code: def update_prompt(self, prompt_text):
          
          if(isinstance(prompt_text, basestring)):
              self._prompt = util_functions.sub_chars(prompt_text)
              ret = self._prompt
          else:
              raise util_functions.InputError(prompt_text, "Invalid prompt. Need to enter a string value.")
          return ret
docs: Update the default prompt string, which is "".
          prompt_text should be a string.
          Returns the prompt as a confirmation.