# Tdd Mocking Isolation

> Techniques for isolating units under test using mocks, stubs, and fakes.

- Skill: `jcorpac/tdd-mocking-isolation` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add jcorpac/tdd-mocking-isolation`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jcorpac/tdd-mocking-isolation/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: jcorpac (https://skillmd.com/u/jcorpac)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/jcorpac/tdd-mocking-isolation

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# 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)
```python
from unittest.mock import MagicMock

# Create a mock for an external API service
service = MagicMock()
service.get_user_data.return_value = {"id": 1, "name": "Jane Doe"}

# Use it in your test
result = my_function(service)
assert result == "Jane Doe"
service.get_user_data.assert_called_once_with(1)
```