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Groups > comp.lang.python > #14859
| From | Dennis Lee Bieber <wlfraed@ix.netcom.com> |
|---|---|
| Subject | Re: How to isolate a constant? |
| Date | 2011-10-22 17:55 -0700 |
| Organization | > Bestiaria Support Staff < |
| References | <f5539538-d079-4be1-b2c0-d98d3fc6a33f@h39g2000prh.googlegroups.com> |
| Newsgroups | comp.lang.python |
| Message-ID | <mailman.2139.1319331340.27778.python-list@python.org> (permalink) |
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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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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