code: def update_list_function(self, list_name, list_func):
          
          if self.get_list_function(list_name) is None:
              raise CloudantArgumentError(113, list_name)
  
          self.lists.__setitem__(list_name, codify(list_func))
docs: Modifies/overwrites an existing list function in the
          locally cached DesignDocument indexes dictionary.
  
          :param str list_name: Name used to identify the list function.
          :param str list_func: Javascript list function.