Test Design: Universal Principles

These principles apply to every test you write, regardless of language or framework. Consult the active stack profile for framework-specific syntax; this skill provides the design thinking.

1. Test Isolation

Each test must be independent. Running tests in any order, or running a single test in isolation, must produce the same result.

Rules:

• No shared mutable state between tests. Each test sets up its own data.
• Use fixtures (pytest fixtures, setUp/tearDown, @Before/@After) for per-test setup.
• Avoid class-level or module-level state that persists across tests.
• Never depend on test execution order.

Anti-pattern:

# BAD: test_update depends on state from test_create
class TestUser:
    user_id = None