<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def change_prompt_format(self, arg, **_):
 """ Change the prompt format. """

        if not arg:
            message = 'Missing required argument, format.'
            return [(None, None, None, message)]

        self.prompt = self.get_prompt(arg)
        return [(None, None, None, "Changed prompt format to %s" % arg)]