Class Variable Access and Assignment

[Magnus Lycka]

Antoon Pardon wrote:
> Op 2005-11-03, Steven D'Aprano schreef <steve at REMOVETHIScyber.com.au>:
> 
> 
>>>There are two possible fixes, either by prohibiting instance variables
>>>with the same name as class variables, which would allow any reference
>>>to an instance of the class assign/read the value of the variable. Or
>>>to only allow class variables to be accessed via the class name itself.
>>
>>There is also a third fix: understand Python's OO model, especially
>>inheritance, so that normal behaviour no longer surprises you.
> 
> 
> No matter wat the OO model is, I don't think the following code
> exhibits sane behaviour:
> 
> class A:
>   a = 1
> 
> b = A()
> b.a += 2
> print b.a
> print A.a
> 
> Which results in
> 
> 3
> 1

On the other hand:

 >>> class C:
...     a = [1]
...
 >>> b=C()
 >>> b.a += [2]
 >>> b.a
[1, 2]
 >>> C.a
[1, 2]

I can understand that Guido was a bit reluctant to introduce
+= etc into Python, and it's important to understand that they
typically behave differently for immutable and mutable objects.