Most uses of OpenAPI I've seen is simple REST API documentation. Many internet standards & protocols are typically developed together with their reference impl

Most uses of OpenAPI I've seen is simple REST API documentation. Many internet standards & protocols are typically developed together with their reference implementation. E.g. the IETF is pretty good at that for things like HTTP. Waterfall just doesn't work for anything moderately complicated. If it's simple, a lot of upfront documentation is not going to be that helpful. If it's not, a proof of concept implementation that irons out all the design mistakes and that proves it is any good would be a good idea. There aren't a lot of good protocols that get developed without those. Anyway, the article is about slapping openapi specs on python web frameworks, which suggests it's being used in its usual role of documenting existing APIs here. And for REST protocols, python is a great tool to prototype those.