attac-t Skill: Decide Mock
"Mock at the boundaries, not everywhere."
The Decision
Mock when:
- External service (payment gateway, email provider)
- Slow dependency (file system, network)
- Non-deterministic (time, randomness)
- You don't control it
Don't mock when:
- In-process dependency (your own classes)
- Database (use transactions instead)
- The mock is more complex than the real thing
- You're testing integration, not isolation
The Heuristic
Ask: "Does this cross a process boundary?"
Yes → Mock it. No → Use the real thing.
The Quick Test
| Ask | Answer | Action |
|---|---|---|
| External API? | Yes | Mock |
| Your own class? | Yes | Real |
| Database? | Yes | Real + transaction |
| Time-dependent? | Yes | Freeze time |
| Non-deterministic? | Yes | Mock or seed |
Real-World Examples
See examples.md.