Although in a twisted world with deferreds you don't really need to specify the return type as you did in your example.
Perhaps it's worth designing the python bindings after features found in recent python versions such as generators and decorators, which wingo wrote about a couple of days ago. You might also want to take a look at xmlrpclib, which is very easy to use, thanks to excessive getattr usage. Not sure if you want to walk down that road for D-BUS, but it's definitely worth looking at.