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To: python-list@python.org
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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References: <kp4jf4$5fu$1@dont-email.me> <mailman.2967.1370872174.3114.python-list@python.org> <kp4lrg$hfi$2@dont-email.me>
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Xref: csiph.com comp.lang.python:47565

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