Agent skill
test-writing-patterns
Test structure, patterns, assertions, and test doubles
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npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/testing/test-writing-patterns-rubrical-studios-gh-pmu-02dc29ee
SKILL.md
Test Writing Patterns
When to Use
- Need test structure guidance
- Questions about test doubles (mock/stub/fake/spy)
- Test organization questions
- Framework-agnostic test patterns
Test Structure Patterns
AAA Pattern (Arrange-Act-Assert)
# Arrange - Set up test data and preconditions
user = User(name="alice")
calculator = Calculator()
# Act - Execute behavior being tested
result = calculator.add(2, 3)
# Assert - Verify expected outcome
assert result == 5
Given-When-Then (BDD Style)
# Given - Preconditions
given_user_exists("alice")
# When - Action
result = login("alice", "password")
# Then - Verification
then_user_is_authenticated(result)
Test Doubles
| Type | Purpose | When to Use |
|---|---|---|
| Mock | Verify interactions | Check method was called |
| Stub | Provide canned responses | Replace dependency output |
| Fake | Working implementation | Lightweight replacement |
| Spy | Record calls | Verify calls while keeping behavior |
Mock Example
python
mock_db = Mock()
service = UserService(db=mock_db)
service.create_user("alice")
mock_db.save.assert_called_once_with("alice")
Stub Example
python
stub_api = Mock()
stub_api.get_user.return_value = {"name": "alice"}
result = service.fetch_user(api=stub_api)
assert result["name"] == "alice"
Assertion Patterns
Good Assertions
- Specific expected values
- Descriptive failure messages
- One concept per assertion
python
assert result == 5, f"Expected 5, got {result}"
assert user.is_active is True
assert "error" in response.json()
Poor Assertions
python
assert result # Not specific
assert True # Always passes
assert result != None # Use is not None
Test Organization
tests/
├── unit/ # Fast, isolated tests
├── integration/ # Component interaction tests
└── e2e/ # Full system tests
Naming Convention
test_[unit]_[scenario]_[expected]
test_add_two_positive_numbers_returns_sum
test_login_invalid_password_returns_error
test_create_user_duplicate_email_raises_exception
Common Test Patterns
Setup/Teardown
setUp()/tearDown()for each test- Fixtures for shared resources
- Clean state between tests
Parameterized Tests
Run same test with different data:
python
@pytest.mark.parametrize("a,b,expected", [(1,2,3), (0,0,0), (-1,1,0)])
def test_add(a, b, expected):
assert add(a, b) == expected
Test Independence
- Tests don't depend on each other
- Tests can run in any order
- Each test sets up its own state
Anti-Patterns
❌ Tests depending on other tests ❌ Testing implementation details ❌ Slow tests in unit suite ❌ No assertions ❌ Commenting out failing tests
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