Test-Driven Development

TDD Cycle (MANDATORY)

+-----------------------------------------+
|  RED -> GREEN -> REFACTOR               |
|     ^                        |          |
|     +------------------------+          |
+-----------------------------------------+

The question is NOT "should I write tests?" but "what tests do I need?"

---

The Three Laws of TDD

1. No production code until you have a failing test
2. No more test than necessary to fail
3. No more code than necessary to pass

---

Detect Your Stack

Before starting, identify which component you're working on:

Working in	Stack	Runner	Test pattern	Details
ui/	TypeScript / React	Vitest + RTL	*.test.{ts,tsx} (co-located)	See vitest skill
prowler/	Python	pytest + moto	*_test.py (suffix) in tests/	See prowler-test-sdk skill
api/	Python / Django	pytest + django	test_*.py (prefix) in api/src/backend/**/tests/	See prowler-test-api skill

---

Phase 0: Assessment (ALWAYS FIRST)

Before writing ANY code:

UI (ui/)

# 1. Find existing tests
fd "*.test.tsx" ui/components/feature/

# 2. Check coverage
pnpm test:coverage -- components/feature/

# 3. Read existing tests

SDK (prowler/)

# 1. Find existing tests
fd "*_test.py" tests/providers/aws/services/ec2/

# 2. Run specific test
uv run pytest tests/providers/aws/services/ec2/ec2_ami_public/ -v

# 3. Read existing tests

API (api/)

# 1. Find existing tests
fd "test_*.py" api/src/backend/api/tests/

# 2. Run specific test
uv run pytest api/src/backend/api/tests/test_models.py -v

# 3. Read existing tests

Decision Tree (All Stacks)

+------------------------------------------+
|     Does test file exist for this code?  |
+----------+-----------------------+-------+
           | NO                    | YES
           v                       v
+------------------+    +------------------+
| CREATE test file |    | Check coverage   |
| -> Phase 1: RED  |    | for your change  |
+------------------+    +--------+---------+
                                 |
                        +--------+--------+
                        | Missing cases?  |
                        +---+---------+---+
                            | YES     | NO
                            v         v
                    +-----------+ +-----------+
                    | ADD tests | | Proceed   |
                    | Phase 1   | | Phase 2   |
                    +-----------+ +-----------+

---

Phase 1: RED - Write Failing Tests

For NEW Functionality

UI (Vitest)

describe("PriceCalculator", () => {
  it("should return 0 for quantities below threshold", () => {
    // Given
    const quantity = 3;

    // When
    const result = calculateDiscount(quantity);

    // Then
    expect(result).toBe(0);
  });
});

SDK (pytest)

class Test_ec2_ami_public:
    @mock_aws
    def test_no_public_amis(self):
        # Given - No AMIs exist
        aws_provider = set_mocked_aws_provider([AWS_REGION_US_EAST_1])

        with mock.patch("prowler...ec2_service", new=EC2(aws_provider)):
            from prowler...ec2_ami_public import ec2_ami_public

            # When
            check = ec2_ami_public()
            result = check.execute()

            # Then
            assert len(result) == 0

API (pytest-django)

@pytest.mark.django_db
class TestResourceModel:
    def test_create_resource_with_tags(self, aws_provider):
        # Given
        provider = aws_provider
        tenant_id = provider.tenant_id

        # When
        resource = Resource.objects.create(
            tenant_id=tenant_id, provider=provider,
            uid="arn:aws:ec2:us-east-1:123456789:instance/i-1234",
            name="test", region="us-east-1", service="ec2", type="instance",
        )

        # Then
        assert resource.uid == "arn:aws:ec2:us-east-1:123456789:instance/i-1234"

Run -> MUST fail: Test references code that doesn't exist yet.

For BUG FIXES

Write a test that reproduces the bug first:

UI: expect(() => render(<DatePicker value={null} />)).not.toThrow();

SDK: assert result[0].status == "FAIL" # Currently returns PASS incorrectly

API: assert response.status_code == 403 # Currently returns 200

Run -> Should FAIL (reproducing the bug).

