python字符串，可以用成对的单引号('......')或双引号("......")来标示（标示类型为字符串）
如果字符串内部存在特殊字符，例如单引号或双引号，遵循以下规则：
1. 如果字符串使用成对的单引号来标示，字符串内部的单引号字符一定要用转义字符，字符串内部的双引号字符可以不用转义字符
2. 如果字符串使用成对的双引号来标示，字符串内部的单引号字符可以不用转义字符，字符串内部的双引号字符一定要用转义字符
示例：
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'
>>> mystring = '"Isn\'t," they said.'
>>> print(mystring)
"Isn't," they said.