test-driven-development

name	test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code. Enforces red-green-refactor and the Iron Law: no production code without a failing test first. Invoke when the operator says 'add tests', 'TDD this', 'write a test for X', or is about to add new behavior to existing code.
source	{"upstream":"obra/superpowers","tag":"v5.0.7","commit":"1f20bef3f59b85ad7b52718f822e37c4478a3ff5","path":"skills/test-driven-development/SKILL.md","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.

NovaCore adaptations (vendored from Superpowers v5.0.7):

• Python is the default stack — pytest is the test runner. For Pine Script, route work through pinescript-developer (no runtime test framework exists for Pine; its review loop is the closest equivalent).
• The Iron Law is preserved. Exceptions still require explicit operator approval — NovaCore's path-choice autonomy does not waive the Iron Law; it only waives asking permission for execution choices within an approved task.
• Upstream phrasing "your human partner" is rendered here as "operator".

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask the operator):

• 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

Wrote 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

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong failure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay green"];
    verify_green -> next;
    next -> red;
}

RED — Write Failing Test

Write one minimal test showing what should happen.

Good:

def test_retries_failed_operations_three_times():
    attempts = {"count": 0}

    def operation():
        attempts["count"] += 1
        if attempts["count"] < 3:
            raise RuntimeError("fail")
        return "success"

    result = retry_operation(operation)

    assert result == "success"
    assert attempts["count"] == 3

Clear name, tests real behavior, one thing.

Bad:

def test_retry_works():
    mock = Mock(side_effect=[Error(), Error(), "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.

pytest path/to/test.py::test_name -v

Confirm:

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

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

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

GREEN — Minimal Code

Write the simplest code that passes.

Good:

def retry_operation(fn):
    last = None
    for i in range(3):
        try:
            return fn()
        except Exception as e:
            last = e
    raise last

Just enough to pass.

Bad:

def retry_operation(fn, max_retries=3, backoff="linear", on_retry=None):
    ...  # YAGNI

Over-engineered.

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

Verify GREEN — Watch It Pass

Mandatory.

pytest path/to/test.py::test_name -v

Confirm:

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

Test fails? Fix the code, not the 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 the next feature.

Good Tests

Quality	Good	Bad
Minimal	One thing. "and" in the name? Split it.	test_validates_email_and_domain_and_whitespace
Clear	Name describes behavior	test_test1
Shows intent	Demonstrates desired API	Obscures what code should do

Why Order Matters

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

Tests written after code pass immediately. Passing immediately proves nothing: might test the 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. No record, can't re-run, easy to forget cases under pressure. Automated tests are systematic — same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is gone. The choice is: delete and rewrite with TDD (high confidence), or keep it and add tests after (low confidence, likely bugs). Working code without real tests is technical debt.

"TDD is dogmatic, pragmatic means adapting"

TDD is pragmatic — finds bugs before commit, prevents regressions, documents behavior, enables refactoring. "Pragmatic" shortcuts = debugging in production = slower.

"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. Tests-first force edge-case discovery before implementing.

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 ≠ systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Unverified code is tech 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 the test. Hard to test = hard to use.
"TDD will slow me down"	TDD faster than debugging.
"Existing code has no tests"	You're improving it. Add tests for existing code.

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"
• "Keep as reference" / "adapt existing code"
• "Already spent X hours, deleting is wasteful"

All of these mean: delete code, start over with TDD.

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? TDD was skipped. Start over.

When Stuck

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask the operator.
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.

Debugging Integration

Bug found? Write a failing test that reproduces it. Follow the TDD cycle. The test proves the fix and prevents regression. Never fix bugs without a test.

Final Rule

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

No exceptions without operator permission.