docs: def write_history_file(self, filename = None): 
          u'''Save a readline history file.'''
          if filename is None:
              filename = self.history_filename
          fp = open(filename, u'wb')
          for line in self.history[-self.history_length:]:
              fp.write(ensure_str(line.get_line_text()))
              fp.write(u'\n')
          fp.close()