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Re: Multi-dimensional list initialization

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Subject Re: Multi-dimensional list initialization
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On 2012-11-06 23:52, Ian Kelly wrote:
> On Tue, Nov 6, 2012 at 3:41 PM, Andrew Robinson
> <andrew3@r3dsolutions.com> wrote:
>>>      Q: What about other mutable objects like sets or dicts?
>>>      A: No, the elements are never copied.
>>
>> They aren't list multiplication compatible in any event! It's a total
>> nonsense objection.
>>
>> If these are inconsistent in my idea -- OBVIOUSLY -- they are inconsistent
>> in Python's present implementation.  You can't even reference duplicate them
>> NOW.
>>
>>>>> { 1:'a', 2:'b', 3:'c' } * 2
>>
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> TypeError: unsupported operand type(s) for *: 'dict' and 'int'
>
> The objection is not nonsense; you've merely misconstrued it.  If
> [[1,2,3]] * 4 is expected to create a mutable matrix of 1s, 2s, and
> 3s, then one would expect [[{}]] * 4 to create a mutable matrix of
> dicts.  If the dicts are not copied, then this fails for the same
> reason
>
>>>      Q: How about if I use delegation to proxy a list?
>>>      A: Oh no, they definitely won't be copied.
>>
>> Give an example usage of why someone would want to do this.  Then we can
>> discuss it.
>
> Seriously?  Read a book on design patterns.  You might start at SO:
>
> http://stackoverflow.com/questions/832536/when-to-use-delegation-instead-of-inheritance
>
>>> Losing consistency in favour of saving a few characters for something as
>>> uncommon as list multiplication is a poor tradeoff. That's why this
>>> proposal has been rejected again and again and again every time it has
>>> been suggested.
>>
>> Please link to the objection being proposed to the developers, and their
>> reasoning for rejecting it.
>> I think you are exaggerating.
>
>>From Google:
>
> http://bugs.python.org/issue1408
> http://bugs.python.org/issue12597
> http://bugs.python.org/issue9108
> http://bugs.python.org/issue7823
>
> Note that in two out of these four cases, the reporter was trying to
> multiply lists of dicts, not just lists of lists.
>
>> Besides, 2D arrays are *not* rare and people *have* to copy internals of
>> them very often.
>> The copy speed will be the same or *faster*, and the typing less -- and the
>> psychological mistakes *less*, the elegance more.
>
> List multiplication is not potentially useful for copying 2D lists,
> only for initializing them.  For copying an existing nested list,
> you're still stuck with either copy.deepcopy() or a list
> comprehension.
>
>> It's hardly going to confuse anyone to say that lists are copied with list
>> multiplication, but the elements are not.
>>
>> Every time someone passes a list to a function, they *know* that the list is
>> passed by value -- and the elements are passed by reference.  People in
>> Python are USED to lists being "the" way to weird behavior that other
>> languages don't do.
>
> Incorrect.  Python uses what is commonly known as call-by-object, not
> call-by-value or call-by-reference.  Passing the list by value would
> imply that the list is copied, and that appends or removes to the list
> inside the function would not affect the original list.  This is not
> what Python does; the list inside the function and the list passed in
> are the same list.  At the same time, the function does not have
> access to the original reference to the list and cannot reassign it by
> reassigning its own reference, so it is not call-by-reference
> semantics either.
>
I prefer the term "reference semantics".

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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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