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Re: How to isolate a constant?

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On 23/10/2011 01:26, Gnarlodious wrote:
> Say this:
>
> class tester():
> 	_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? And how to prevent a classwide
> constant from ever getting changed?
>
'_someList' is part of the class itself.

This:

     someList = self._someList

just creates a new _reference to the list and this:

     someList += "X"

appends the items of the sequence "X" to the list.

Note that a string is also a sequence of characters, so:

 >>> x = []
 >>> x += "XY"
 >>> x
['X', 'Y']

Python will copy something only when you tell it to copy. A simple way
of copying a list is to slice it:

     someList = self._someList[:]

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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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