Re: Multi-dimensional list initialization

Date	2012-11-05 21:51 -0800
From	Andrew Robinson <andrew3@r3dsolutions.com>
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>
Newsgroups	comp.lang.python
Message-ID	<mailman.3311.1352181444.27098.python-list@python.org> (permalink)

Show all headers | 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.

:)

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