test-driven-development

name	test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code

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 user):

• 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.

```python def test_retries_failed_operations_3_times(): attempts = 0

def operation():
    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
</Good>

<Bad>
```python
def test_retry_works(mocker):
    mock = mocker.patch("module.operation",
        side_effect=[RuntimeError(), RuntimeError(), "success"])
    retry_operation(mock)
    assert mock.call_count == 3

Vague name, tests mock not code

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

# Go
go test ./path/to/...

# Python
uv run pytest path/to/test_file.py

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.

```go func RetryOperation[T any](fn func() (T, error)) (T, error) { var zero T for i := 0; i < 3; i++ { result, err := fn() if err == nil { return result, nil } if i == 2 { return zero, err } } return zero, errors.New("unreachable") } ``` Just enough to pass ```go func RetryOperation[T any]( fn func() (T, error), opts ...RetryOption, // YAGNI ) (T, error) { // configurable retries, backoff, callbacks... } ``` Over-engineered

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

Verify GREEN - Watch It Pass

MANDATORY.

go test ./path/to/...
uv run pytest path/to/test_file.py

Confirm:

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

Test fails? Fix code, not test.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Already manually tested"	Ad-hoc ≠ systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Keeping unverified code is debt.
"Keep as reference"	You'll adapt it. That's testing after. Delete means delete.
"Need to explore first"	Fine. Throw away exploration, start with TDD.
"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
• "Just this once"
• "Keep as reference"
• "Already spent X hours"
• "This is different because..."

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

When Stuck

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

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
• 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.

Final Rule

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

No exceptions without the user's permission.