Test-Driven Development (TDD)

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

RED-GREEN-REFACTOR Cycle

RED - Write Failing Test

Write one minimal test showing what should happen.

Python (pytest):

def test_retries_failed_operations_3_times():
    """Should retry failed operations exactly 3 times before succeeding."""
    attempts = []
    
    def operation():
        attempts.append(1)
        if len(attempts) < 3:
            raise ValueError("fail")
        return "success"
    
    result = retry_operation(operation)
    
    assert result == "success"
    assert len(attempts) == 3

Flutter (test package):

test('retries failed operations 3 times', () async {
  int attempts = 0;
  
  Future<String> operation() async {
    attempts++;
    if (attempts < 3) throw Exception('fail');
    return 'success';
  }
  
  final result = await retryOperation(operation);
  
  expect(result, equals('success'));
  expect(attempts, equals(3));
});

Requirements:

• One behavior per test
• Clear descriptive name
• Real code (no mocks unless unavoidable)
• Tests behavior, not implementation

Verify RED - Watch It Fail

MANDATORY. Never skip.

Python:

pytest path/to/test_module.py::test_retries_failed_operations_3_times -v

Flutter:

flutter test test/retry_test.dart

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

Python:

def retry_operation(fn, max_retries=3):
    """Retry operation up to max_retries times."""
    for i in range(max_retries):
        try:
            return fn()
        except Exception as e:
            if i == max_retries - 1:
                raise
    raise RuntimeError("unreachable")

Flutter:

Future<T> retryOperation<T>(Future<T> Function() fn, {int maxRetries = 3}) async {
  for (int i = 0; i < maxRetries; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i == maxRetries - 1) rethrow;
    }
  }
  throw StateError('unreachable');
}

Just enough to pass. Don't add features, refactor other code, or "improve" beyond the test.

Verify GREEN - Watch It Pass

MANDATORY.

Python:

pytest path/to/test_module.py -v

Flutter:

flutter test

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Good Tests

Python-Specific Guidance

Test structure:

# Arrange
user = User(email="test@example.com")

# Act
result = user.validate()

# Assert
assert result.is_valid is True
assert result.errors == []

Fixtures for setup:

@pytest.fixture
def sample_user():
    return User(email="test@example.com", age=25)

def test_adult_user(sample_user):
    assert sample_user.is_adult() is True

Parametrize for multiple cases:

@pytest.mark.parametrize("email,expected", [
    ("valid@example.com", True),
    ("invalid", False),
    ("", False),
])
def test_email_validation(email, expected):
    assert is_valid_email(email) == expected

Flutter-Specific Guidance

Widget tests:

testWidgets('LoginButton shows loading spinner when pressed', (tester) async {
  await tester.pumpWidget(MaterialApp(home: LoginButton()));
  
  // Initially shows text
  expect(find.text('Login'), findsOneWidget);
  expect(find.byType(CircularProgressIndicator), findsNothing);
  
  // Tap button
  await tester.tap(find.byType(ElevatedButton));
  await tester.pump();
  
  // Now shows spinner
  expect(find.byType(CircularProgressIndicator), findsOneWidget);
});

Golden tests for UI:

testWidgets('ProfileCard matches golden', (tester) async {
  await tester.pumpWidget(MaterialApp(
    home: ProfileCard(user: testUser),
  ));
  
  await expectLater(
    find.byType(ProfileCard),
    matchesGoldenFile('profile_card.png'),
  );
});

Mock dependencies with mockito:

@GenerateMocks([ApiClient])
void main() {
  test('fetches user data from API', () async {
    final mockClient = MockApiClient();
    when(mockClient.getUser(any))
        .thenAnswer((_) async => User(id: '1', name: 'Alice'));
    
    final service = UserService(mockClient);
    final user = await service.fetchUser('1');
    
    expect(user.name, equals('Alice'));
    verify(mockClient.getUser('1')).called(1);
  });
}

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

Common Rationalizations

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix

Bug: Empty email accepted

RED

def test_rejects_empty_email():
    result = submit_form({"email": ""})
    assert result.error == "Email required"

Verify RED

$ pytest test_form.py::test_rejects_empty_email -v
FAILED: AssertionError: assert None == 'Email required'

GREEN

def submit_form(data):
    if not data.get("email", "").strip():
        return FormResult(error="Email required")
    # ...

Verify GREEN

$ pytest test_form.py -v
PASSED

REFACTOR

Extract validation for multiple fields if needed.

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

See testing-anti-patterns.md to avoid:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies
• Incomplete mocks that hide structural issues

Platform-Specific Test Commands

Python:

# Run all tests
pytest

# Run specific test file
pytest tests/test_user.py

# Run specific test
pytest tests/test_user.py::test_validates_email

# Run with coverage
pytest --cov=src tests/

# Run in watch mode (with pytest-watch)
ptw

Flutter:

# Run all tests
flutter test

# Run specific test file
flutter test test/user_test.dart

# Run with coverage
flutter test --coverage

# Run in watch mode
flutter test --watch

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.