def esmtpAUTHRequired(self, code=-1, resp=None):
        """
        Fail because authentication is required, but the server does not support
        any schemes we support.

        @param code: The server status code from the most recently received
            server message.
        @type code: L{int}

        @param resp: The server status response from the most recently received
            server message.
        @type resp: L{bytes}
        """
        tmp = []

        for a in self.authenticators:
            tmp.append(a.getName().upper())

        auth = b"[%s]" % b", ".join(tmp)

        self.sendError(AUTHRequiredError(502, b"Server does not support Client "
            b"Authentication schemes %s" % auth, self.log.str()))

        Fail because authentication is required, but the server does not support
        any schemes we support.

        @param code: The server status code from the most recently received
            server message.
        @type code: L{int}

        @param resp: The server status response from the most recently received
            server message.
        @type resp: L{bytes}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/smtp.py