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 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
# Use exec tool to run the specific test
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.

## 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. 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 (high confidence)
- Keep it and add tests after (low confidence, likely bugs)

The "waste" is keeping code you can't trust.

**"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. 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. 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 the test. Hard to test = hard to use. |
| "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. |
| "Manual test faster" | Manual doesn't prove edge cases. You'll re-test every change. |
| "Existing cod