test-driven-development

name: Test Driven Development description: Implement features using the Red-Green-Refactor cycle to ensure testability and correctness from the start.

Test Driven Development (TDD)

Goal

Write code that is proven to work the moment it is written, preventing regression and ensuring high design quality.

When to Use

• Implementing complex business logic.
• Fixing a bug (write the failing test first).
• Creating utility functions or data transformations.
• NOT strictly required for boilerplate/scaffolding, but recommended.

Instructions

1. Red (Write a Failing Test)

Write a test that defines the expected behavior.

• It must fail. If it passes, your test is broken or the feature already exists.
• Compilation Error: Even not compiling counts as "Red".

def test_calculate_total():
    # We haven't written the function yet
    result = calculate_total([10, 20]) 
    assert result == 30

2. Green (Make it Pass)

Write the minimum amount of code to pass the test.

• Do not optimize.
• Do not worry about clean code yet.
• Hardcoding values is acceptable if it passes the specific test (you will generalize in the next step).

def calculate_total(items):
    return sum(items)

3. Refactor (Clean Up)

Improve the code while keeping the test green.

• Rename variables.
• Extract functions.
• Optimize performance.
• Remove duplication.

4. Repeat

Add the next test case (e.g., test_calculate_total_empty_list) and repeat.

Constraints

✅ Do

• DO: Run tests after every change.
• DO: Write only enough code to pass the current failing test.
• DO: Treat test code with the same quality standards as production code.

❌ Don't

• DON'T: Write the implementation first, then "backfill" tests.
• DON'T: Skip the "Red" phase. You must verify the test can fail.
• DON'T: Refactor while the test is failing. Get to Green first.

Output Format

• Test files (tests/unit/test_xyz.py).
• Implementation files.

Dependencies

• backend/testing-flask/SKILL.md OR frontend/testing-frontend/SKILL.md