Imported from bobmatnyc/claude-mpm-skills (universal/testing/testing-anti-patterns/SKILL.md). Install upstream with npx skills add bobmatnyc/claude-mpm-skills --skill testing-anti-patterns. Copyright stays with the author.

Testing Anti-Patterns

Overview

Tests must verify real behavior, not mock behavior. Mocks are a means to isolate, not the thing being tested.

Core principle: Test what the code does, not what the mocks do.

Following strict TDD prevents these anti-patterns. See the Test-Driven Development skill (available in the skill library) for the complete TDD workflow.

When to Use This Skill

Activate this skill when:

• Writing or changing tests - Verify tests cover real behavior
• Adding mocks - Ensure mocking is necessary and correct
• Reviewing test failures - Check if mock behavior is the issue
• Tempted to add test-only methods - STOP and reconsider
• Tests feel overly complex - Sign of over-mocking

The Iron Laws

1. NEVER test mock behavior
2. NEVER add test-only methods to production classes
3. NEVER mock without understanding dependencies
4. NEVER create incomplete mocks
5. NEVER treat tests as afterthought

Core Anti-Pattern Categories

1. Testing Mock Behavior

Asserting on mock elements instead of real behavior. Fix: Test real component or don't mock it. → core-anti-patterns.md

2. Test-Only Methods in Production

Methods in production classes only used by tests. Fix: Move to test utilities. → core-anti-patterns.md

3. Mocking Without Understanding

Mocking without understanding dependencies/side effects. Fix: Understand first, mock minimally. → core-anti-patterns.md

4. Incomplete Mocks

Partial mocks missing fields downstream code needs. Fix: Mirror complete API structure. → completeness-anti-patterns.md

5. Tests as Afterthought

Implementation "complete" without tests. Fix: TDD - write test first. → completeness-anti-patterns.md

Quick Detection Checklist

Run this checklist before committing any test:

Language-agnostic checks:

□ Am I asserting on mock behavior instead of real behavior?
  → TypeScript: testId='*-mock', expect(mock).toHaveBeenCalled()
  → Python: mock.assert_called(), mock.call_count
  → If yes: STOP - Test real behavior or unmock

□ Does this method only exist for tests?
  → TypeScript: destroy(), reset(), clear() only in *.test.ts
  → Python: _set_mock_*, _for_testing only in test_*.py
  → If yes: STOP - Move to test utilities

□ Do I fully understand what I'm mocking?
  → If no: STOP - Run with real impl first, then mock minimally

□ Is my mock missing fields the real API has?
  → TypeScript: Partial<T>, incomplete objects
  → Python: Mock() with few attributes, missing nested fields
  → If yes: STOP - Mirror complete API structure

□ Did I write implementation before test?
  → If yes: STOP - Delete impl, write test first (TDD)

□ Is mock setup >50% of test code?
  → If yes: Consider integration test with real components

See: detection-guide.md for comprehensive red flags and warning signs.

The Bottom Line

Mocks are tools to isolate, not things to test.

Testing mock behavior indicates a problem. Fix: Test real behavior or question why mocking is necessary.

TDD prevents these patterns. Write test first → Watch fail → Minimal implementation → Pass → Refactor.

Navigation

Detailed Anti-Pattern Analysis

• Core Anti-Patterns - Patterns 1-3: Mock behavior, test-only methods, uninformed mocking
• Completeness Anti-Patterns - Patterns 4-5: Incomplete mocks, tests as afterthought

Detection & Prevention

• Detection Guide - Red flags, warning signs, gate functions
• TDD Connection - How test-driven development prevents these patterns

Language-Specific Examples

• Python Examples - Complete Python/pytest guide covering all 5 anti-patterns with unittest.mock and pytest-mock patterns, fixture best practices, and pytest-specific detection. Load when working with Python tests.

Related Skills

When using this skill, consider these complementary skills (if deployed in your skill bundle):

• test-driven-development: Complete TDD workflow and red-green-refactor cycle

  • Use case: Implementing TDD discipline to prevent anti-patterns
  • Integration: TDD workflow prevents most anti-patterns by design
  • Status: Recommended - basic anti-patterns covered in this skill

• verification-before-completion: Definition of "done" and verification protocols

  • Use case: Ensuring tests are part of completion criteria
  • Integration: Tests must pass before work is considered complete
  • Status: Recommended - testing mindset reinforcement

Note: All skills are independently deployable. This skill is fully functional without them.

Key Reminders

1. Mocks isolate, don't prove - Test real code, not mocks
2. Production ignores tests - No test-only methods
3. Understand before mocking - Know dependencies and side effects
4. Complete mocks only - Mirror full API structure
5. Tests ARE implementation - Not optional afterthought

Red Flags - STOP

STOP immediately when:

• Testing mock behavior
  • TypeScript: Asserting on *-mock test IDs, expect(mock).toHaveBeenCalled()
  • Python: mock.assert_called(), mock.call_count without real behavior checks
• Adding test-only methods
  • TypeScript: destroy(), reset() only in *.test.ts
  • Python: _set_mock_*, _for_testing with "For testing only" docstrings
• Mocking without understanding
  • Adding @patch or vi.mock() "just to be safe"
  • Creating mocks from memory instead of API docs
• Incomplete mocks
  • TypeScript: Partial<T>, missing nested objects
  • Python: Mock() for data objects, missing required fields
• Tests as afterthought
  • Saying "tests can wait" or "ready for testing"
  • Implementation commits before test commits

When mocks become too complex: Consider integration tests with real components. Often simpler and more valuable.

Integration with Other Skills

Prerequisite: Test-Driven Development skill - TDD prevents anti-patterns (recommended for complete workflow) Complementary: Verification-Before-Completion skill - Tests = done (ensures proper testing discipline) Domain-specific: webapp-testing, backend-testing for framework patterns (see skill library if available)