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Re: How to isolate a constant?

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On Sat, Oct 22, 2011 at 5:26 PM, Gnarlodious <gnarlodious@gmail.com> wrote:
> Say this:
>
> class tester():

Style note: either have it explicitly subclass `object`, or don't
include the parens at all. Empty parens for the superclasses is just
weird.

>        _someList = [0, 1]
>        def __call__(self):
>                someList = self._someList
>                someList += "X"
>                return someList
>
> test = tester()
>
> But guess what, every call adds to the variable that I am trying to
> copy each time:
> test()
>> [0, 1, 'X']
> test()
>> [0, 1, 'X', 'X']
>
>
> Can someone explain this behavior?

The line `someList = self._someList` does NOT copy the list. It make
`someList` point to the same existing list object. Hence,
modifications to that object from either variable will affect the
other.
Similarly, `someList += "X"` modifies someList *in-place*; it does not
produce a new list object.

The upshot is that you're just modifying and returning references to
*the same list* repeatedly, never producing a new list object.

> And how to prevent a classwide
> constant from ever getting changed?

Python doesn't have any language-enforced notion of constants. So,
short of writing/using a library to try and enforce such a notion,
you're out of luck. You could use an immutable datatype (e.g. a tuple)
instead of a mutable one (e.g. a list) as some level of safeguard
though.

Cheers,
Chris
--
http://rebertia.com

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Thread

How to isolate a constant? Gnarlodious <gnarlodious@gmail.com> - 2011-10-22 17:26 -0700
  Re: How to isolate a constant? Chris Rebert <clp2@rebertia.com> - 2011-10-22 17:41 -0700
    Re: How to isolate a constant? Gnarlodious <gnarlodious@gmail.com> - 2011-10-22 18:01 -0700
      Re: How to isolate a constant? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-10-22 22:12 -0700
      Re: How to isolate a constant? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-10-23 05:32 +0000
      Re: How to isolate a constant? Paul Rudin <paul.nospam@rudin.co.uk> - 2011-10-23 11:23 +0100
  Re: How to isolate a constant? MRAB <python@mrabarnett.plus.com> - 2011-10-23 01:46 +0100
    Re: How to isolate a constant? Alan Meyer <ameyer2@yahoo.com> - 2011-10-25 15:50 -0400
      Re: How to isolate a constant? Ian Kelly <ian.g.kelly@gmail.com> - 2011-10-25 14:05 -0600
      Re: How to isolate a constant? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2011-10-25 17:08 -0700
        Re: How to isolate a constant? Mel <mwilson@the-wire.com> - 2011-10-25 22:48 -0400
      Re: How to isolate a constant? Ian Kelly <ian.g.kelly@gmail.com> - 2011-10-25 18:30 -0600
  Re: How to isolate a constant? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2011-10-22 17:55 -0700

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