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

>>>