Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #32806

Re: Multi-dimensional list initialization

Path csiph.com!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path <andrew3@r3dsolutions.com>
X-Original-To python-list@python.org
Delivered-To python-list@mail.python.org
X-Spam-Status OK 0.011
X-Spam-Evidence '*H*': 0.98; '*S*': 0.00; 'operator': 0.03; 'none,': 0.05; 'received:64.202.165': 0.07; 'python': 0.09; 'compact': 0.09; 'logic': 0.09; 'modifies': 0.09; 'notation': 0.09; 'underlying': 0.09; 'backward': 0.16; 'benjamin': 0.16; 'clone': 0.16; 'copied.': 0.16; 'elements,': 0.16; 'instantiate': 0.16; 'integers,': 0.16; 'multiplied': 0.16; 'operation.': 0.16; 'reason.': 0.16; 'received:64.202.165.33': 0.16; 'received:smtpauth11.prod.mesa1.secureserver.net': 0.16; 'shallow': 0.16; 'wrote:': 0.17; 'element': 0.17; 'implementing': 0.17; 'typing': 0.17; '>>>': 0.18; 'otherwise,': 0.20; 'bit': 0.21; 'modifying': 0.22; 'subject:skip:i 10': 0.22; 'example': 0.23; 'programming': 0.23; 'elements': 0.23; 'header:In-Reply- To:1': 0.25; 'header:User-Agent:1': 0.26; 'skip:[ 10': 0.26; 'common': 0.26; 'implemented': 0.27; 'plain': 0.27; 'reflect': 0.27; 'subject:list': 0.28; 'chris': 0.28; 'initial': 0.28; 'run': 0.28; 'post': 0.28; 'behaviour': 0.29; 'closer': 0.29; 'factor': 0.29; 'routine': 0.29; 'array': 0.29; "we're": 0.30; 'stuff': 0.30; 'lists': 0.31; 'point': 0.31; 'could': 0.32; 'problem': 0.33; 'anyone': 0.33; 'to:addr:python-list': 0.33; 'everyone': 0.33; "can't": 0.34; 'wrong': 0.34; 'list': 0.35; 'needed': 0.35; 'whatever': 0.35; 'lists.': 0.35; 'nov': 0.35; 'so,': 0.35; 'doing': 0.35; 'pm,': 0.35; 'something': 0.35; 'list.': 0.35; 'really': 0.36; 'but': 0.36; "wasn't": 0.36; 'anything': 0.36; 'enough': 0.36; 'bad': 0.37; 'level': 0.37; 'being': 0.37; 'why': 0.37; 'quite': 0.37; 'rather': 0.37; 'far': 0.37; 'subject:: ': 0.38; 'fact': 0.38; 'object': 0.38; 'things': 0.38; 'to:addr:python.org': 0.39; 'short': 0.39; 'where': 0.40; 'subject:-': 0.40; 'think': 0.40; 'claim': 0.60; 'easy': 0.60; 'most': 0.61; "you've": 0.61; 'received:network': 0.61; '(that': 0.62; 'relatively': 0.62; 'worth': 0.63; 'different': 0.63; 'received:unknown': 0.63; 'ever': 0.63; 'skip:n 10': 0.63; 'more': 0.63; 'within': 0.64; '10.': 0.64; 'behavior': 0.64; 'making': 0.64; 'copies.': 0.65; 'realise': 0.65; 'header:Reply-To:1': 0.68; 'further,': 0.71; 'reply-to:no real name:2**0': 0.72; 'low': 0.83; 'observed': 0.84; 'oscar': 0.84; 'mean.': 0.91; 'from.': 0.93
Date Mon, 05 Nov 2012 21:51:24 -0800
From Andrew Robinson <andrew3@r3dsolutions.com>
User-Agent Mozilla/5.0 (X11; Linux i686; rv:8.0) Gecko/20111126 Thunderbird/8.0
MIME-Version 1.0
To python-list@python.org
Subject Re: Multi-dimensional list initialization
References <mailman.3267.1352096879.27098.python-list@python.org> <50978323$0$6908$e4fe514c@news2.news.xs4all.nl> <CAHVvXxQR2J2tyFcQZd7bNYTAeetwM+LfoV4UcYrkRfeNVOj+pg@mail.gmail.com> <CAPTjJmqzQi2TEjme_we-KfmG7vxOfRWoKG=zL=SGHYGqiFf9_Q@mail.gmail.com> <CAHVvXxQNCxpACM=tkKir0f3iVqui4oVGnyS-GywO+dtGacKAdQ@mail.gmail.com>
In-Reply-To <CAHVvXxQNCxpACM=tkKir0f3iVqui4oVGnyS-GywO+dtGacKAdQ@mail.gmail.com>
Content-Type text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding 7bit
X-BeenThere python-list@python.org
X-Mailman-Version 2.1.15
Precedence list
Reply-To andrew3@r3dsolutions.com
List-Id General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe <http://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive <http://mail.python.org/pipermail/python-list/>
List-Post <mailto:python-list@python.org>
List-Help <mailto:python-list-request@python.org?subject=help>
List-Subscribe <http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups comp.lang.python
Message-ID <mailman.3311.1352181444.27098.python-list@python.org> (permalink)
Lines 70
NNTP-Posting-Host 2001:888:2000:d::a6
X-Trace 1352181444 news.xs4all.nl 6925 [2001:888:2000:d::a6]:58935
X-Complaints-To abuse@xs4all.nl
Xref csiph.com comp.lang.python:32806

