code: def transformer(self):
          
          if self.factory:
              return self.factory
          elif self.type_:
              return self.type_
          else:
              return None
docs: Return the class for this field that transforms non-Entity objects
          (e.g., dicts or binding objects) into Entity instances.
  
          Any non-None value returned from this method should implement a
          from_obj() and from_dict() method.
  
          Returns:
              None if no type_ or factory is defined by the field. Return a class
              with from_dict and from_obj methods otherwise.