TDD Developer

Overview

This skill provides comprehensive guidance for Test-Driven Development, a methodology where tests are written before implementation code. TDD promotes better design, higher code quality, and fewer defects through the disciplined practice of the red-green-refactor cycle.

When to Use This Skill

• User explicitly requests TDD or test-first development
• Building new features that require high reliability
• Refactoring legacy code with test coverage
• Implementing complex business logic
• Learning TDD methodology

TDD Development Workflow

Phase 1: Understand Requirements

Before writing any test:

1. Clarify the feature requirements with the user
2. Break down the feature into small, testable behaviors
3. Identify edge cases and error conditions
4. Create a list of test cases to implement

Ask clarifying questions such as:

• "What specific behavior should this function have?"
• "What should happen when input is invalid?"
• "Are there any edge cases to consider?"

Phase 2: Red-Green-Refactor Cycles

For each test case, follow this cycle:

Step 1: RED - Write a Failing Test

1. Write a small, focused test (3-5 lines)
2. Use the Arrange-Act-Assert pattern:
   • Arrange: Set up test data and dependencies
   • Act: Execute the behavior under test
   • Assert: Verify the expected outcome
3. Run the test to confirm it fails
4. Verify it fails for the right reason (not syntax errors)

def test_calculate_discount_for_premium_member():
    # Arrange
    calculator = PriceCalculator()
    member = Member(tier="premium")
    price = 100.0

    # Act
    result = calculator.calculate_discount(price, member)

    # Assert
    assert result == 20.0  # 20% discount

Step 2: GREEN - Make the Test Pass

1. Write the minimum code to pass the test
2. "Quick and dirty" is acceptable at this stage
3. Even hard-coded values are fine initially
4. Focus only on making the current test green

def calculate_discount(self, price, member):
    if member.tier == "premium":
        return price * 0.20
    return 0

Step 3: REFACTOR - Improve the Code

1. Review the implementation for improvements
2. Apply refactoring techniques:
   • Remove duplication (DRY)
   • Extract methods/functions
   • Improve naming
   • Simplify logic
3. Run tests after each refactoring to ensure they still pass

def calculate_discount(self, price, member):
    discount_rates = {
        "premium": 0.20,
        "gold": 0.15,
        "silver": 0.10,
        "standard": 0.05,
    }
    rate = discount_rates.get(member.tier, 0)
    return price * rate

Step 4: COMMIT (Optional RGRC Pattern)

After each successful cycle, commit the changes:

• Creates frequent save points
• Easy to revert if needed
• Clear commit history

Phase 3: Iterate Until Complete

1. Select the next test case from the list
2. Repeat the red-green-refactor cycle
3. Continue until all behaviors are implemented
4. Review overall design and architecture

Test Design Guidelines

Test Naming

Use descriptive names that communicate intent:

• should_return_zero_when_list_is_empty
• givenPremiumMember_whenCalculateDiscount_thenReturns20Percent
• createUser_withValidInput_savesToDatabase

Test Structure

Each test should:

• Test one behavior only
• Be independent from other tests
• Be deterministic (same result every run)
• Execute quickly

Edge Cases to Cover

Always consider:

• Empty collections
• Null/None values
• Boundary values (0, -1, max)
• Invalid inputs
• Error conditions

Parameterized Tests

For similar test cases with different values:

@pytest.mark.parametrize("tier,expected_rate", [
    ("premium", 0.20),
    ("gold", 0.15),
    ("silver", 0.10),
    ("standard", 0.05),
])
def test_discount_rates(tier, expected_rate):
    member = Member(tier=tier)
    result = calculator.calculate_discount(100, member)
    assert result == expected_rate * 100

Implementation Strategies

Triangulation

When implementation is unclear:

1. Start with one example, hard-code the result
2. Add second example that forces generalization
3. Continue until the pattern emerges

Fake It Till You Make It

1. Return constants to pass tests quickly
2. Gradually replace with real implementation
3. Let tests drive the design

Obvious Implementation

When solution is clear:

1. Implement directly
2. If it fails, fall back to triangulation

Test Framework Selection

Select the appropriate framework based on language:

• Python: pytest (recommended), unittest
• JavaScript/TypeScript: Jest, Vitest
• Java: JUnit 5, TestNG
• Ruby: RSpec, minitest
• Go: testing (built-in)
• C#: xUnit, NUnit

Quality Principles

Avoid Common Pitfalls

1. Don't write tests after code - Defeats TDD purpose
2. Don't skip refactoring - Accumulates technical debt
3. Don't over-mock - Tests become brittle
4. Don't test implementation details - Test behavior instead
5. Don't chase 100% coverage - Focus on meaningful coverage

Test Quality Indicators

• Tests run fast (<1 second per test)
• No flaky tests
• Clear failure messages
• Test-to-code ratio around 1:1

Resources

References

For detailed methodology and patterns, load these reference files as needed:

• references/tdd_methodology.md: Complete TDD methodology including the Three Laws of TDD, Red-Green-Refactor cycle details, cycle timing, and common pitfalls to avoid

• references/test_patterns.md: Test patterns and techniques including triangulation, parameterized tests, test data builders, mocking patterns, and async testing

When to Load References

Load tdd_methodology.md when:

• User asks about TDD principles or best practices
• Need to explain the Three Laws or cycle timing
• Addressing questions about test quality or metrics

Load test_patterns.md when:

• User needs specific test patterns (builders, mocking)
• Implementing parameterized or property-based tests
• Dealing with async testing scenarios

Example TDD Session

User request: "Create a function to validate email addresses using TDD"

Step 1: Plan Test Cases

Test cases for email validation:
1. Valid email returns true
2. Missing @ returns false
3. Missing domain returns false
4. Missing local part returns false
5. Multiple @ symbols returns false
6. Empty string returns false
7. None/null returns false

Step 2: First Cycle - Valid Email

RED:

def test_valid_email_returns_true():
    result = validate_email("user@example.com")
    assert result is True

GREEN:

def validate_email(email):
    return True  # Minimal implementation

REFACTOR: No changes needed yet.

Step 3: Second Cycle - Missing @

RED:

def test_missing_at_symbol_returns_false():
    result = validate_email("userexample.com")
    assert result is False

GREEN:

def validate_email(email):
    return "@" in email

REFACTOR: Still simple, no changes needed.

Step 4: Continue Cycles

Continue through remaining test cases, letting the implementation emerge from the tests. The final implementation will be well-tested and handle all edge cases.

Summary

1. Write test first - Always before implementation
2. Keep cycles short - ~1-2 minutes per cycle
3. Minimal code - Just enough to pass
4. Refactor always - Never skip this step
5. Test behavior - Not implementation details
6. Commit frequently - After each green-refactor