def check_filter_validity(self, reply, filter_str):
        """Check for the validify of the filter. """
        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
Check for the validify of the filter.