code: def _check_subnet(self, name):
          
          # Subnets only exist in VPCs, so we don't need to worry about
          # the EC2 Classic case here.
          subnets = self._vpc_connection.get_all_subnets(
              filters={'vpcId': self._vpc_id})
  
          matching_subnets = [
              subnet
              for subnet
               in subnets
               if name in [subnet.tags.get('Name'), subnet.id]
          ]
          if len(matching_subnets) == 0:
              raise SubnetError(
                  "the specified subnet %s does not exist" % name)
          elif len(matching_subnets) == 1:
              return matching_subnets[0].id
          else:
              raise SubnetError(
                  "the specified subnet name %s matches more than "
                  "one subnet" % name)
docs: Checks if the subnet exists.
  
          :param str name: name of the subnet
          :return: str - subnet id of the subnet
          :raises: `SubnetError` if group does not exist