test-writing-patterns

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)

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

Given-When-Then Pattern (BDD Style)

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

Test Organization

File Organization

Production:
  src/services/user_service

Tests:
  tests/services/test_user_service

Test Naming

test_[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

Type	Purpose	When to Use
Stub	Provide predetermined responses	Control dependency behavior
Mock	Verify interactions/calls	Check method was called
Fake	Working simplified implementation	Integration testing
Spy	Record calls while delegating	Need real behavior + verification

Selection Guide

Need 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

Good: 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

Smell	Fix
Test does too much	Split into focused tests
Tests are brittle	Test behavior, not implementation
Tests are slow	Use test doubles, optimize setup
Tests are unclear	Better naming, clear AAA structure
Tests depend on each other	Ensure test isolation

Parameterized Tests

test_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