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copy on write

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I wonder whether this is normal behaviour.

I would expect equal sign to copy values from right to left. However, it 
seems there is a copy-on-write mechanism that is not working.

Anyone can explain and provide a working example?

Thanks,
-Eduardo

$ 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.
 >>> class n:
...     def __init__(self, id, cont):
...         self.id = id;
...         self.cont = cont;
...
 >>> r={'a':1};
 >>> d={};
 >>> d['x']=r;
 >>> d['y']=r;
 >>> x1 = n('x',d['x']);
 >>> y1 = n('y',d['y']);
 >>> x1.cont['a']=2;
 >>> y1.cont
{'a': 2}
 >>>

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copy on write Eduardo Suarez-Santana <esuarez@itccanarias.org> - 2012-01-13 11:33 +0000
  Re: copy on write Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-01-13 12:10 +0000
    Re: copy on write Chris Angelico <rosuav@gmail.com> - 2012-01-13 23:30 +1100
      Re: copy on write Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-01-13 13:04 +0000
        Re: copy on write Ethan Furman <ethan@stoneleaf.us> - 2012-01-13 10:40 -0800
          Re: copy on write 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-13 14:26 -0800
          Re: copy on write 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-13 14:26 -0800
        Re: copy on write John O'Hagan <research@johnohagan.com> - 2012-02-02 14:18 +1100
        Re: copy on write Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-02-02 01:34 -0500
        Re: copy on write John O'Hagan <research@johnohagan.com> - 2012-02-02 19:11 +1100
          Re: copy on write Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-02 09:16 +0000
            Re: copy on write Hrvoje Niksic <hniksic@xemacs.org> - 2012-02-02 11:53 +0100
              Re: copy on write MRAB <python@mrabarnett.plus.com> - 2012-02-02 16:28 +0000
              Re: copy on write Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-02-02 12:21 -0500
            Re: copy on write John O'Hagan <research@johnohagan.com> - 2012-02-03 01:17 +1100
            Re: copy on write Terry Reedy <tjreedy@udel.edu> - 2012-02-02 12:25 -0500
            Re: copy on write John O'Hagan <research@johnohagan.com> - 2012-02-03 14:08 +1100
              Re: copy on write Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-03 05:04 +0000
                Re: copy on write Chris Angelico <rosuav@gmail.com> - 2012-02-03 16:28 +1100
                Re: copy on write Rick Johnson <rantingrickjohnson@gmail.com> - 2012-02-03 07:35 -0800
                Re: copy on write Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2012-02-03 10:08 +0100
                Re: copy on write John O'Hagan <research@johnohagan.com> - 2012-02-03 21:47 +1100
                Re: copy on write Wolfram Hinderer <wolfram.hinderer@googlemail.com> - 2012-02-05 06:09 -0800
                Re: copy on write "OKB (not okblacke)" <brenNOSPAMbarn@NObrenSPAMbarn.net> - 2012-02-03 16:15 +0000
      Re: copy on write Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-02-02 11:42 +0100
    Re: copy on write Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-01-13 08:50 -0500
      Re: copy on write Grant Edwards <invalid@invalid.invalid> - 2012-01-13 15:13 +0000
        Re: copy on write Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-01-13 11:48 -0500
          Re: copy on write Neil Cerutti <neilc@norwich.edu> - 2012-01-13 16:54 +0000
            Re: copy on write Grant Edwards <invalid@invalid.invalid> - 2012-01-13 18:15 +0000
              Re: copy on write Chris Angelico <rosuav@gmail.com> - 2012-01-14 05:26 +1100
                Re: copy on write Grant Edwards <invalid@invalid.invalid> - 2012-01-13 19:30 +0000
                Re: copy on write Neil Cerutti <neilc@norwich.edu> - 2012-01-13 20:11 +0000
            Re: copy on write Evan Driscoll <edriscoll@wisc.edu> - 2012-01-13 13:24 -0600
              Re: copy on write Neil Cerutti <neilc@norwich.edu> - 2012-01-13 21:20 +0000
                Re: copy on write Evan Driscoll <edriscoll@wisc.edu> - 2012-01-13 16:48 -0600
                Re: copy on write 88888 Dihedral <dihedral88888@googlemail.com> - 2012-02-02 05:33 -0800
                Re: copy on write Evan Driscoll <edriscoll@wisc.edu> - 2012-02-02 15:20 -0600
                Re: copy on write 88888 Dihedral <dihedral88888@googlemail.com> - 2012-02-02 05:33 -0800
                Re: copy on write 88888 Dihedral <dihedral88888@googlemail.com> - 2012-02-03 14:16 -0800
                Re: copy on write 88888 Dihedral <dihedral88888@googlemail.com> - 2012-02-03 14:16 -0800
          Re: copy on write Rick Johnson <rantingrickjohnson@gmail.com> - 2012-02-01 19:51 -0800
            Re: copy on write Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-02 05:31 +0000

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