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()