def _append(self, signature, fields=(), response=None):
self.packer.pack_struct(signature, fields)<EOL>self.output_buffer.chunk()<EOL>self.output_buffer.chunk()<EOL>self.responses.append(response)<EOL>
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