The callback will be a stream level handler, and will not
execute in its own thread. If you must have a thread, have the
callback function raise a custom event, which can be processed
by another event handler, which may run in an individual thread,
like so:
def handle_reply(self, iq):
self.event('custom_event', iq)
def do_long_operation_in_thread(self, iq):
...
self.add_event_handler('custom_event', self.do_long_operation_in_thread)
...take out already prepared iq stanza...
iq.send(callback=self.handle_reply)