def _flushReceiveBIO(self):
        """
        Try to receive any application-level bytes which are now available
        because of a previous write into the receive BIO.  This will take
        care of delivering any application-level bytes which are received to
        the protocol, as well as handling of the various exceptions which
        can come from trying to get such bytes.
        """
        # Keep trying this until an error indicates we should stop or we
        # close the connection.  Looping is necessary to make sure we
        # process all of the data which was put into the receive BIO, as
        # there is no guarantee that a single recv call will do it all.
        while not self._lostTLSConnection:
            try:
                bytes = self._tlsConnection.recv(2 ** 15)
            except WantReadError:
                # The newly received bytes might not have been enough to produce
                # any application data.
                break
            except ZeroReturnError:
                # TLS has shut down and no more TLS data will be received over
                # this connection.
                self._shutdownTLS()
                # Passing in None means the user protocol's connnectionLost
                # will get called with reason from underlying transport:
                self._tlsShutdownFinished(None)
            except Error:
                # Something went pretty wrong.  For example, this might be a
                # handshake failure during renegotiation (because there were no
                # shared ciphers, because a certificate failed to verify, etc).
                # TLS can no longer proceed.
                failure = Failure()
                self._tlsShutdownFinished(failure)
            else:
                if not self._aborted:
                    ProtocolWrapper.dataReceived(self, bytes)

        # The received bytes might have generated a response which needs to be
        # sent now.  For example, the handshake involves several round-trip
        # exchanges without ever producing application-bytes.
        self._flushSendBIO()

        Try to receive any application-level bytes which are now available
        because of a previous write into the receive BIO.  This will take
        care of delivering any application-level bytes which are received to
        the protocol, as well as handling of the various exceptions which
        can come from trying to get such bytes.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/tls.py