python：将切片的字符串与“is”等于False的字符串进行比较

Question

将包含多个字符的切片字符串与另一个字符串is等于False进行比较。见Why does comparing strings in Python using either '==' or 'is' sometimes produce a different result?

但只比较1个字符切片与is等于True。为什么呢？

>>>> 'ab'[1:] is 'b'
True
>>>> 'abb'[1:] is 'bb'
False