I wonder if it makes more sense e.g. for macOS to create a Swift UI and an existing python code base as external process which is called on request.

Does somebody knows which API interface specification is most suitable for that kind of communication on a single PC: Swift <-> Python ? For web it easy (e.g. REST) but there has to be other options on a fully controllable code base on your PC (seems my desktop programming days are long gone.. otherwise I would know myself)