docs: def list_subnets(self, tag_values=None):
  
          '''
              a method to discover the list of subnets on AWS EC2
  
          :param tag_values: [optional] list of tag values
          :return: list of strings with subnet ids
          '''
  
          title = '%s.list_subnets' % self.__class__.__name__
  
      # validate inputs
          input_fields = {
              'tag_values': tag_values
          }
          for key, value in input_fields.items():
              if value:
                  object_title = '%s(%s=%s)' % (title, key, str(value))
                  self.fields.validate(value, '.%s' % key, object_title)
              
      # add tags to method arguments
          kw_args = {}
          tag_text = ''
          if tag_values:
              kw_args = {
                  'Filters': [ { 'Name': 'tag-value', 'Values': tag_values } ]
              }
              from labpack.parsing.grammar import join_words
              plural_value = ''
              if len(tag_values) > 1:
                  plural_value = 's'
              tag_text = ' with tag value%s %s' % (plural_value, join_words(tag_values))
              
      # request instance details from AWS
          self.iam.printer('Querying AWS region %s for subnets%s.' % (self.iam.region_name, tag_text))
          subnet_list = []
          try:
              if kw_args:
                  response = self.connection.describe_subnets(**kw_args)
              else:
                  response = self.connection.describe_subnets()
          except:
              raise AWSConnectionError(title)
          response_list = []
          if 'Subnets' in response:
              response_list = response['Subnets']
  
      # construct list of subnets from response
          for sub_dict in response_list:
              subnet_list.append(sub_dict['SubnetId'])
  
      # report results and return list
          if subnet_list:
              print_out = 'Found subnet'
              if len(subnet_list) > 1:
                  print_out += 's'
              from labpack.parsing.grammar import join_words
              print_out += ' %s.' % join_words(subnet_list)
              self.iam.printer(print_out)
          else:
              self.iam.printer('No subnets found.')
  
          return subnet_list