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Date: Fri, 13 Jan 2012 13:07:05 +0100
From: Jean-Michel Pichavant <jeanmichel@sequans.com>
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To: Eduardo Suarez-Santana <esuarez@itccanarias.org>
Subject: Re: copy on write
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:18911

Eduardo Suarez-Santana wrote:
> El 13/01/12 11:33, Eduardo Suarez-Santana escribió:
>> I wonder whether this is normal behaviour.
>>
> Even simpler:
>
> $ python
> Python 2.7.2 (default, Oct 31 2011, 11:54:55)
> [GCC 4.5.3] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> r={'a':1};
> >>> d={};
> >>> d['x']=r;
> >>> d['y']=r;
> >>> d['x']['a']=3
> >>> d['y']
> {'a': 3}
> >>>
>
yes it is.

 >>> d['x']=r;
 >>> d['y']=r;

means that both d['x'] and d['y'] name the same object r. If you change 
r, you'll see these changes wheter using d['x'] or d['y'].

The operator '=' does not copy objects, it binds an object to a name, 
and an object can have multiple names.
Use the dictionary copy method to copy a dictionary:

d['x'] = r.copy()

JM