For SD I have a ServiceDescription with a fixed set of attributes. These should be read only for the user. The obvious way of doing this is to use the @Property decorator as in the Parrot example in http://docs.python.org/library/functions.html#property (the _voltage can be set of course but voltage can't) In my case I then have, where sd is a ServiceDescription, sd.uid, sd.url, sd.name etc. I will also have sd.relatedServices which is a list which I would always create even if empty to allow people to use it without checking first. For the data fields obtained via sd.get_data in the spec I would make sd.data (also read only) return a dictionary (maybe empty). With this: "print sd.data" works without any effort. The dictionary will contain anything you want - but normally it will be simple things like strings and numbers or lists of them. I would also suggest not implementing the async stuff in python. Threads are easy enough for the user to do in python - and you can put exactly what you want in each thread. Steve