string.casefold和string.lower 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？他们各自的应用场景？
对 Unicode 的时候用 casefold
string.casefold官方说明：
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".
The casefolding algorithm is described in section 3.13 of the Unicode Standard
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'：
s = 'ß'
s.lower() # 'ß'
s.casefold() # 'ss'string.lower官方说明：
Return a copy of the string with all the cased characters [4] converted to lowercase.
The lowercasing algorithm used is described in section 3.13 of the Unicode Standard
参考
https://docs.python.org/3/library/stdtypes.html#str.casefold
https://segmentfault.com/q/1010000004586740/a-1020000004586838
总结
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()