Test Driven Development

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