Re: Explanation of list reference

From	Ben Finney <ben+python@benfinney.id.au>
Subject	Re: Explanation of list reference
Date	2014-02-15 15:20 +1100
References	(3 earlier) <871tz5piy0.fsf@elektro.pacujo.net> <mailman.6934.1392409097.18130.python-list@python.org> <87vbwho1i0.fsf@elektro.pacujo.net> <mailman.6938.1392412611.18130.python-list@python.org> <87mwhtnzdu.fsf@elektro.pacujo.net>
Newsgroups	comp.lang.python
Message-ID	<mailman.6957.1392438051.18130.python-list@python.org> (permalink)

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Marko Rauhamaa <marko@pacujo.net> writes:

> Chris Angelico <rosuav@gmail.com>:
> > Distinguishing "small values" from "big values" leads to the obvious
> > question: Which is which? And why doesn't this work?
>
> This is related to the recent id(string) question on this forum.
>
> Unfortunately neither the "everything is a reference" model nor the
> "small/big" model help you predict the value of an "is" operator in the
> ambiguous cases.

You should never need to predict the result of an ‘is’ operation. (More
precisely, for *some* cases you can predict it, but for other cases you
can't.)

The Python implementation is free to behave unpredictably for the state
of object identity. It may have some objects that conceptually may be
different (i.e. you'd predict the ‘is’ operation would return False)
actually be the same object (i.e. the ‘is’ operation would return True).

Exactly the same case may behave differently in this regard on other
Python implementations, or different versions of the same Python
implementation, or even exactly the same version of the Python
implementation under different circumstances.

The management of object identity is an implementation detail, not to be
relied on in your Python code. So, if your teaching method depends on
general principles for predicting object identity, you're already
losing.

> Explaining Python's memory model at some level is necessary right off
> the bat. However, it is far from easy to understand.

The use of the term “variable”, and all the implications that has for
“variables contain values” etc., is IMO too confusing, and its use in
the Python documentation is not helping this.

Rather, I recommend that people teaching Python should avoid the term
“variable” entirely, and use the term “reference” which much more
accurately implies Python's data model. An addressable item in a
container is a reference, as is a name. Assignment binds a reference to
a value. And so on.

This article by the Effbot corrects similar misconceptions
<URL:http://effbot.org/zone/python-objects.htm>, and is a valuable
approach to teaching the Python data model.

-- 
 \       “Theology is the effort to explain the unknowable in terms of |
  `\                         the not worth knowing.” —Henry L. Mencken |
_o__)                                                                  |
Ben Finney

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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