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Re: Newbie: question regarding references and class relationships

From Rui Maciel <rui.maciel@gmail.com>
Newsgroups comp.lang.python
Subject Re: Newbie: question regarding references and class relationships
Date 2013-06-10 15:38 +0100
Organization A noiseless patient Spider
Message-ID <kp4o3u$pv$1@dont-email.me> (permalink)
References <kp4jf4$5fu$1@dont-email.me> <mailman.2967.1370872174.3114.python-list@python.org> <kp4lrg$hfi$2@dont-email.me> <mailman.2969.1370874511.3114.python-list@python.org>

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Peter Otten wrote:

> Rui Maciel wrote:
> 
>> Peter Otten wrote:
>> 
>>> Don't add
>>> 
>>>>position = []
>>> 
>>> to your code. That's not a declaration, but a class attribute and in the
>>> long run it will cause nothing but trouble.
>> 
>> Why's that?
> 
> Especially with mutable attributes it's hard to keep track whether you are
> operating on the instance or the class:
> 
>>>> class Point:
> ...     position = []
> ...     def __init__(self, x, y, z):
> ...             self.position = [x, y, z]
> ...
>>>> a = Point(1, 2, 3)
>>>> b = Point(10, 20, 30)
>>>> a.position
> [1, 2, 3]
>>>> del a.position
>>>> a.position
> [] # did you expect that?
>>>> del b.position
>>>> b.position.extend(["did you expect that?"])
>>>> a.position
> ['did you expect that?']
>>>> del a.position
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: position


How do you guarantee that any object of a class has a specific set of 
attributes?


Rui Maciel

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Thread

Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 14:18 +0100
  Re: Newbie: question regarding references and class relationships Roy Smith <roy@panix.com> - 2013-06-10 09:35 -0400
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 14:57 +0100
      Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 15:00 +0100
      Re: Newbie: question regarding references and class relationships Ian Kelly <ian.g.kelly@gmail.com> - 2013-06-10 13:56 -0600
        Re: Newbie: question regarding references and class relationships Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-11 17:02 -0700
  Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 15:49 +0200
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 14:59 +0100
      Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 16:28 +0200
        Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 15:38 +0100
          Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 17:05 +0200
            Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 16:20 +0100
              Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 17:55 +0200
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 17:09 +0100
                Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 19:07 +0200
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 18:42 +0100
                Re: Newbie: question regarding references and class relationships Dave Angel <davea@davea.name> - 2013-06-10 15:36 -0400
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 21:31 +0100
                Re: Newbie: question regarding references and class relationships Terry Jan Reedy <tjreedy@udel.edu> - 2013-06-10 15:51 -0400
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 21:13 +0100
                Re: Newbie: question regarding references and class relationships Terry Jan Reedy <tjreedy@udel.edu> - 2013-06-10 18:07 -0400
                Re: Newbie: question regarding references and class relationships Grant Edwards <invalid@invalid.invalid> - 2013-06-10 22:39 +0000
                Re: Newbie: question regarding references and class relationships Chris Angelico <rosuav@gmail.com> - 2013-06-11 08:54 +1000
                Re: Newbie: question regarding references and class relationships Tim Chase <python.list@tim.thechases.com> - 2013-06-10 18:24 -0500
                Re: Newbie: question regarding references and class relationships Dave Angel <davea@davea.name> - 2013-06-10 19:26 -0400
                Re: Newbie: question regarding references and class relationships Jason Swails <jason.swails@gmail.com> - 2013-06-10 23:49 -0400
  Re: Newbie: question regarding references and class relationships Terry Jan Reedy <tjreedy@udel.edu> - 2013-06-10 15:54 -0400
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 21:09 +0100
  Re: Newbie: question regarding references and class relationships Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-11 16:50 -0700
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-13 13:06 +0100
      Re: Newbie: question regarding references and class relationships Chris Angelico <rosuav@gmail.com> - 2013-06-14 00:07 +1000
        Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-13 19:23 +0100

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