is用来判断两个对象是不是同一个对象，通过id()函数取对象地址进行判断。python会缓存一些小的整数，以及只包含字母数字以及下划线的字符串。所以在对于这些值的时候，is判断为True。
>>> a = 10 >>> b = 10 >>> a is b True >>> a = 257 >>> b = 257 >>> a is b False >>> a = "123" >>> b = "123" >>> a is b True >>> a = "urifsdka" >>> b = "urifsdka" >>> a is b True >>> a = "siru289ur2jfkjhsfa98uasfjh293ruhfkjsdhf9w28u3rhhskfj_fhihfiwehfkjsahfushf" >>> b = "siru289ur2jfkjhsfa98uasfjh293ruhfkjsdhf9w28u3rhhskfj_fhihfiwehfkjsahfushf" >>> a is b #对于很长的字符串也是如此 True >>> a = "a b" #包含空格 >>> b = "a b" >>> a is b False >>> a = "ab " #包含空格 >>> b = "ab " >>> a is b False >>> a = 10.0 >>> b = 10.0 >>> a is b False >>> a = "a#b" >>> b = "a#b" >>> a is b False >>>
原文：https://www.cnblogs.com/luoheng23/p/10988032.html