Show key headers only | View raw

On 11/05/2012 06:30 PM, Oscar Benjamin wrote:
> On 6 November 2012 02:01, Chris Angelico<rosuav@gmail.com>  wrote:
>> On Tue, Nov 6, 2012 at 12:32 PM, Oscar Benjamin
>> <oscar.j.benjamin@gmail.com>  wrote:
>>> I was just thinking to myself that it would be a hard thing to change
>>> because the list would need to know how to instantiate copies of all
>>> the different types of the elements in the list. Then I realised it
>>> doesn't. It is simply a case of how the list multiplication operator
>>> is implemented and whether it chooses to use a reference to the same
>>> list or make a copy of that list. Since all of this is implemented
>>> within the same list type it is a relatively easy change to make
>>> (ignoring backward compatibility concerns).
>>>
>>> I don't see this non-copying list multiplication behaviour as
>>> contradictory but has anyone ever actually found a use for it?
>> Stupid example of why it can't copy:
>>
>> bad = [open("test_file")] * 4
>>
>> How do you clone something that isn't Plain Old Data? Ultimately,
>> that's where the problem comes from. It's easy enough to clone
>> something that's all scalars (strings, integers, None, etc) and
>> non-recursive lists/dicts of scalars, but anything more complicated
>> than that is rather harder.
> That's not what I meant. But now you've made me realise that I was
> wrong about what I did mean. In the case of
>
>     stuff = [[obj] * n] * m
>
> I thought that the multiplication of the inner list ([obj] * n) by m
> could create a new list of lists using copies. On closer inspection I
> see that the list being multiplied is in fact [[obj] * n] and that
> this list can only know that it is a list of lists by inspecting its
> element(s) which makes things more complicated.
>
> I retract my claim that this change would be easy to implement.
>
>
> Oscar
Hi Oscar,

In general, people don't use element multiplication (that I have *ever* 
seen) to make lists where all elements of the outer most list point to 
the same sub-*list* by reference.  The most common use of the 
multiplication is to fill an array with a constant, or short list of 
constants;  Hence, almost everyone has  to work around the issue as the 
initial poster did by using a much longer construction.

The most compact notation in programming really ought to reflect the 
most *commonly* desired operation.  Otherwise, we're really just making 
people do extra typing for no reason.

Further, list comprehensions take quite a bit longer to run than low 
level copies; by a factor of roughly 10. SO, it really would be worth 
implementing the underlying logic -- even if it wasn't super easy.

I really don't think doing a shallow copy of lists would break anyone's 
program.
The non-list elements, whatever they are, can be left as reference 
copies -- but any element which is a list ought to be shallow copied.  
The behavior observed in the opening post where modifying one element of 
a sub-list, modifies all elements of all sub-lists is never desired as 
far as I have ever witnessed.

