code: def prepare_fields_attribute(attribute_name, attributes, class_name):
          
          attribute = attributes.get(attribute_name)
          if not attribute:
              attribute = tuple()
          elif isinstance(attribute, std_collections.Iterable):
              attribute = tuple(attribute)
          else:
              raise errors.Error('{0}.{1} is supposed to be a list of {2}, '
                                 'instead {3} given', class_name, attribute_name,
                                 fields.Field, attribute)
          return attribute
docs: Prepare model fields attribute.