python 的 string.casefold 和 string.lower 方法区别？

python 3.3　引入了string.casefold 方法，其效果和 string.lower 非常类似，都可以把字符串变成小写，那么它们之间有什么区别？什么时候该用string.casefold而非string.lower ？？

In [5]: name = 'Xu Zhoufeng'

In [6]: name.casefold()
Out[6]: 'xu zhoufeng'

In [7]: cname = 'Yu Dongfeng'

In [8]: cname.lower()
Out[8]: 'yu dongfeng'

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对 Unicode 的时候用 casefold

Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. For example, the German lowercase letter 'ß' is equivalent to "ss". Since it is already lowercase, lower() would do nothing to 'ß'; casefold() converts it to "ss".

lower() 只对 ASCII 也就是 'A-Z'有效，但是其它一些语言里面存在小写的情况就没办法了。文档里面举得例子是德语中'ß'的小写是'ss'（这个我也不懂）：

s = 'ß'
s.lower() #  'ß'
s.casefold() # 'ss'

总结来说，汉语 & 英语环境下面，继续用 lower()没问题；要处理其它语言且存在大小写情况的时候再用casefold()。

https://docs.python.org/3/library/stdtypes.html#str.casefold