def proto_version(self, vnum):
        """
        Protocol message: (version version-number)

        Check to make sure that both ends of the protocol are speaking
        the same version dialect.

        @param vnum: The version number.
        """

        if vnum != self.version:
            raise ProtocolError("Version Incompatibility: %s %s" % (self.version, vnum))

        Protocol message: (version version-number)

        Check to make sure that both ends of the protocol are speaking
        the same version dialect.

        @param vnum: The version number.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/spread/pb.py