爲什麼不能用「is」來比較數字？

可能重複：
Python 「is」 operator behaves unexpectedly with integers 爲什麼不能用「is」來比較數字？

我通常使用type(x) == type(y)進行比較，如果類型相同。然後使用x==y來比較數值是否相等。

但是，有人提議可以只使用z1 is z2來比較z1和z2是否包含具有完全相同值的相同類型的數字。在很多情況下，它會成功（特別是對於正面整數）。

但是，有時相同的數字（主要是負整數）可能有幾個不同的實例。這是python的預期行爲嗎？

例如：

>>> for x in range(-20,125): 
    z1=x 
    z2=int(float(x)) 
    if z1 is not z2: 
     print "z1({z1}; type = {typez1}; id={idz1}) is not z2({z2}; type = {typez2}; id={idz2})".format(z1=z1,typez1=type(z1),idz1=id(z1),z2=z2,typez2=type(z2),idz2=id(z2)) 


z1(-20; type = <type 'int'>; id=33869592) is not z2(-20; type = <type 'int'>; id=33870384) 
z1(-19; type = <type 'int'>; id=33870480) is not z2(-19; type = <type 'int'>; id=33870408) 
z1(-18; type = <type 'int'>; id=32981032) is not z2(-18; type = <type 'int'>; id=33870384) 
z1(-17; type = <type 'int'>; id=33871368) is not z2(-17; type = <type 'int'>; id=33870408) 
z1(-16; type = <type 'int'>; id=33869712) is not z2(-16; type = <type 'int'>; id=33870384) 
z1(-15; type = <type 'int'>; id=33869736) is not z2(-15; type = <type 'int'>; id=33870408) 
z1(-14; type = <type 'int'>; id=33869856) is not z2(-14; type = <type 'int'>; id=33870384) 
z1(-13; type = <type 'int'>; id=33869280) is not z2(-13; type = <type 'int'>; id=33870408) 
z1(-12; type = <type 'int'>; id=33868464) is not z2(-12; type = <type 'int'>; id=33870384) 
z1(-11; type = <type 'int'>; id=33868488) is not z2(-11; type = <type 'int'>; id=33870408) 
z1(-10; type = <type 'int'>; id=33869616) is not z2(-10; type = <type 'int'>; id=33870384) 
z1(-9; type = <type 'int'>; id=33871344) is not z2(-9; type = <type 'int'>; id=33870408) 
z1(-8; type = <type 'int'>; id=33869064) is not z2(-8; type = <type 'int'>; id=33870384) 
z1(-7; type = <type 'int'>; id=33870336) is not z2(-7; type = <type 'int'>; id=33870408) 
z1(-6; type = <type 'int'>; id=33870360) is not z2(-6; type = <type 'int'>; id=33870384) 
>>> x 
124 
>>> print x 
124 
>>> import sys 
>>> print sys.version 
2.7.2+ (default, Oct 4 2011, 20:06:09) 
[GCC 4.6.1]

來源

2012-07-23 Wang

http://stackoverflow.com/questions/306313/python-is-operator-behaves-unexpectedly-with-integers也有你的答案 – 2012-07-23 18:43:00

誰提出的？他們應該被打了。 – delnan 2012-07-23 18:43:04

對於一小部分整數，這是真實的（在CPython中）。不應該依賴這種行爲，因爲它不能保證在python實現中是相同的。 – 2012-07-23 18:56:16