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Re: Newbie: question regarding references and class relationships

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From Peter Otten <__peter__@web.de>
Subject Re: Newbie: question regarding references and class relationships
Date Mon, 10 Jun 2013 16:28:30 +0200
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Xref csiph.com comp.lang.python:47565

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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

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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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