code: def _append(self, signature, fields=(), response=None):
          
          self.packer.pack_struct(signature, fields)
          self.output_buffer.chunk()
          self.output_buffer.chunk()
          self.responses.append(response)
docs: Add a message to the outgoing queue.
  
          :arg signature: the signature of the message
          :arg fields: the fields of the message as a tuple
          :arg response: a response object to handle callbacks