Path: csiph.com!news.mixmin.net!weretis.net!feeder1.news.weretis.net!feeder.erje.net!1.eu.feeder.erje.net!ecngs!feeder2.ecngs.de!217.188.199.168.MISMATCH!takemy.news.telefonica.de!telefonica.de!newsfeed.xs4all.nl!newsfeed8.news.xs4all.nl!nzpost1.xs4all.net!not-for-mail
To: python-list@python.org
From: jmp <jeanmichel@sequans.com>
Subject: Re: Question re class variable
Date: Tue, 29 Sep 2015 13:02:12 +0200
References: <3948d9cd-24b1-4a3b-8ed0-46bb60a8d738@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0
In-Reply-To: <3948d9cd-24b1-4a3b-8ed0-46bb60a8d738@googlegroups.com>
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.233.1443524549.28679.python-list@python.org>
Lines: 58
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:97202

On 09/29/2015 11:27 AM, plewto@gmail.com wrote:
> I have a perplexing problem with Python 3 class variables.

Your problem is that when assigning values to your class attribute, you 
are actually creating a instance attribute.


class Foo:
   bar = "I'm a class attribute"
   def __init__(self):
     self.bar = "I'm an instance attribute"

   def foo(self):
     print self.bar
     print Foo.bar
     # this is how you set a class attribute from an instance
     Foo.bar = "I am still a class attribute"
     print Foo.bar

Foo.foo()

I'm an instance attribute
I'm a class attribute
I am still a class attribute


What can be confusing is that assuming you never use the same name for a 
class an instance attribute (that would be bad code), you can access
your class attribute from the instance:

class Foo:
   bar = "I'm a class attribute"
   def foo(self):
     # python will look into the class scope if not found in the instance
     print self.bar # this is not an assignment so we're fine

Foo.foo()
I'm an class attribute

As side note and unrelated topic, your are using name mangling 
(attribute starting with __), are you sure you need it ? You need a 
strong motive to use this feature otherwise you're making things 
difficult for yourself without any benefit.

Finally here's how I'd code your id, to give some idea on alternative ways:

class GameObject:

   @property
   def id(self):
     return id(self) #use the builtin id function

print GameObject().id

Cheers,

JM