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name: test-writing-patterns description: Guide experienced developers on test structure, patterns, assertions, and test doubles for effective test-driven development license: Complete terms in LICENSE.txt

Test Writing Patterns

Version: {{VERSION}}

When to Use

• User needs 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)

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

Given-When-Then Pattern (BDD Style)

CopyGIVEN: Initial context/preconditions
WHEN: Action/event occurs
THEN: Expected outcomes

Test Organization

File Organization

CopyProduction:
  src/services/user_service

Tests:
  tests/services/test_user_service

Test Naming

Copytest_[unit]_[scenario]_[expected]

Examples:
- test_add_positive_numbers_returns_sum
- test_get_user_when_not_found_returns_null

Assertion Strategies

Single Concept Per Test

Good: All assertions verify the same concept Poor: Multiple unrelated assertions in one test

Common Assertion 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

Types

Selection Guide

CopyNeed to control response? -> Stub
Need to verify call made? -> Mock
Need working but simple 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

Test Data Strategies

Explicit Test Data

CopyGood: user = create_user(name="Alice", age=30)
Poor: user = create_user(name=random_string())

Minimal Test Data

Use simplest data that tests the behavior.

Testing Strategies by Type

Unit Tests

• Test single unit
• Fast execution
• No external dependencies

Integration Tests

• Test multiple units together
• May use real dependencies

End-to-End Tests

• Test complete user workflows
• Use real system

Test Coverage

Coverage shows:

• Which code is executed by tests
• Which code is NOT executed

Coverage does NOT mean:

• All behaviors tested
• Tests are good quality
• Code is correct

Test Smells

Parameterized Tests

Copytest_add_numbers:
  parameters:
    (2, 3, 5)
    (-2, -3, -5)
    (0, 0, 0)

  for each (a, b, expected):
    assert add(a, b) == expected

Resources

• resources/aaa-pattern-template.md
• resources/test-doubles-guide.md
• resources/assertion-patterns.md
• resources/test-organization-examples.md

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End of Test Writing Patterns Skill