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Tests as documentation
When developers want to use a tool or library, they learn by reading the documentation or guides that contain code samples they can try, or by following tutorials to build a basic application.
Test cases can essentially act as code samples and form part of the documentation. In fact, tests are the most comprehensive set of code samples there are, covering every use case that the application cares about.
Although tests provide the best form of documentation, tests alone are not enough. Test cases do not provide context for the code, such as how it fits into the overall business goals, or convey the rationale behind its implementation. Therefore, tests should be supplemented by inline comments and automatically-generated, as well as manually-written, documentation.