Accurate by design
An API should state what it will do and do what it has said.
You should not have to read the code behind the API to call it. There should not be a “tribal knowledge” on the behavior of the API.
It should not have unintended consequences not described in the API.
One litmus test to do is to ask if none of the engineers are in the team anymore whether they would be any context lost not conveyed by the API.
Software Engineering from the Frontlines Course on Maven
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I've fallen in love with FastAPI recently, and one of the reasons I enjoy using it is because it automatically generates Swagger documentation from the function declarations. There's no need to mess with additional configuration or documentation--the functions themselves are the source of truth.