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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: id() and is operator
Date: Mon, 23 Feb 2015 01:02:13 -0500
References: <87f18c68-120d-44f2-bd34-6f73c69365da@googlegroups.com> <871tlhiqpz.fsf@elektro.pacujo.net>
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Xref: csiph.com comp.lang.python:86192

On 2/22/2015 4:25 PM, Marko Rauhamaa wrote:
> LJ <luisjosenovoa@gmail.com>:
>
>>>>> id(b[0])
>> 45855552
> [...]
>>>>> id(b[2])
>> 45855552

>> Please correct me if I am wrong, but according to this b[2] and b[0]
>> are the same object. Now,
>>
>>>>> b[0] is b[2]
>> False
>
> This is a true statement:
>
>     If X is Y, then id(X) == id(Y).
>
> However, this is generally not a true statement:
>
>     If X is Y, then id(X) is id(Y).

If X and Y exist at the *same time*, then (X is Y) == (id(X) is id(Y)). 
  Since X and Y in the example above do not exist at the same time, it 
is nonsensical to compare them.

-- 
Terry Jan Reedy