Class Variable Access and Assignment

[Antoon Pardon]

Op 2005-11-03, Stefan Arentz schreef <stefan.arentz at gmail.com>:
> Antoon Pardon <apardon at forel.vub.ac.be> writes:
>
>> Op 2005-11-03, venk schreef <venkatasubramanian at gmail.com>:
>> > You see,
>> >      The seen behavior is due to the result of python's name
>> > binding,scoping scheme.
>> 
>> I know what causes the behaviour. But I still think it is
>> not sane behaviour.
>> 
>> 
>> > ...
>> >
>> > the same thing happens in the case of b.a = b.a + 2 .... search for b.a
>> > not found, read the value from the enclosing scope (of the class
>> > object).... then assign b.a to the local scope, with the value 3.
>> 
>> This is an explanation depending on a specific implementation.
>> 
>> Now can you give me a language design argument that supports the 
>> idea that in "b.a = b.a + 2" b.a refers to two different objects.
>> 
>> And even if you could do that, can you give such an argument that
>> in "b.a += 2" that one occurence of b.a should refer to two different
>> objects.
>
> Your problem is a namespace conflict together with a certain
> stubborness about lookup order :-)
>
> It is really simple. When you say b.a then the instance variable 'a'
> is looked up first. If it does not exist then a class variable lookup
> is done.

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?

> 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

> It is all according to how things have been in Python for a long time.

Unsane behaviour for a long time is still unsane behaviour.

> The real issue here is that you should propery name class variables so
> that there can't be any confusion about class or instance scope. I use
> all uppercase identifiers for class variables for example.

The fact that this can be regarded as unwise coding, doesn't imply
it is sane behaviour of python. Variable shadowing happens. I don't
consider it sane behaviour if the same reference in a line gets
resolved in different name spaces

-- 
Antoon Pardon