test-driven-development

name	test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code
source	https://github.com/Ethanon/developer.ai
license	MIT

Test-Driven Development (TDD)

Overview

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 the project owner):

• 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

RED - Write Failing Test

Write one minimal test showing what should happen.

Good:

def test_retries_failed_operations_three_times() -> None:
    attempts = 0

    def operation() -> str:
        nonlocal attempts
        attempts += 1
        if attempts < 3:
            raise RuntimeError('fail')
        return 'success'

    result = retry_operation(operation)

    assert result == 'success'
    assert attempts == 3

Clear name, tests real behavior, one thing.

Bad:

def test_retry_works() -> None:
    mock_op = MagicMock(side_effect=[RuntimeError(), RuntimeError(), 'success'])
    retry_operation(mock_op)
    assert mock_op.call_count == 3

Vague name, tests mock not code.

Requirements:

• One behavior
• Clear name (test_<behavior>, not test_<function>)
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

pytest tests/test_module.py::test_retries_failed_operations_three_times -v

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not import errors or 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.

Good:

from typing import Callable, TypeVar

T = TypeVar('T')

def retry_operation(fn: Callable[[], T], max_attempts: int = 3) -> T:
    for attempt in range(max_attempts):
        try:
            return fn()
        except Exception:
            if attempt == max_attempts - 1:
                raise
    raise RuntimeError('unreachable')

Just enough to pass.

Bad:

def retry_operation(
    fn: Callable[[], T],
    max_attempts: int = 3,
    backoff: Literal['linear', 'exponential'] = 'linear',
    on_retry: Callable[[int], None] | None = None,
    exceptions: tuple[type[Exception], ...] = (Exception,),
) -> T:
    # YAGNI
    ...

Over-engineered.

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

Verify GREEN - Watch It Pass

MANDATORY.

pytest tests/test_module.py -v

Confirm:

• Test passes
• Other tests still pass
• Output clean (no warnings, no deprecation notices)

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

Quality	Good	Bad
Minimal	One thing. "and" in name? Split it.	test_validates_email_and_domain_and_whitespace
Clear	Name describes behavior	test1, test_foo
Shows intent	Demonstrates desired API	Obscures what code should do

pytest Specifics

Fixtures over setup/teardown:

# GOOD
@pytest.fixture
def user_repo(tmp_path: Path) -> UserRepository:
    return UserRepository(db_path=tmp_path / 'test.db')

def test_saves_user(user_repo: UserRepository) -> None:
    user = User(name='Alice', email='[e-mail]')
    user_repo.save(user)
    assert user_repo.find_by_email('[e-mail]') == user

Parametrize for multiple cases:

@pytest.mark.parametrize('email', ['', 'notanemail', '@nodomain'])
def test_rejects_invalid_email(email: str) -> None:
    result = validate_email(email)
    assert result.is_err()

Fake time, don't sleep:

def test_expires_after_ttl(freezer: FrozenDateTimeFactory) -> None:
    cache = Cache(ttl_seconds=60)
    cache.set('key', 'value')
    freezer.tick(61)
    assert cache.get('key') is None

Use freezegun / time-machine; never time.sleep() in tests.

Why Order Matters

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

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

"I already manually tested all the edge cases"

Manual testing is ad-hoc. No record, can't re-run, easy to forget under pressure.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. Keeping code you can't trust is the real waste.

"Tests after achieve the same goals"

Tests-after answer "What does this do?" Tests-first answer "What should this do?" Tests-after are biased by your implementation.

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Tests after achieve same goals"	Tests-after = "what does this do?" Tests-first = "what should this do?"
"Already manually tested"	Ad-hoc is not systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Keeping unverified code is technical debt.
"Keep as reference, write tests first"	You'll adapt it. That's testing after. Delete means delete.
"Need to explore first"	Fine. Throw away exploration, start with TDD.
"Test hard = design unclear"	Listen to test. Hard to test = hard to use.
"TDD will slow me down"	TDD faster than debugging. Pragmatic = test-first.

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"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "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() -> None:
    result = submit_form({'email': ''})
    assert result['error'] == 'Email required'

Verify RED

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

GREEN

def submit_form(data: dict[str, str]) -> dict[str, str | None]:
    if not data.get('email', '').strip():
        return {'error': 'Email required'}
    ...

Verify GREEN

$ pytest tests/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 or import error)
• Wrote minimal code to pass each test
• All tests pass (pytest)
• Type checker passes (mypy/pyright)
• Output pristine (no 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

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask the project owner.
Test too complicated	Design too complicated. Simplify interface.
Must mock everything	Code too coupled. Use dependency injection or pass a Protocol.
Test setup huge	Extract fixtures. Still complex? Simplify design.

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

When adding mocks or test utilities, read testing-anti-patterns.md to avoid common pitfalls:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies

Final Rule

Production code -> test exists and failed first
Otherwise -> not TDD

No exceptions without the project owner's explicit approval.