Re: How to isolate a constant?

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From	Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject	Re: How to isolate a constant?
Date	Sat, 22 Oct 2011 17:55:26 -0700
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On Sat, 22 Oct 2011 17:26:22 -0700 (PDT), Gnarlodious
<gnarlodious@gmail.com> declaimed the following in
gmane.comp.python.general:

> 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:

	You never copied it... You just created a temporary local reference
to the same object

	_someList = [0, 1]

creates a mutable list object containing two elements; it then binds the
/name/ "_someList" to that object.

	someList = self._someList

finds the OBJECT to which "self._someList" is bound, and binds a second
name "someList" to the same object.

	someList += "X"

retrieves the OBJECT to which "someList" is bound, mutates it by
appending an "X".

	Nowhere do you ever create a NEW list object. Try

	someList = self._someList[:]

which finds the object to which "self._someList" is bound, extracts a
sublist [:] (from first element to last element) -- this is a NEW list
-- and binds "someList" to that new object.

> Can someone explain this behavior? And how to prevent a classwide
> constant from ever getting changed?

	"Ever"? Can't really be done with Python -- if you know it exists,
you can get to it and rebind it, or mutate it if it is a mutable object.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.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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