code: def _begin_stream(self, command: Command):
          '''Start data stream transfer.'''
          begin_reply = yield from self._commander.begin_stream(command)
  
          self._response.reply = begin_reply
  
          self.event_dispatcher.notify(self.Event.begin_transfer, self._responsef _begin_stream(self, command: Command):
          '''Start data stream transfer.'''
          begin_reply = yield from self._commander.begin_stream(command)
  
          self._response.reply = begin_reply
  
          self.event_dispatcher.notify(self.Event.begin_transfer, self._response)
docs: Start data stream transfer.