I'm not familiar with Connexion (this was my first time hearing about it), but it seems, from reading its README, that the two approaches are polar opposites: in Connexion you write the schema first and that is used to hook up your API to your business logic, whereas in molten you write your API using normal Python code and idioms and an OpenAPI schema is generated from that code. As a user of Molten, you don't need to know anything about OpenAPI to be able to use it.