I think he addressed that (or a very similar circumstance) while talking about the stdlib heap, and again in response to the question on "bag of related functio

I think he addressed that (or a very similar circumstance) while talking about the stdlib heap, and again in response to the question on "bag of related functions." I don't think he'd take issue with your position. Alternatively, if you are working with things that you expect to follow a certain interface, Python does not really require a formal definition. Duck typing should suffice.