def _flushSendBIO(self):
        """
        Read any bytes out of the send BIO and write them to the underlying
        transport.
        """
        try:
            bytes = self._tlsConnection.bio_read(2 ** 15)
        except WantReadError:
            # There may be nothing in the send BIO right now.
            pass
        else:
            self.transport.write(bytes)

        Read any bytes out of the send BIO and write them to the underlying
        transport.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/tls.py