Test Writing Patterns

Version: v0.18.0

Test structure patterns, assertion strategies, test doubles, and organizational practices.

When to Use

• Need guidance on test structure
• Questions about test organization
• Need test double (mock/stub/fake) guidance
• Uncertainty about assertion strategies

Test Structure Patterns

AAA Pattern (Arrange-Act-Assert)

ARRANGE: Set up test conditions and inputs
ACT: Execute the behavior being tested
ASSERT: Verify the expected outcome

Given-When-Then (BDD Style)

GIVEN = ARRANGE (preconditions)
WHEN = ACT (action/event)
THEN = ASSERT (expected outcomes)

Test Organization

File Organization: Mirror production structure Naming: test_[unit]_[scenario]_[expected] Suite Organization: By test type (unit/integration/e2e) or by feature

Assertion Strategies

Single Concept Per Test: One assertion or set of related assertions Good Assertions: Specific, include messages, test behavior not implementation

Common Types:

• Equality: assert actual == expected
• Truthiness: assert condition is true
• Comparison: assert value > 0
• Collection: assert item in collection
• Exception: assert raises(ExpectedException)

Test Doubles

Selection:

• Need controlled response? → Stub
• Need to verify call made? → Mock
• Need simple working version? → Fake
• Need real behavior + verification? → Spy

Test Isolation

Each test should:

• Set up its own data
• Clean up after itself
• Run in any order
• Not depend on other tests

Achieving Isolation:

• Setup/Teardown per test
• Fixtures (fresh for each test)
• Database transactions/rollback

Test Data Strategies

Prefer: Explicit, minimal, clear data Avoid: Random data, excessive fields

Test Data Builders:

user = UserBuilder().with_name("Alice").with_role("admin").build()

Test Types

Test Smells and Fixes

Coverage Guidelines

• Focus on meaningful tests, not coverage numbers
• High coverage of critical paths
• Use coverage to find untested code
• TDD naturally produces good coverage

End of Test Writing Patterns Skill