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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Thu, 5 May 2011 10:44:39 -0600
Subject: Re: What other languages use the same data model as Python?
To: John Nagle <nagle@animats.com>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:4758

On Thu, May 5, 2011 at 9:41 AM, John Nagle <nagle@animats.com> wrote:
> On 5/5/2011 3:06 AM, Gregory Ewing wrote:
>>
>> John Nagle wrote:
>>
>>> A reasonable compromise would be that "is" is treated as "=3D=3D" on
>>> immutable objects.
>>
>> That wouldn't work for tuples, which can contain references
>> to other objects that are not immutable.
>
> =A0 =A0Such tuples are still identical, even if they
> contain identical references to immutable objects.

>>> a =3D (1, 2, [3, 4, 5])
>>> b =3D (1, 2, [3, 4, 5])
>>> a =3D=3D b
True
>>> a is b  # Using the proposed definition
True
>>> a[0] is b[0]
True
>>> a[1] is b[1]
True
>>> a[2] is b[2]
False
>>> a[2].append(6)
>>> a
(1, 2, [3, 4, 5, 6])
>>> b
(1, 2, [3, 4, 5])
>>> a =3D=3D b
False
>>> a is b
False

Thus a and b cannot be used interchangeably even though "a is b"
originally returned True.