def _checkHandshakeStatus(self):
        """
        Ask OpenSSL to proceed with a handshake in progress.

        Initially, this just sends the ClientHello; after some bytes have been
        stuffed in to the C{Connection} object by C{dataReceived}, it will then
        respond to any C{Certificate} or C{KeyExchange} messages.
        """
        # The connection might already be aborted (eg. by a callback during
        # connection setup), so don't even bother trying to handshake in that
        # case.
        if self._aborted:
            return
        try:
            self._tlsConnection.do_handshake()
        except WantReadError:
            self._flushSendBIO()
        except Error:
            self._tlsShutdownFinished(Failure())
        else:
            self._handshakeDone = True
            if IHandshakeListener.providedBy(self.wrappedProtocol):
                self.wrappedProtocol.handshakeCompleted()

        Ask OpenSSL to proceed with a handshake in progress.

        Initially, this just sends the ClientHello; after some bytes have been
        stuffed in to the C{Connection} object by C{dataReceived}, it will then
        respond to any C{Certificate} or C{KeyExchange} messages.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/tls.py