def dbus_signal(dbus_interface):
    def decorator(func):
        def handler(self, *args, **kwargs):
            func(self, *args, **kwargs)
            # however you emit a dbus signal ...
            emit_signal(self, dbus_interface, func.func_name, *args, **kwargs)
        handler.func_name = func.func_name
        return handler
    return decorator

This decorator wraps the function object so that you can perform a signal emission after running the function. The assignment to the func_name attribute is so that the proxy function reports the right name in stack traces, etc.

def dbus_method(interface_name):
    def decorator(func):
        func.dbus_interface = interface_name
        return func
    return decorator

After the class is constructed, you can check for all methods with dbus_interface attributes to work out what interfaces to provide. Furthermore, if you use a custom meta-class for dbus.Object, you can do some extra processing at class creation time.