class attribute confusion

[Steven D'Aprano]

On Sat, 04 Dec 2010 15:00:43 +0100, Omar Abo-Namous wrote:

>>> I think this behaviour is totally wrong, since it seems
>>> A.__init__(self) is changing the value inside of A() not inside of the
>>> object variable 'self' (that should be x or y)!!
>> It's not wrong at all.  You expect "mylist" to behave as an instance
>> attribute, but you defined it as a class attribute.  Instance
>> attributes are naturally initialised in the __init__() method.
>>
> Could you please point me to a reference in the doc??

http://docs.python.org/reference/datamodel.html

In the section about classes:

"Class attribute assignments update the class’s dictionary ..."

and in the section about class instances:

"Attribute assignments and deletions update the instance’s dictionary, 
never a class’s dictionary."

In this specific example, you also have to realise that mylist.append() 
mutates the list in place, and doesn't create a new list. It doesn't 
matter whether the list comes from a global variable, a local variable, 
an instance attribute or a class attribute, append is always an inplace 
operation.


-- 
Steven