This is in error, this should actually read: 


        si_prop = ServiceInstanceType(left_virtual_network = ":".join(self._left_vn_fq_name), 
                                      management_virtual_network = ":".join(self._mgmt_vn_fq_name),
                                      right_virtual_network = ":".join(self._right_vn_fq_name_vn))