"is" and "=="

[Dagur Páll Ammendrup]

Thomas Wouters wrote:

>On Tue, Mar 11, 2003 at 04:46:26PM +0000, Dagur Páll Ammendrup wrote:
>  
>
>>I was wondering when == should be used and when "is". I like using "is" 
>>when comparing strings and stuff but maybe I shouldn't?
>>    
>>
>
>Yes, you shouldn't. 'is' tests for identity; whether you have two references
>to the same object. This often looks the same as testing for equality when
>doing small tests with small immutable objects (like small integers or short
>strings) in the interactive interpreter, but easily breaks for real-world
>comparisons:
>
>  
>
>>>>"spam" is "spam"
>>>>        
>>>>
>1                                                                               
>  
>
>>>>"spam" is "sp" + "am"
>>>>        
>>>>
>0                                                                               
>  
>
>>>>101 is 101
>>>>        
>>>>
>1                                                                               
>  
>
>>>>101 is 100 + 1                                                              
>>>>        
>>>>
>0                                                                               
>
>The 'id' function might make it clearer for you:
>
>  
>
>>>>id("spam"), id("spam")
>>>>        
>>>>
>(135743896, 135743896)                                                          
>  
>
>>>>id("spam"), id("sp" + "am")
>>>>        
>>>>
>(135743896, 135743944)                                                          
>  
>
>>>>id(101), id(100 + 1)                                                       
>>>>        
>>>>
>(135388108, 135388048)                                                          
>
>The reason the 'is' test succeeds so often for simple tests is because
>Python caches strings (in particular ones created from string-literals, like
>"spam" and "sp" above) and small numbers internally, which is safe to do
>since they are immutable. You should not rely on this behaviour, though.
>
>There are very few uses of 'is' in normal user code...
>(much like 'from module import *' :)
>
>  
>
Ah ok, I wish it was the same as ==, it just looks nicer :-)