code: def check_filter_validity(self, reply, filter_str):
          
          try:
              f_ind = reply.index(filter_str)
              l_ind = reply.rindex(filter_str)
          except Exception:
              fail_reason = vdp_const.filter_failure_reason % (reply)
              LOG.error("%s", fail_reason)
              return False, fail_reason
          if f_ind != l_ind:
              # Currently not supported if reply contains a filter keyword
              fail_reason = vdp_const.multiple_filter_failure_reason % (reply)
              LOG.error("%s", fail_reason)
              return False, fail_reason
          return True, None
docs: Check for the validify of the filter.