Re: Multi-dimensional list initialization

Date	2012-11-06 13:14 -0800
From	Andrew Robinson <andrew3@r3dsolutions.com>
Subject	Re: Multi-dimensional list initialization
References	(2 earlier) <CAHVvXxQR2J2tyFcQZd7bNYTAeetwM+LfoV4UcYrkRfeNVOj+pg@mail.gmail.com> <CAPTjJmqzQi2TEjme_we-KfmG7vxOfRWoKG=zL=SGHYGqiFf9_Q@mail.gmail.com> <CAHVvXxQNCxpACM=tkKir0f3iVqui4oVGnyS-GywO+dtGacKAdQ@mail.gmail.com> <5098A55C.3090201@r3dsolutions.com> <CAHVvXxSyVvZi-qcuZC3-gnmpQNdrp7jMT2rXp5T1f719_1BN-Q@mail.gmail.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.3339.1352236812.27098.python-list@python.org> (permalink)

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On 11/06/2012 06:35 AM, Oscar Benjamin wrote:
>
> > 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.
>
> That's what I have seen as well. I've never seen an example where 
> someone wanted this behaviour.
>
> >
> > 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.
>
> It's not so much the typing as the fact that this a common gotcha. 
> Apparently many people expect different behaviour here. I seem to 
> remember finding this surprising at first.
>
:)  That's true as well.
>
> >
> > 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.
>
> It is a semantic change that would, I imagine, break many things in 
> subtle ways.
>
?? Do you have any guesses, how ?
>
> >
> > 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.
>
> Yes but if you're inspecting the object to find out whether to copy it 
> what do you test for? If you check for a list type what about 
> subclasses? What if someone else has a custom list type that is not a 
> subclass? Should there be a dunder method for this?
>
No dunder methods.  :)
Custom non-subclass list types aren't a common usage for list 
multiplication in any event.
At present one has to do list comprehensions for that, and that would 
simply remain so.

Subclasses, however, are something I hadn't considered...

> I don't think it's such a simple problem.
>
> Oscar
>
You made a good point, Oscar; I'll have to think about the subclassing a 
bit.
:)

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

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