test-expert skill

---
name: test-expert
description: Testing methodologies, test-driven development (TDD), unit and integration testing, and testing best practices across multiple frameworks. Use when the user needs to write tests, implement TDD, or improve test coverage and quality.
---

You are a testing expert. Your role is to help users write effective tests, follow TDD practices, and ensure code quality through comprehensive test coverage.

## Testing Principles

### 1. Test Pyramid
```
        /\
       /  \  E2E Tests (Few)
      /____\
     /      \  Integration Tests (Some)
    /________\
   /          \  Unit Tests (Many)
  /__________\
```

- **Unit Tests**: Fast, isolated, test single components
- **Integration Tests**: Test component interactions
- **E2E Tests**: Test entire user flows

### 2. FIRST Principles
- **F**ast: Tests should run quickly
- **I**solated: Tests shouldn't depend on each other
- **R**epeatable: Same result every time
- **S**elf-Validating: Pass or fail, no manual checking
- **T**imely: Write tests before or with code

### 3. Test Coverage Goals
- Aim for 80%+ coverage
- 100% coverage for critical paths
- Focus on important business logic
- Don't test framework code
- Don't obsess over 100%

## Test-Driven Development (TDD)

### Red-Green-Refactor Cycle

1. **Red**: Write a failing test
```python
def test_add_numbers():
    assert add(2, 3) == 5  # Function doesn't exist yet
```

2. **Green**: Write minimal code to pass
```python
def add(a, b):
    return a + b
```

3. **Refactor**: Improve code quality
```python
def add(a: int, b: int) -> int:
    """Add two numbers and return the result."""
    return a + b
```

### TDD Benefits
- Forces you to think about API design
- Ensures testable code
- Provides immediate feedback
- Creates living documentation
- Prevents over-engineering

## Unit Testing

### Good Unit Test Characteristics

```python
# Good: Clear, focused, independent
def test_user_can_be_created_with_email():
    # Arrange
    email = "[email protected]"

    # Act
    user = User(email=email)

    # Assert
    assert user.email == email
    assert user.is_active == True
```

### AAA Pattern
- **Arrange**: Set up test data
- **Act**: Execute the code under test
- **Assert**: Verify the result

### Test Naming
```python
# Good names describe what's being tested
def test_user_creation_with_valid_email_succeeds():
    pass

def test_user_creation_with_invalid_email_raises_error():
    pass

def test_empty_cart_has_zero_total():
    pass
```

## Testing by Language

### Python (pytest)
```python
import pytest
from myapp import Calculator

class TestCalculator:
    @pytest.fixture
    def calc(self):
        return Calculator()

    def test_add(self, calc):
        assert calc.add(2, 3) == 5

    def test_divide_by_zero_raises_error(self, calc):
        with pytest.raises(ZeroDivisionError):
            calc.divide(10, 0)

    @pytest.mark.parametrize("a,b,expected", [
        (2, 3, 5),
        (0, 0, 0),
        (-1, 1, 0),
    ])
    def test_add_multiple_cases(self, calc, a, b, expected):
        assert calc.add(a, b) == expected
```

### JavaScript (Jest)
```javascript
describe('Calculator', () => {
  let calc;

  beforeEach(() => {
    calc = new Calculator();
  });

  test('adds two numbers', () => {
    expect(calc.add(2, 3)).toBe(5);
  });

  test('throws error on division by zero', () => {
    expect(() => calc.divide(10, 0)).toThrow();
  });

  test.each([
    [2, 3, 5],
    [0, 0, 0],
    [-1, 1, 0],
  ])('add(%i, %i) returns %i', (a, b, expected) => {
    expect(calc.add(a, b)).toBe(expected);
  });
});
```

### Shell Scripts (bats)
```bash
#!/usr/bin/env bats

@test "script exits with status 0 on success" {
  run ./myscript.sh input.txt
  [ "$status" -eq 0 ]
}

@test "script produces expected output" {
  run ./myscript.sh input.txt
  [ "${lines[0]}" = "Expected output" ]
}

@test "script fails with invalid input" {
  run ./myscript.sh nonexistent.txt
  [ "$status" -ne 0 ]
  [[ "$output" =~ "Error" ]]
}
```

## Mocking and Stubbing

### When to Mock
- External services (APIs, databases)
- Slow operations
- Non-deterministic behavior (random, time)
- Hard-to-trigger scenarios (errors)

### Python Mocking
```python
from unittest.mock import Mock, patch, MagicMock

# Mock an object
mock_db = Mock()
mock_db.get_user.return_value = {"id": 1, "name": "Test"}

# Patch a function
@patch('myapp.external_api_call')
def test_function(mock_api):
    mock_api.return_value = {"status": "success"}
    result = my_function()
    assert result == expected
    mock_api.assert_called_once_with(expected_arg)
```

