def cbConsumer(cons):
            """
            Called after the file was opended for reading.

            Prepare the data transfer channel and send the response
            to the command channel.
            """
            if not self.binary:
                cons = ASCIIConsumerWrapper(cons)

            d = self.dtpInstance.registerConsumer(cons)

            # Tell them what to doooo
            if self.dtpInstance.isConnected:
                self.reply(DATA_CNX_ALREADY_OPEN_START_XFR)
            else:
                self.reply(FILE_STATUS_OK_OPEN_DATA_CNX)

            return d

            Called after the file was opended for reading.

            Prepare the data transfer channel and send the response
            to the command channel.
            
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/ftp.py