find usage for elasticache security groups

def _find_usage_security_groups(self):
        """find usage for elasticache security groups"""
        num_groups = 0
        # If EC2-Classic isn't available (e.g., a new account)
        # this method will fail with:
        #   Code:    "InvalidParameterValue"
        #   Message: "Use of cache security groups is not permitted in
        #             this API version for your account."
        #   Type:    "Sender"
        try:
            paginator = self.conn.get_paginator(
                'describe_cache_security_groups')
            for page in paginator.paginate():
                for secgroup in page['CacheSecurityGroups']:
                    num_groups += 1
        except ClientError as ex:
            if ex.response['Error']['Code'] != 'InvalidParameterValue':
                raise ex
            logger.debug("caught ClientError checking ElastiCache security "
                         "groups (account without EC2-Classic?)")

        self.limits['Security Groups']._add_current_usage(
            num_groups,
            aws_type='WS::ElastiCache::SecurityGroup'
        )