Re: Explanation of list reference

From	Terry Reedy <tjreedy@udel.edu>
Subject	Re: Explanation of list reference
Date	2014-02-15 15:40 -0500
References	(13 earlier) <mailman.6955.1392435220.18130.python-list@python.org> <cf89a78f-3d3a-4df5-9a9a-c0bda8359d8b@googlegroups.com> <CALwzidkCwc+x7sS11cqcND-sa8YM3T4DP3Ly0dNwAX0_gS4V_A@mail.gmail.com> <mailman.6964.1392442298.18130.python-list@python.org> <dfe26f72-10bc-466d-8086-2a0e32880aae@googlegroups.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.7021.1392496901.18130.python-list@python.org> (permalink)

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On 2/15/2014 1:07 AM, Rustom Mody wrote:
> On Saturday, February 15, 2014 10:50:35 AM UTC+5:30, Ian wrote:
>> On Fri, Feb 14, 2014 at 9:24 PM, Rustom Mody  wrote:
>>> In the case of physical objects like dice there is a fairly
>>> unquestionable framing that makes identity straightforward --
>>> 4-dimensional space-time coordiantes. If the space-time coordinates of
>>> 2 objects are all equal then the objects are identical, else not.
>>> Now we analogize the space-time identity of physical objects to
>>> computer identity of computer objects (so-called) and all sorts of
>>> problems ensue.
>>> To start with we say two objects are identical if they have the same
>>> memory address.
>
>> This is false.  It happens to hold for CPython, but that's an
>> implementation detail.  The definition of object identity does not
>> depend on memory address.  It also doesn't have anything to do with
>> space-time coordinates.  The concept of object identity is an
>> abstraction, not an analogy from physics.
>
>> The language reference states, "Every object has an identity, a type
>> and a value. An object's identity never changes once it has been
>> created; you may think of it as the object's address in memory."
>> Okay, so that quote does bring up memory address, but in my
>> interpretation that's just an analogy to introduce the concept.  The
>> more important part of that sentence is the first part, which ties an
>> object's identity to its creation.  If two objects share the same
>> creation, then they're the same object.
>
> Whats the notion of object identity is the question.
> Ok so you reject the memory addr as an 'implementation detail'
> Then you are obliged to provide some other way of understanding object-identity

'Identity' is an abstraction. For immutable objects, it is essentially 
irrelevant. For mutable objects, it is essential in the following sense. 
  A and B are the same object if mutating A necessarily mutates B and 
different if they can be independently mutated.

The id() of an object is a fixed integer that is temporally unique. An 
implementation might use the id to access objects. When one executes 
Python mentally, one might skip implementing id(). Its main use is to 
test an implementation. Most beginners should ignore it.

