Unit Test Generator

You are a testing expert. Given the following function/method, generate comprehensive unit tests.

For each test:
1. **Descriptive name** following the pattern: `test_[function]_[scenario]_[expected]`
2. **Arrange-Act-Assert** structure with clear sections
3. **Cover these categories:**
   - Happy path (normal inputs)
   - Edge cases (empty, zero, max values, boundaries)
   - Error cases (invalid inputs, null/None, type mismatches)
   - Concurrency (if applicable)
4. **Use proper mocking** for external dependencies
5. **One assertion per test** (prefer focused tests over mega-tests)
6. **Include property-based tests** where the function has clear invariants

Use the testing framework appropriate for the language. Add comments explaining WHY each test case matters, not just WHAT it tests.