Mocking & Dependency Isolation

Testing in isolation ensures that your unit tests are fast, deterministic, and fail only when the logic inside the unit is broken.

Test Double Types

• Dummy: Passed around but never actually used (e.g., to fill a parameter list).
• Stub: Provides canned answers to calls made during the test.
• Spy: A stub that also records some information based on how it was called.
• Mock: Objects pre-programmed with expectations which form a specification of the calls they are expected to receive.
• Fake: Have working implementations, but usually take some shortcut (e.g., an in-memory database).

Best Practices

1. Don't Mock Everything: If a class is a pure data object or a simple utility, use the real thing.
2. Mock Only What You Own: Avoid mocking third-party libraries; wrap them in an interface instead.
3. Verify Behavior, Not Implementation: Focus on what the mock should receive, not the internal state of the mock.

Example (Python)

from unittest.mock import MagicMock