find usage for route tables

def _find_usage_route_tables(self):
        """find usage for route tables"""
        # Route tables per VPC
        tables = defaultdict(int)
        for table in self.conn.describe_route_tables()['RouteTables']:
            tables[table['VpcId']] += 1
            # Entries per route table
            routes = [
                r for r in table['Routes']
                if r['Origin'] != 'EnableVgwRoutePropagation'
            ]
            self.limits['Entries per route table']._add_current_usage(
                len(routes),
                aws_type='AWS::EC2::RouteTable',
                resource_id=table['RouteTableId']
            )
        for vpc_id in tables:
            self.limits['Route tables per VPC']._add_current_usage(
                tables[vpc_id],
                aws_type='AWS::EC2::VPC',
                resource_id=vpc_id,
            )