Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!news.mixmin.net!news.musoftware.de!wum.musoftware.de!fu-berlin.de!uni-berlin.de!not-for-mail
From: Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com>
Newsgroups: comp.lang.python
Subject: Re: class object's attribute is also the instance's attribute?
Date: Thu, 30 Aug 2012 13:22:19 +0200
Lines: 41
Message-ID: <503F4CEB.9070408@dominolaser.com>
References: <3830e549-cb6d-4bcf-af45-f7c83ad2b65e@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
X-Trace: news.uni-berlin.de J8WCM9hFxzaOIyDgljCl5QsTqPKU84+c+wrDt7j499984eyzgzFQKUgAY2Wg==
X-Orig-Path: satorlaser.homedns.org!not-for-mail
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:14.0) Gecko/20120713 Thunderbird/14.0
In-Reply-To: <3830e549-cb6d-4bcf-af45-f7c83ad2b65e@googlegroups.com>
Xref: csiph.com comp.lang.python:28104

Am 30.08.2012 12:55, schrieb 陈伟:
> class A(object):
>      d = 'it is a doc.'
>
> t = A()
>
> print t.__class__.d
> print t.d
>
> the output is same.

You could go even further:

print id(t.__class__.d)
print id(t.d)

which should show you that they are not just equal but identical.


> so it means class object's attribute is also the instance's
> attribute.is it right?

Yes. This is even useful sometimes:

class Point(object):
     x = 0
     y = 0

This will cause every Point to have two attributes x and y that have a 
default value 0.


Note that setting this attribute on an instance does not change the 
class' attribute, just in that that was what confused you. However, if 
the attribute references a mutable type (e.g. a list) this can cause 
problems because the instance (see id() above) is the same and thus 
modifications affect both the class and all instances.


Uli