test-unit_skill

Overview

This skill helps create focused unit tests for individual functions, methods, and classes with clear mocking, assertions, and prioritized test plans. It emphasizes isolation, complete assertions, and coverage of both happy paths and error conditions. The output is a prioritized test plan suitable for immediate implementation in a test suite.

How this skill works

It inspects function signatures, dependencies, and side effects to identify what to mock or stub and which behaviors to assert. It produces a checklist and a prioritized test matrix (Must/Should/Could/Won't) covering return values, state changes, exceptions, side effects, boundaries, and failure modes. The skill outputs concise test cases and setup/teardown guidance to ensure independence and reproducibility.

When to use it

• Writing unit tests for newly implemented functions or recently refactored code
• Before adding integration tests to ensure core logic is verified in isolation
• When introducing mocks/stubs for external dependencies (I/O, network, databases)
• Preparing regression tests for bug fixes to prevent reintroduction
• Validating edge cases, error handling, and boundary conditions

Best practices

• Isolate a single behavior per test and mock all external dependencies
• Keep tests independent: avoid shared mutable state and use explicit setup/teardown
• Assert both return values and observable side effects, including collaborator calls
• Cover happy paths first with representative inputs, then add error and boundary tests
• Prioritize tests using Must/Should/Could/Won't to focus effort on preventing regressions

Example use cases

• Create Must-priority tests asserting core return values and primary side effects for a payment processing function
• Write Should tests for typical invalid inputs, null handling, and dependency failures
• Add Could tests for rare concurrency scenarios and extended boundary conditions
• Produce a test checklist for a refactor to guide developers on required mock coverage and assertions
• Generate setup/teardown snippets for shell-invoked helpers and environment-controlled tests