Names:**
- Test what is being tested
- Include expected behavior
- Use clear language
- Follow conventions

**Example:**
```python
def test_calculate_total_returns_sum_of_items():
    # Test implementation
    pass

def test_calculate_total_handles_empty_list():
    # Test implementation
    pass
```

## Test Data Management

### Test Fixtures

**Reusable Test Data:**
- Create test fixtures
- Use factories
- Isolate test data
- Clean up after tests

**Example:**
```python
@pytest.fixture
def user():
    return User(email="test@example.com", password="password")

def test_user_login(user):
    assert login(user.email, user.password) is True
```

### Test Isolation

**Independent Tests:**
- Each test is independent
- No shared state
- Can run in any order
- Easy to debug

## Continuous Testing

### CI/CD Integration

**Automated Testing:**
- Run tests on every commit
- Fast feedback
- Prevent broken code
- Automated deployment

### Test Execution

**Execution Strategy:**
- Run fast tests first
- Parallel execution
- Fail fast
- Comprehensive reporting

## Best Practices

1. **Write Tests First**: TDD approach
2. **Test Behavior**: Focus on what, not how
3. **Keep Tests Simple**: One assertion per test
4. **Use Descriptive Names**: Clear test names
5. **Maintain Tests**: Update with code changes
6. **Test Edge Cases**: Boundary conditions
7. **Mock External Dependencies**: Isolate units
8. **Run Tests Frequently**: Continuous testing

## Common Mistakes

### Testing Implementation

**Problem:**
- Testing how, not what
- Brittle tests
- Break on refactoring

**Solution:**
- Test behavior
- Test public interface
- Focus on outcomes

### Over-Mocking

**Problem:**
- Too many mocks
- Tests don't reflect reality
- Hard to maintain

**Solution:**
- Mock external dependencies
- Use real objects when possible
- Balance isolation and realism

### Slow Tests

**Problem:**
- Tests take too long
- Slow feedback
- Reduced test frequency

**Solution:**
- Optimize test execution
- Use test doubles
- Parallel execution
- Separate slow tests

## References

- [Testing Strategies](https://martinfowler.com/articles/practical-test-pyramid.html)
- [Test-Driven Development](https://www.agilealliance.org/glossary/tdd/)

