How to best explain a "subtle" difference between Python and Perl ?

[Palindrom]

On Aug 13, 2:12 am, Jonathan Gardner <jgard... at jonathangardner.net>
wrote:
> On Aug 12, 9:17 am, Palindrom <demarc... at gmail.com> wrote:
>
>
>
> > Hi everyone !
>
> > I'd like to apologize in advance for my bad english, it's not my
> > mother tongue...
>
> > My girlfriend (who is a newbie in Python, but knows Perl quite well)
> > asked me this morning why the following code snippets didn't give the
> > same result :
>
> > ### Python ###
>
> > liste = [1,2,3]
>
> > def foo( my_list ):
> >     my_list = []
>
> > foo(liste)
>
> > print liste# she expected liste to be an empty list
>
> > ### Perl ###
>
> > @lst =(1,2,3);
> > $liste =\@lst;
> > foo($liste);
> > print "@lst\n";
>
> > sub foo {
> >  my($my_list)=@_;
> >  @{$my_list}=()
>
> > }
>
> > I have to admit that I don't know how to clearlyexplainto her the
> > differences between these results.
> > Could someone please help us understand these difference between
> > Python and Perl ?
>
> David Ullrich gives a great and complete answer. I want to focus on
> some of the subtleties.
>
> Perl and C share a lot in common. There are "direct" variables, things
> like numbers and arrays. These aren't references to object, but the
> object is itself stored in the variable. That is, you can't talk about
> the thing that is '@lst' without creating a reference to it.
>
> Python, on the other hand, doesn't have direct variables. Well, it
> does, it is just that all of the direct variables are really pointers
> or references to the values they reference.
>
> Imagine a perl where you are only allowed to use scalars, and
> specifically, scalars that are references to object but not references
> to references. That is the Python variable system.
>
> To be more concrete...
>
> This statement cannot be expressed in Python:
>
>  @lst = (1, 2, 3);    # perl
>
> However, you can create an arrayref (perl) / list (Python) and assign
> a scalar (perl) / variable (Python) to reference it:
>
>  $liste = [1, 2, 3];  # perl
>  liste = [1, 2, 3]    # Python
>
> Likewise, this statement cannot be expressed in Python:
>
>  $refref = \$ref;     # perl
>
> Although you can cheat in both perl and Python to get a similar
> result:
>
>  $refref = [$ref]     # perl
>  refref = [ref]       # python
>
> As far as the functions, the Python version and the perl version are
> doing two completely different things. David explains how to write a
> Python version that does what the perl version is doing. If you wanted
> a perl version that did what your python version did, it would look
> like this:
>
>  sub foo {
>    my ($my_list) =  @_;
>    $my_list = [];
>    return undef;
>  }
>
> Is Python's variable system better than perl's? It depends on which
> way you prefer. As for me, being a long-time veteran of perl and
> Python, I don't think having a complicated variable system such as
> perl's adds anything to the language. Python's simplicity in this
> regard is not only sufficient, but preferable.

Thank you Jonathan for your extensive and clear response !

I agree with you, Python's variable system is eazier to understand.
Regards.