test-driven-development

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SKILL.md (first lines)

# 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 bash 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.

From Astroreen/astrocode, MIT.

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