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---
name: testing-guidance
description: "Useful testing guidance for developers."
---
## Naming
Name tests to describe behaviour: `should_return_error_when_input_is_empty`, `test_parse_handles_nested_objects`. Use `describe`/`it` blocks for grouping where the language supports them. Follow `given_when_then` or `should_when`. Test names are specifications — a reader should understand the expected behaviour without reading the body.
## Assertions
Assert exact expected values, not truthiness (`assert result == 42`, not `assert result`). Use snapshot testing for complex structured output and property-based testing for functions with wide input ranges. Include descriptive failure messages. Always test error paths and edge cases, not just the happy path.
## Independence
Tests must be independent and idempotent — runnable in any order, in parallel. No shared mutable state between tests. Use factories or fixtures for setup. Clean up created resources (files, DB rows, env vars) after each test. Never rely on execution order.
## Anti-patterns
- Do not test mock behaviour instead of real behaviour.
- Do not add test-only methods to production code.
- Do not mock what you don't own — wrap it and test the wrapper.
- Do not test implementation details — test observable behaviour.
- Do not write tests that pass when the code is broken. If a test never fails, it is not testing anything.
