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In my experience, it's just easier to take their API descriptor file (JSON IIRC) and generate a client from it.

This is what boto3 does

Wouldn't that be WADL?

No, WADL is XML. You can see some of the data files in the boto3 repository: https://github.com/boto/boto3/tree/develop/boto3/data

It is somewhat similar in concept

WADL is an description language for REST services.