Test Driven Development

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 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

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\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

RED - Write Failing Test

Write one minimal test showing what should happen.

Python

def test_retries_failed_operations_3_times(): attempts = [0] # mutable for closure def operation(): attempts[0] += 1 if attempts[0] < 3: raise Exception('fail') return 'success'

result = retry_operation(operation)

assert result == 'success'
assert attempts[0] == 3

C#

[Fact] public void RetriesFailedOperations3Times() { var attempts = 0; Func operation = () => { attempts++; if (attempts < 3) throw new Exception("fail"); return "success"; };

var result = RetryOperation(operation);

Assert.Equal("success", result);
Assert.Equal(3, attempts);

}

TypeScript/Node.js

test('retries failed operations 3 times', async () => { let attempts = 0; const operation = () => { attempts++; if (attempts < 3) throw new Error('fail'); return 'success'; };

const result = await retryOperation(operation);

expect(result).toBe('success'); expect(attempts).toBe(3); });

Clear name, tests real behavior, one thing
</Good>

<Bad>
```python

# Testing mock behavior instead of real behavior
def test_retry_works():
    mock = Mock(side_effect=[Exception(), Exception(), 'success'])
    retry_operation(mock)
    assert mock.call_count == 3  # ❌ Tests mock, not code

Vague name, tests mock not code - see testing-anti-patterns.md

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)
• Never test mock behavior (see Anti-Pattern 1 in testing-anti-patterns.md)

Verify RED - Watch It Fail

MANDATORY. Never skip.

# Python
pytest path/to/test_module.py::test_name -v

# C#
dotnet test --filter TestName

# TypeScript/Node.js (Jest)
npm test path/to/test.test.ts

# TypeScript/Node.js (Vitest)
npx vitest path/to/test.test.ts

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): for i in range(3): try: return fn() except Exception as e: if i == 2: raise

C#

public static T RetryOperation(Func fn) { for (int i = 0; i < 3; i++) { try { return fn(); } catch (Exception e) { if (i == 2) throw; } } throw new InvalidOperationException("unreachable"); }

TypeScript

async function retryOperation(fn: () => Promise): Promise { for (let i = 0; i < 3; i++) { try { return await fn(); } catch (e) { if (i === 2) throw e; } } throw new Error('unreachable'); }

Just enough to pass
</Good>

<Bad>
```python

# YAGNI - Over-engineered before test demands it
def retry_operation(fn, max_retries=3, backoff='linear',
    # Features not tested yet
    pass

Over-engineered

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

# C#
dotnet test

# TypeScript/Node.js
npm test  # or: npx vitest

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

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.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behavior (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"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. 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).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

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"
• "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

# Python
def test_rejects_empty_email():
    result = submit_form({'email': ''})
    assert result['error'] == 'Email required'

# C#
[Fact]
public void RejectsEmptyEmail() {
    var result = SubmitForm(new FormData { Email = "" });
    Assert.Equal("Email required", result.Error);
}

# TypeScript
test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});

Verify RED

# Output shows test fails (language-specific format)
FAIL: expected 'Email required', got undefined/None/null

GREEN

# Python
def submit_form(data):
    if not data.get('email', '').strip():
        return {'error': 'Email required'}
    # ...

# C#
public FormResult SubmitForm(FormData data) {
    if (string.IsNullOrWhiteSpace(data.Email)) {
        return new FormResult { Error = "Email required" };
    }
    // ...
}

# TypeScript
function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}

Verify GREEN

PASS  # All tests green

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

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 your human partner's permission.

Related Skills

Uses:

• senior-software-developer: Provides language-specific patterns and quality standards used during implementation
• verification-before-completion: Ensures test evidence before claiming success

Used by:

• subagent-driven-development: Subagents follow TDD for each task

Relationship:

• TDD is the methodology (test-first process)
• senior-software-developer provides the patterns (quality standards)
• For complex multi-step projects, use writing-plans → subagent-driven-development instead