---
name: Test-Driven Development
slug: test-driven-development-3
category: Quality
description: "A rigorous test-driven development workflow enforcing red-green-refactor cycles: write failing tests first, watch them fail, then write minimal code, and refactor. Ideal for AI coding agents to ensure code quality and prevent untested production code."
github: "https://github.com/HezaoHezao/poirot/tree/master/poirot/backend/agents/skill/builtin_skills/core/test-driven-development"
language: Python
stars: 185
forks: 8
install: "npx degit https://github.com/HezaoHezao/poirot/tree/master/poirot/backend/agents/skill/builtin_skills/core/test-driven-development ~/.claude/skills/test-driven-development"
installs_to: ~/.claude/skills/test-driven-development
source_path: poirot/backend/agents/skill/builtin_skills/core/test-driven-development/SKILL.md
collection_size: 37
category_size: 1354
collection_url: "https://dirskills.com/collections/HezaoHezao/poirot"
added: 2026-08-11T07:20:43.291Z
last_synced: 2026-08-11T07:20:43.291Z
canonical_url: "https://dirskills.com/skills/test-driven-development-3"
---

# Test-Driven Development

A rigorous test-driven development workflow enforcing red-green-refactor cycles: write failing tests first, watch them fail, then write minimal code, and refactor. Ideal for AI coding agents to ensure code quality and prevent untested production code.

**Install:**

```bash
npx degit https://github.com/HezaoHezao/poirot/tree/master/poirot/backend/agents/skill/builtin_skills/core/test-driven-development ~/.claude/skills/test-driven-development
```

## README

# 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.

## When to Use

**Always:**
- New features
- Bug fixes
- Refactoring
- Behavior changes

**Exceptions (ask the user first):**
- 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.

**Good test:**
```python
def test_retries_failed_operations_3_times():
    attempts = 0
    def operation():
        nonlocal attempts
        attempts += 1
        if attempts < 3:
            raise Exception('fail')
        return 'success'

    result = retry_operation(operation)

    assert result == 'success'
    assert attempts == 3
```
Clear name, tests real behavior, one thing.

**Bad test:**
```python
def test_retry_works():
    mock = MagicMock()
    mock.side_effect = [Exception(), Exception(), 'success']
    result = retry_operation(mock)
    assert result == 'success'  # What about retry count? Timing?
```
Vague name, tests mock not real code.

**Requirements:**
- One behavior per test
- Clear descriptive name ("and" in name? Split it)
- Real code, not mocks (unless truly unavoidable)
- Name describes behavior, not implementation

### Verify RED — Watch It Fail

**MANDATORY. Never skip.**

```bash
pytest tests/test_feature.py::test_specific_behavior -v
```

Confirm:
- Test fails (not errors from typos)
- Failure message is expected
- Fails because the feature is missing

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

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

### GREEN — Minimal Code

Write the simplest code to pass the test. Nothing more.

**Good:**
```python
def add(a, b):
    return a + b  # Nothing extra
```

**Bad:**
```python
def add(a, b):
    result = a + b
    logging.info(f"Adding {a} + {b} = {result}")  # Extra!
    return result
```

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

**Cheating is OK in GREEN:**
- Hardcode return values
- Copy-paste
- Duplicate code
- Skip edge cases

We'll fix it in REFACTOR.

### Verify GREEN — Watch It Pass

**MANDATORY.**

```bash
# Run the specific test
pytest tests/test_feature.py::test_specific_behavior -v

# Then run ALL tests to check for regressions
pytest tests/ -q
```

Confirm:
- Test passes
- Other tests still pass
- Output pristine (no errors, warnings)

**Test fails?** Fix the code, not the test.
**Other tests fail?** Fix regressions now.

### REFACTOR — Clean Up

After green only:
- Remove duplication
- Improve names
- Extract helpers
- Simplify expressions

Keep tests green throughout. Don't add behavior.

**If tests fail during refactor:** Undo immediately. Take smaller steps.

### Repeat

Next failing test for next behavior. One cycle at a time.

## Avoid Horizontal Slices

Do **not** write all tests first and then all implementation. That is horizontal
slicing. Use vertical tracer bullets instead:

```text
WRONG:
  RED:   test1, test2, test3, test4
  GREEN: impl1, impl2, impl3, impl4

RIGHT:
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
```

A tracer bullet is one end-to-end behavior slice. It proves the path works,
teaches you about the interface, and keeps each next test grounded.

## 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.

**"Deleting X hours of work is wasteful"**

Sunk cost fallacy. The time is already gone. Your choice now:
- Delete and rewrite with TDD (high confidence)
- Keep it and add tests after (low confidence, likely bugs)

## 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, 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. |
| "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. |

## Red Flags — STOP and Start Over

If you catch yourself doing any of these, delete the code and restart with TDD:

- Code before test
- Test after implementation
- Test passes immediately on first run
- Can't explain why test failed
- Tests added "later"
- Rationalizing "just this once"
- "I already manually tested it"
- "Keep as reference" or "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? You skipped TDD. Start over.

## When Stuck

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

## Testing Anti-Patterns

- **Testing mock behavior instead of real behavior** — mocks should verify
  interactions, not replace the system under test
- **Testing implementation details** — test behavior/results, not internal calls
- **Happy path only** — always test edge cases, errors, and boundaries
- **Brittle tests** — tests should verify behavior, not structure; refactoring
  shouldn't break them

## Final Rule

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

No exceptions without the user's explicit permission.