Class Variable Access and Assignment

Bengt Richter bokr at oz.net
Sat Nov 5 03:11:51 CET 2005


On 4 Nov 2005 11:09:36 GMT, Antoon Pardon <apardon at forel.vub.ac.be> wrote:
[...]
>
>Take the code:
>
>  lst[f()] += 1
>
>Now let f be a function with a side effect, that in succession
>produces the positive integers starting with one.
>
>What do you think this should be equivallent to:
>
>  t = f()
>  lst[t] = lst[t] + 1
>
>or
>
>  lst[f()] = lst[f()] + 1
>
>If you think the environment can change between references then I
>suppose you prefer the second approach.
>
I am quite sympathetic to your probe of python semantics, but I
don't think the above is an argument that should be translated
to attribute assignment. BTW, ISTM that augassign (+=) is
a red herring here, since it's easy to make a shared class variable
that is augassigned apparently as you want, e.g.,

 >>> class shared(object):
 ...     def __init__(self, v=0): self.v=v
 ...     def __get__(self, *any): return self.v
 ...     def __set__(self, _, v): self.v = v
 ...
 >>> class B(object):
 ...     a = shared(1)
 ...
 >>> b=B()
 >>> b.a
 1
 >>> B.a
 1
 >>> b.a += 2
 >>> b.a
 3
 >>> B.a
 3
 >>> vars(b)
 {}
 >>> vars(b)['a'] = 'instance attr'
 >>> vars(b)
 {'a': 'instance attr'}
 >>> b.a
 3
 >>> b.a += 100
 >>> b.a
 103
 >>> B.a
 103
 >>> B.a = 'this could be prevented'
 >>> b.a
 'instance attr'
 >>> B.a
 'this could be prevented'

The spelled out attribute update works too
 >>> B.a = shared('alpha')
 >>> b.a
 'alpha'
 >>> b.a = b.a + ' beta'
 >>> b.a
 'alpha beta'
 >>> B.a
 'alpha beta'

But the instance attribute we forced is still there
 >>> vars(b)
 {'a': 'instance attr'}

You could have shared define __add__ and __iadd__ and __radd__ also,
for confusion to taste ;-)

Regards,
Bengt Richter



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