code: def update_show_function(self, show_name, show_func):
          
          if self.get_show_function(show_name) is None:
              raise CloudantArgumentError(114, show_name)
  
          self.shows.__setitem__(show_name, show_func)
docs: Modifies/overwrites an existing show function in the
          locally cached DesignDocument shows dictionary.
  
          :param show_name: Name used to identify the show function.
          :param show_func: Javascript show function.