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[Python-Dev] Re: closure semantics

Brett C. bac at OCF.Berkeley.EDU
Fri Oct 24 21:16:41 EDT 2003 
David Eppstein wrote:

> In article <bncf99$366$1 at sea.gmane.org>,
>  "Terry Reedy" <tjreedy at udel.edu> wrote:
> 
> 
>>>A, B, and C *are* instance variables.  Why do you think they aren't?
>>
>>What?  They are class attributes that live in the class dictionary,
>>not the instance dictionary.
> 
> 
> They are instance variables on the class object, which is an instance of 
> type 'class'.
> 

I think the confusion that is brewing here is how Python masks class 
attributes when you do an assignment on an instance::

 >>> class foo(object):
...     A = 42
...
[12213 refs]
 >>> bar = foo()
[12218 refs]
 >>> bar.A
42
[12220 refs]
 >>> bar.A = 13
[12223 refs]
 >>> foo.A
42
[12223 refs]
 >>> bar.A
13

Python's resolution order checks the instance first and then the class 
(this is ignoring a data descriptor somewhere in this chain; for the 
details read Raymond's essay on descriptors @ 
http://users.rcn.com/python/download/Descriptor.htm#invoking-descriptors ).

-Brett
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