def switchToBodyMode(self, decoder):
        """
        Switch to body parsing mode - interpret any more bytes delivered as
        part of the message body and deliver them to the given decoder.
        """
        if self.state == BODY:
            raise RuntimeError(u"already in body mode")

        self.bodyDecoder = decoder
        self.state = BODY
        self.setRawMode()

        Switch to body parsing mode - interpret any more bytes delivered as
        part of the message body and deliver them to the given decoder.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/_newclient.py