def _prepareTLS(self, certificate, verifyAuthorities):
        """
        Used by StartTLSCommand to put us into the state where we don't
        actually send things that get sent, instead we buffer them.  see
        L{_sendBoxCommand}.
        """
        self._startingTLSBuffer = []
        if self.hostCertificate is not None:
            raise OnlyOneTLS(
                "Previously authenticated connection between %s and %s "
                "is trying to re-establish as %s" % (
                    self.hostCertificate,
                    self.peerCertificate,
                    (certificate, verifyAuthorities)))

        Used by StartTLSCommand to put us into the state where we don't
        actually send things that get sent, instead we buffer them.  see
        L{_sendBoxCommand}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/amp.py