The underlying implementation of Python can check an object type 
trivially, and the only routine needed is a shallow list copy.  So, no 
it really isn't a complicated operation to do shallow copies of lists.

:)

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Multi-dimensional list initialization Demian Brecht <demianbrecht@gmail.com> - 2012-11-04 22:27 -0800
  Re: Multi-dimensional list initialization Hans Mulder <hansmu@xs4all.nl> - 2012-11-05 10:13 +0100
    Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-06 01:32 +0000
    Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-06 13:01 +1100
    Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-06 02:30 +0000
    Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-05 21:51 -0800
      Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-06 09:04 +0000
        RE: Multi-dimensional list initialization "Shambhu Rajak" <shambhu.rajak@calsoftinc.com> - 2012-11-06 18:57 +0530
        Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 14:41 -0800
          Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-07 01:55 +0000
            Re: Multi-dimensional list initialization Demian Brecht <demianbrecht@gmail.com> - 2012-11-06 22:56 -0800
            Re: Multi-dimensional list initialization wxjmfauth@gmail.com - 2012-11-07 00:57 -0800
            Re: Multi-dimensional list initialization Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-07 22:27 +0000
            Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-07 16:39 -0700
            Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-08 00:09 -0700
            Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-08 08:58 -0700
            Re: Multi-dimensional list initialization Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-09 01:39 +0000
            Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-09 17:07 +1100
              Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-09 06:37 +0000
                Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-09 17:59 +1100
                Re: Multi-dimensional list initialization Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-09 07:27 +0000
                Re: Multi-dimensional list initialization rusi <rustompmody@gmail.com> - 2012-11-09 07:05 -0800
                Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-10 02:23 +1100
            Re: Multi-dimensional list initialization Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-11-09 14:34 -0500
            RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-09 20:31 +0000
            Re: Multi-dimensional list initialization Ethan Furman <ethan@stoneleaf.us> - 2012-11-09 13:49 -0800
        RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-06 23:39 +0000
        Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-06 16:52 -0700
        Re: Multi-dimensional list initialization MRAB <python@mrabarnett.plus.com> - 2012-11-07 00:23 +0000
          Re: Multi-dimensional list initialization rusi <rustompmody@gmail.com> - 2012-11-06 20:11 -0800
          Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-07 05:05 +0000
            Re: Multi-dimensional list initialization Roy Smith <roy@panix.com> - 2012-11-07 00:12 -0500
              Re: Multi-dimensional list initialization Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2012-11-07 18:32 +1300
                RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-07 15:57 +0000
            Re: Multi-dimensional list initialization Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2012-11-07 10:52 +0200
            Re: Multi-dimensional list initialization MRAB <python@mrabarnett.plus.com> - 2012-11-07 17:17 +0000
              Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-07 22:36 +0000
        Re: Multi-dimensional list initialization Ethan Furman <ethan@stoneleaf.us> - 2012-11-07 07:23 -0800
        Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-07 14:01 -0700
          Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-08 00:00 +0000
            Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-08 00:30 +0000
              Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-08 03:47 +0000
            Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-07 20:51 -0800
            Re: Multi-dimensional list initialization wrw@mac.com - 2012-11-08 08:26 -0500
        Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-07 16:24 -0800
          Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-08 04:20 +0000
    Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-06 17:07 +1100
    Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 00:21 -0800
    Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-06 02:19 -0700
    RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-06 17:32 +0000
    Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 13:14 -0800
    Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 13:19 -0800
    Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-06 15:46 -0700
      Re: Multi-dimensional list initialization Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2012-11-07 18:34 +1300
        Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-07 14:00 +0000
        Re: Multi-dimensional list initialization Ethan Furman <ethan@stoneleaf.us> - 2012-11-07 06:47 -0800
        Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-07 23:06 +0000
        Re: Multi-dimensional list initialization Greg Ewing <greg.ewing@canterbury.ac.nz> - 2012-11-08 14:29 +1300
  Re: Multi-dimensional list initialization wxjmfauth@gmail.com - 2012-11-05 01:55 -0800

csiph-web