说明：
字符串就是一系列字符，在Python中用引号括起来的都是字符串，引号可以是单引号，也可以是双引号，比如：“This is a book.” ‘This is an apple.’
接下来简单说明下关于字符串大小写的变换。
操作：
casefold() 所有字母小写
lower() 所有字母小写
upper() 所有字母大写
capitalize() 所有字母小写
title() 所有首字母大写，其余字母小写
swapcase() 大小写字母替换
这几个方法都是生成新的字符串，不对原字符串内容进行修改。
demo = ‘tHis iS a GOod boOK.‘ print(demo.casefold()) print(demo.lower()) print(demo.upper()) print(demo.capitalize()) print(demo.title()) print(demo.swapcase())
输出：
this is a good book. this is a good book. THIS IS A GOOD BOOK. This is a good book. This Is A Good Book. ThIS Is A goOD BOok.
原文：https://www.cnblogs.com/mengjinxiang/p/12368482.html