# Testing Basics

> Essential testing patterns for any project. Use when: writing tests, setting up testing, or fixing test issues. Triggers: "test", "testing", "unit test", "write tests".

- Skill: `majiayu000/testing-basics` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add majiayu000/testing-basics`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/testing-basics/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/testing-basics

---


# Testing Basics

Core testing patterns that apply to any language or framework.

## Test Structure

### AAA Pattern
```
Arrange → Act → Assert
```

```python
# Arrange
user = User(name="Alice")

# Act
result = user.greet()

# Assert
assert result == "Hello, Alice!"
```

## Test Types

| Type | Scope | Speed | When to use |
|------|-------|-------|-------------|
| Unit | Single function/class | Fast | Core logic |
| Integration | Multiple components | Medium | APIs, DB |
| E2E | Full system | Slow | Critical paths |

## Naming Tests

```
test_[what]_[condition]_[expected]
```

Examples:
- `test_login_valid_credentials_returns_token`
- `test_checkout_empty_cart_raises_error`
- `test_search_no_results_returns_empty_list`

## What to Test

### Do Test
- Business logic
- Edge cases
- Error handling
- Public interfaces

### Don't Test
- Framework code
- Simple getters/setters
- Implementation details
- External services directly

## Test Isolation

Each test should:
- Run independently
- Not depend on order
- Clean up after itself
- Not share state

## Mocking

Mock external dependencies, not internal logic:

```python
# Good: mock external service
@patch('app.services.email.send')
def test_signup_sends_welcome_email(mock_send):
    signup(user)
    mock_send.assert_called_once()

# Bad: mock internal implementation
@patch('app.models.user.User._validate')  # Don't do this
```

## Coverage

- Aim for meaningful coverage, not 100%
- Cover happy paths first
- Then error cases
- Then edge cases

## Quick Checklist

Before committing:
- [ ] Tests pass locally
- [ ] New code has tests
- [ ] No flaky tests added
- [ ] Test names are descriptive