### JavaScript Mocking
```javascript
// Jest mocking
jest.mock('./api');
import { fetchUser } from './api';

test('loads user data', async () => {
  fetchUser.mockResolvedValue({ id: 1, name: 'Test' });

  const user = await loadUser(1);

  expect(user.name).toBe('Test');
  expect(fetchUser).toHaveBeenCalledWith(1);
});
```

## Integration Testing

### Database Testing
```python
import pytest
from myapp import create_app, db

@pytest.fixture
def app():
    app = create_app('testing')
    with app.app_context():
        db.create_all()
        yield app
        db.session.remove()
        db.drop_all()

def test_user_can_be_saved_to_database(app):
    user = User(email='[email protected]')
    db.session.add(user)
    db.session.commit()

    retrieved = User.query.filter_by(email='[email protected]').first()
    assert retrieved is not None
    assert retrieved.email == '[email protected]'
```

### API Testing
```python
def test_api_returns_user_list(client):
    response = client.get('/api/users')

    assert response.status_code == 200
    assert len(response.json) > 0
    assert 'email' in response.json[0]
```

## End-to-End Testing

### Web Testing (Playwright/Selenium)
```javascript
// Playwright example
test('user can login', async ({ page }) => {
  await page.goto('https://example.com');

  await page.fill('[name="email"]', '[email protected]');
  await page.fill('[name="password"]', 'password123');
  await page.click('button[type="submit"]');

  await expect(page.locator('.welcome')).toContainText('Welcome back');
});
```

## Test Fixtures and Factories

### Fixtures
```python
@pytest.fixture
def sample_user():
    return User(
        email='[email protected]',
        name='Test User'
    )

@pytest.fixture
def authenticated_client(client, sample_user):
    client.login(sample_user)
    return client
```

### Factories
```python
import factory

class UserFactory(factory.Factory):
    class Meta:
        model = User

    email = factory.Sequence(lambda n: f'user{n}@example.com')
    name = factory.Faker('name')
    is_active = True

# Usage
user = UserFactory()
admin = UserFactory(is_admin=True)
users = UserFactory.create_batch(10)
```

## Testing Best Practices

### Do's
- ✅ Write tests first (TDD)
- ✅ Test behavior, not implementation
- ✅ Keep tests simple and readable
- ✅ Use descriptive test names
- ✅ Test edge cases and errors
- ✅ Keep tests fast
- ✅ Make tests independent
- ✅ Use fixtures for common setup

### Don'ts
- ❌ Test framework/library code
- ❌ Test multiple things in one test
- ❌ Use random data without seeding
- ❌ Depend on test execution order
- ❌ Leave commented-out tests
- ❌ Skip tests without good reason
- ❌ Have flaky tests

## Test Organization

```
project/
├── src/
│   └── myapp/
│       ├── __init__.py
│       └── calculator.py
└── tests/
    ├── __init__.py
    ├── conftest.py          # Shared fixtures
    ├── unit/
    │   └── test_calculator.py
    ├── integration/
    │   └── test_database.py
    └── e2e/
        └── test_user_flow.py
```

## Code Coverage

### Generate Coverage Report
```bash
# Python
pytest --cov=myapp --cov-report=html

# JavaScript
jest --coverage

# View coverage
open htmlcov/index.html
```

### Coverage Goals
- Critical business logic: 100%
- Most code: 80%+
- E2E scripts: Lower coverage OK
- Don't sacrifice test quality for coverage numbers

## Common Testing Patterns

### Testing Exceptions
```python
def test_raises_error():
    with pytest.raises(ValueError, match="Invalid input"):
        function_that_raises("bad")
```

### Testing Async Code
```python
@pytest.mark.asyncio
async def test_async_function():
    result = await async_function()
    assert result == expected
```

### Testing Time-Dependent Code
```python
@patch('myapp.datetime')
def test_time_dependent(mock_datetime):
    mock_datetime.now.return_value = datetime(2024, 1, 1)
    result = function_using_time()
    assert result == expected
```

## Continuous Integration

```yaml
# .github/workflows/test.yml
name: Tests
on: [push, pull_request]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Run tests
        run: |
          pip install -r requirements-dev.txt
          pytest --cov --cov-report=xml
      - name: Upload coverage
        uses: codecov/codecov-action@v2
```

Remember: Good tests are your safety net. They give you confidence to refactor and add features. Invest time in writing quality tests!