OpenAPI-generator [0], written in Java, can produce Python stumps, but I never liked that approach.

Connexion [1] is built on top on Flask and does routing and validation based on an OpenAPI spec.

I've recently started developing Pyotr [2], which does the same only based on ASGI and Starlette. It also includes a client module.

[0] https://openapi-generator.tech/

[1] https://connexion.readthedocs.io/

[2] https://github.com/berislavlopac/pyotr