code: def set_input_fields(self, input_fields):
          
          if not (isinstance(input_fields, basestring) or
                  isinstance(input_fields, types.ListType)):
              raise ValueError("input_fields must be a string or a list")
          self.input_fields = input_fields
          self.generate_json_paths()
          return self
docs: Given a scalar or ordered list of strings generate JSONPaths
          that describe how to access the values necessary for the Extractor