test-driven-development

name	test-driven-development
description	Red-Green-Refactor: write tests first, then code to make them pass.
version	1.0.0
author	Open-Agent
triggers	["test","tdd","unit test","pytest","assert","coverage","spec","behavior"]
metadata	{"hermes":{"tags":["testing","tdd","quality","code-design"],"related_skills":["systematic-debugging","writing-plans"]}}

Test-Driven Development

The Cycle

1. RED    → Write failing test
2. GREEN  → Write minimal code to pass
3. REFACTOR → Clean up while keeping tests green

When to Use TDD

• New features — define behavior upfront
• Bug fixes — write test that reproduces bug first
• Refactoring — ensure behavior preserved
• API design — tests as spec

How to Write Tests

1. Name Clearly

def test_user_can_login_with_valid_credentials():
    ...

def test_login_fails_with_wrong_password():
    ...

2. Test One Thing

• Each test = one behavior
• No multiple assertions that could fail for different reasons

3. AAA Pattern

def test_something():
    # Arrange
    setup_state()
    
    # Act
    result = do_something()
    
    # Assert
    assert result == expected

4. Meaningful Assertions

# Bad
assert user.is_valid()

# Good  
assert user.has_permission("read")
assert user.email_verified == True

Common Mistakes

• Writing tests after code (not TDD)
• Testing implementation details
• Over-mocking
• Tests that depend on each other
• No edge case coverage

Quick Wins

• Start with edge cases: empty, null, zero
• Test error paths
• Test boundary conditions