Class Variable Access and Assignment
apardon at forel.vub.ac.be
Thu Nov 3 16:02:24 CET 2005
Op 2005-11-03, Stefan Arentz schreef <stefan.arentz at gmail.com>:
> Antoon Pardon <apardon at forel.vub.ac.be> writes:
>> Fine, we have the code:
>> b.a += 2
>> We found the class variable, because there is no instance variable,
>> then why is the class variable not incremented by two now?
> Because it really is executed as:
> b.a = b.a + 2
That is an explanation, not a reason.
> 1. get 't'b.a and store it in a temporary 't' (found the instance)
> 2. add 2 to 't'
> 3. store 't' in 'b.a'
> The last operation stores it into an instance variable.
[ I think you mean '(found the class variable)' in line 1 ]
All you are doing here is explain how the current implemantation treats
this. You are not giving arguments for why the result of this
implementation should be considered sane behaviour.
>> > Remember, Python is a dynamic language.
>> So? Python being a dynamic language doesn't prevent the following to fail:
>> >>> a=1
>> >>> def f():
>> ... a += 2
>> >>> f()
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in ?
>> File "<stdin>", line 2, in f
>> UnboundLocalError: local variable 'a' referenced before assignment
> See the 'global' keyword.
You missed the point. If python being a dynamic language would be the
answer to the interaction between instance and class variable, why
then is the interaction between local and global variable different,
why wouldn't the f code be executed as follows
1. get a and store in in a temporary 't' (found the global)
2. add 2 to 't'
3. store 't' in 'a'
The last operation storing it in f's local namespace.
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