マクロ展開とかに便利。
>>> spam_macros = [('egg', 'egg and spam'), ('spam', 'spam with bacon'), ('ham', 'ham and spam and egg')]
>>> reduce(lambda x, y: x.replace(y[0], y[1]), spam_macros, 'egg and bacon and ham')
'egg and spam with bacon and bacon and ham and spam and egg'
>>>