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Re: using "*" to make a list of lists with repeated (and independent) elements

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On Wed, Sep 26, 2012 at 3:20 PM, TP <TP@frenoespam.fr.invalid> wrote:
> Hi everybody,
>
> I have tried, naively, to do the following, so as to make lists quickly:
>
>>>> a=[0]*2
>>>> a
> [0, 0]
>>>> a[0]=3
>>>> a
> [3, 0]
>
> All is working fine, so I extended the technique to do:
>
>>>> a=[[0]*3]*2
>>>> a
> [[0, 0, 0], [0, 0, 0]]
>>>> a[0][0]=2
>>>> a
> [[2, 0, 0], [2, 0, 0]]
>
> The behavior is no more expected!
> The reason is probably that in the first case, 0 is an integer, not a list,
> so Python copies two elements that are independent.
> In the second case, the elements are [0,0,0], which is a list; when Python
> copies a list, he copies in fact the *pointer* to the list, such that we
> obtain this apparently strange behavior.

Mostly correct.  When you do [foo] * 3 it extends the list with the
*same objects* no matter what type they are.  In the case of integers,
it doesn't matter that it's the same objects, because integers are
immutable.  Lists are mutable, however, and so it becomes apparent
that the same objects are repeated when you try to modify one of the
lists.

> In these conditions, how to make this list [[0,0,0],[0,0,0]] with "*"
> without this behavior?

Use a list comprehension:

a = [[0] * 3 for _ in range(2)]

This way the expression `[0] * 3` is re-evaluated at each position in
the outer list, rather than evaluated just once and then copied.

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using "*" to make a list of lists with repeated (and independent) elements TP <TP@frenoespam.fr.invalid> - 2012-09-26 23:20 +0200
  Re: using "*" to make a list of lists with repeated (and independent) elements Ian Kelly <ian.g.kelly@gmail.com> - 2012-09-26 15:39 -0600
  Re: using "*" to make a list of lists with repeated (and independent) elements Paul Rubin <no.email@nospam.invalid> - 2012-09-26 14:43 -0700
    Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-26 15:07 -0700
      Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-26 15:28 -0700
        Re: using "*" to make a list of lists with repeated (and independent) elements Tim Chase <python.list@tim.thechases.com> - 2012-09-26 17:45 -0500
          Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-26 15:53 -0700
          Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-26 15:53 -0700
          Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-27 14:24 -0700
          Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-27 14:24 -0700
          Re: using "*" to make a list of lists with repeated (and independent) elements Ramchandra Apte <maniandram01@gmail.com> - 2012-09-29 06:46 -0700
            Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-29 10:01 -0700
              Re: using "*" to make a list of lists with repeated (and independent) elements Ian Kelly <ian.g.kelly@gmail.com> - 2012-09-29 11:18 -0600
                Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-29 11:50 -0700
                Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-29 11:50 -0700
              Re: using "*" to make a list of lists with repeated (and independent) elements Chris Angelico <rosuav@gmail.com> - 2012-09-30 03:41 +1000
            Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-29 10:01 -0700
          Re: using "*" to make a list of lists with repeated (and independent) elements Ramchandra Apte <maniandram01@gmail.com> - 2012-09-29 06:46 -0700
  Re: using "*" to make a list of lists with repeated (and independent) elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-26 14:45 -0700

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