-- 
Terry Jan Reedy

Thread

Explanation of list reference dave em <daveandem2000@gmail.com> - 2014-02-14 10:08 -0800
  Re: Explanation of list reference Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-02-14 20:26 +0200
    Re: Explanation of list reference dave em <daveandem2000@gmail.com> - 2014-02-14 10:54 -0800
      Re: Explanation of list reference Denis McMahon <denismfmcmahon@gmail.com> - 2014-02-14 19:20 +0000
      Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 21:56 +0200
        Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-14 13:17 -0700
          Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 22:58 +0200
            Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 08:16 +1100
              Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 23:43 +0200
                Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-14 19:00 -0500
                Re: Explanation of list reference dave em <daveandem2000@gmail.com> - 2014-02-14 16:26 -0800
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:07 +0000
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 11:00 +0200
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 11:57 +1100
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 17:55 -0800
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 13:08 +1100
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 18:34 -0800
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 13:42 +1100
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 19:14 -0800
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 14:33 +1100
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 20:24 -0800
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 15:37 +1100
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:54 +0000
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 18:19 +1100
                Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-14 22:20 -0700
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 07:03 +0000
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 16:31 +1100
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 22:07 -0800
                Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-15 17:19 +1100
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 22:31 -0800
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 17:23 +1100
                Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 01:08 -0700
                Re: Explanation of list reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-15 12:42 +0000
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 15:44 +0200
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-16 06:00 -0800
                Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-16 11:29 -0500
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-16 08:50 -0800
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 20:01 +0200
                Re: Explanation of list reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-16 18:36 +0000
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 05:52 +0000
                Re: Explanation of list reference Terry Reedy <tjreedy@udel.edu> - 2014-02-15 15:40 -0500
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 02:27 +0000
                Re: Explanation of list reference Grant Edwards <invalid@invalid.invalid> - 2014-02-15 19:02 +0000
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:44 +0000
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 18:05 +1100
                Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 00:29 +1300
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:15 +0000
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-14 22:24 -0800
                Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 00:32 +1300
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:37 +1100
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 11:10 +0200
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 10:31 +0000
                Re: Explanation of list reference Christian Gollwitzer <auriocus@gmx.de> - 2014-02-15 10:44 +0100
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 12:10 +0200
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 10:53 +0000
                Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-17 11:40 +1300
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 05:57 +0000
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:21 +1100
                Re: Explanation of list reference Alister <alister.ware@ntlworld.com> - 2014-02-16 19:16 +0000
                Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-15 15:20 +1100
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 11:31 +0200
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 12:13 +0200
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 11:17 +0000
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 14:07 +0200
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 14:41 +0000
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 18:29 +0200
                Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-15 12:02 -0500
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-15 09:30 -0800
                Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 11:37 -0700
                Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 11:45 -0700
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 19:05 +0000
                Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 10:11 -0700
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 22:20 +0200
                Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-15 14:20 -0700
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-15 09:47 -0800
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 19:17 +0000
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 23:01 +0200
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 02:25 +0000
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 13:18 +0200
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:31 +1100
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 10:40 +0000
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 22:17 +1100
            Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-14 23:20 +0200
            Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 10:08 +0200
              Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-16 01:28 -0700
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 12:52 +0200
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 22:24 +1100
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 12:04 +0000
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-16 18:17 +0200
              Re: Explanation of list reference Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-02-16 10:35 +0200
              Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-16 01:35 -0700
              Re: Explanation of list reference Alan Bawden <alan@scooby-doo.csail.mit.edu> - 2014-02-16 03:38 -0500
              Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-16 08:40 +0000
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 21:18 +1100
              Re: Explanation of list reference Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2014-02-16 12:10 -0500
        Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-14 15:45 -0500
      Re: Explanation of list reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-02-14 13:12 -0700
      Re: Explanation of list reference Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-02-14 22:36 +0200
        Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 01:03 +1300
        Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-15 23:21 +1100
  Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-14 13:42 -0500
  Re: Explanation of list reference Ryan Gonzalez <rymg19@gmail.com> - 2014-02-14 12:31 -0600
    Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 05:36 +0000
      Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 17:07 +1100
        Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-15 06:48 +0000
          Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 17:57 +1100
            Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-16 01:18 +1300
              Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-15 23:24 +1100
                Re: Explanation of list reference Marko Rauhamaa <marko@pacujo.net> - 2014-02-15 15:25 +0200
                Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-16 00:59 +1100
                Re: Explanation of list reference Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-17 11:54 +1300
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 10:02 +1100
                Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-16 18:46 -0500
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 12:26 +1100
                Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-17 10:05 +1100
                Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-16 18:43 -0500
                Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-17 11:04 +1100
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 06:31 +0000
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-17 05:43 -0800
                Re: Explanation of list reference Ned Batchelder <ned@nedbatchelder.com> - 2014-02-16 20:43 -0500
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 12:59 +1100
                Re: Explanation of list reference Roy Smith <roy@panix.com> - 2014-02-16 22:28 -0500
                Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-17 14:52 +1100
                Re: Explanation of list reference Rustom Mody <rustompmody@gmail.com> - 2014-02-16 20:35 -0800
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 08:03 +0000
                Re: Explanation of list reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-17 06:21 +0000
                Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-17 18:11 +1100
                Re: Explanation of list reference Rotwang <sg552@hotmail.co.uk> - 2014-02-17 18:24 +0000
          Re: Explanation of list reference Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2014-02-15 14:30 -0500
  Re: Explanation of list reference Terry Reedy <tjreedy@udel.edu> - 2014-02-14 16:37 -0500
  Re: Explanation of list reference pecore@pascolo.net - 2014-02-14 23:34 +0100
  Re: Explanation of list reference Christian Gollwitzer <auriocus@gmx.de> - 2014-02-15 10:38 +0100
  Re: Explanation of list reference Ben Finney <ben+python@benfinney.id.au> - 2014-02-16 01:20 +1100
  Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 01:26 +1100
  Re: Explanation of list reference Joshua Landau <joshua@landau.ws> - 2014-02-15 20:44 +0000
  Re: Explanation of list reference Chris Angelico <rosuav@gmail.com> - 2014-02-16 09:27 +1100

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