For REFACTORING

Capture ALL current behavior BEFORE refactoring:

# Any stack: run ALL existing tests, they should PASS
# This is your safety net - if any fail after refactoring, you broke something

Run -> All should PASS (baseline).

---

Phase 2: GREEN - Minimum Code

Write the MINIMUM code to make the test pass. Hardcoding is valid for the first test.

UI:

// Test expects calculateDiscount(100, 10) === 10
function calculateDiscount() {
  return 10; // FAKE IT - hardcoded is valid for first test
}

Python (SDK/API):

# Test expects check.execute() returns 0 results
def execute(self):
    return []  # FAKE IT - hardcoded is valid for first test

This passes. But we're not done...

---

Phase 3: Triangulation (CRITICAL)

One test allows faking. Multiple tests FORCE real logic.

Add tests with different inputs that break the hardcoded value:

Scenario	Required?
Happy path	YES
Zero/empty values	YES
Boundary values	YES
Different valid inputs	YES (breaks fake)
Error conditions	YES

UI:

it("should calculate 10% discount", () => {
  expect(calculateDiscount(100, 10)).toBe(10);
});

// ADD - breaks the fake:
it("should calculate 15% on 200", () => {
  expect(calculateDiscount(200, 15)).toBe(30);
});

it("should return 0 for 0% rate", () => {
  expect(calculateDiscount(100, 0)).toBe(0);
});

Python:

def test_single_public_ami(self):
    # Different input -> breaks hardcoded empty list
    assert len(result) == 1
    assert result[0].status == "FAIL"

def test_private_ami(self):
    assert result[0].status == "PASS"

Now fake BREAKS -> Real implementation required.

---

Phase 4: REFACTOR

Tests GREEN -> Improve code quality WITHOUT changing behavior.

• Extract functions/methods
• Improve naming
• Add types/validation
• Reduce duplication

Run tests after EACH change -> Must stay GREEN.

---

Quick Reference

+------------------------------------------------+
|                 TDD WORKFLOW                    |
+------------------------------------------------+
| 0. ASSESS: What tests exist? What's missing?   |
|                                                |
| 1. RED: Write ONE failing test                 |
|    +-- Run -> Must fail with clear error       |
|                                                |
| 2. GREEN: Write MINIMUM code to pass           |
|    +-- Fake It is valid for first test         |
|                                                |
| 3. TRIANGULATE: Add tests that break the fake  |
|    +-- Different inputs, edge cases            |
|                                                |
| 4. REFACTOR: Improve with confidence           |
|    +-- Tests stay green throughout             |
|                                                |
| 5. REPEAT: Next behavior/requirement           |
+------------------------------------------------+

---

Anti-Patterns (NEVER DO)

# ANY language:

# 1. Code first, tests after
def new_feature(): ...  # Then writing tests = USELESS

# 2. Skip triangulation
# Single test allows faking forever

# 3. Test implementation details
assert component.state.is_loading == True   # BAD - test behavior, not internals
assert mock_service.call_count == 3         # BAD - brittle coupling

# 4. All tests at once before any code
# Write ONE test, make it pass, THEN write the next

# 5. Giant test methods
# Each test should verify ONE behavior

---

Commands by Stack

UI (ui/)

pnpm test                           # Watch mode
pnpm test:run                       # Single run (CI)
pnpm test:coverage                  # Coverage report
pnpm test ComponentName             # Filter by name

SDK (prowler/)

uv run pytest tests/path/ -v              # Run specific tests
uv run pytest tests/path/ -v -k "test_name"  # Filter by name
uv run pytest -n auto tests/              # Parallel run
uv run pytest --cov=./prowler tests/      # Coverage

API (api/)

uv run pytest -x --tb=short                           # Run all (stop on first fail)
uv run pytest api/src/backend/api/tests/test_file.py     # Specific file
uv run pytest -k "test_name" -v                       # Filter by name