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[Python-Dev] Why are class dictionaries not accessible?

Guido van Rossum guido at python.org
Thu Jun 23 16:40:51 EDT 2016 
"Er, among our chief weapons are fear, surprise, ctypes, gc, and fanatical
devotion to the Pope!"

On Thu, Jun 23, 2016 at 1:03 PM, Devin Jeanpierre <jeanpierreda at gmail.com>
wrote:

> On Thu, Jun 23, 2016 at 8:19 AM, Guido van Rossum <guido at python.org>
>  wrote:
>>
>> It was a long time when I wrote this, but IIRC the breakage could express
>> itself as a segfault or other C-level crash due to some internal state
>> invariant of the type object being violated, not just an exception. The
>> existence of ctypes notwithstanding, we take C-level crashes very seriously.
>>
>
> Big digression: one can still obtain the dict if they really want to, even
> without using ctypes. I suppose don't actually mutate it unless you want to
> segfault.
>
> >>> import gc
> >>> class A(object): pass
> >>> type(A.__dict__)
> <class 'mappingproxy'>
> >>> type(gc.get_referents(A.__dict__)[0])
> <class 'dict'>
> >>> gc.get_referents(A.__dict__)[0]['abc'] = 1
> >>> A.abc
> 1
> >>>
>
> (One can also get it right from A, but A can have other references, so
> maybe that's less reliable.)
>
> I think I wanted this at the time so I could better measure the sizes of
> objects. sys.getsizeof(A.__dict__) is very different
> from sys.getsizeof(gc.get_referents(A.__dict__)[0]), and also different
> from sys.getsizeof(A). For example:
>
> >>> import gc
> >>> class A(object): pass
> >>> sys.getsizeof(A); sys.getsizeof(A.__dict__);
> sys.getsizeof(gc.get_referents(A.__dict__)[0])
> 976
> 48
> 288
> >>> for i in range(10000): setattr(A, 'attr_%s' % i, i)
> >>> sys.getsizeof(A); sys.getsizeof(A.__dict__);
> sys.getsizeof(gc.get_referents(A.__dict__)[0])
> 976
> 48
> 393312
>
> (Fortunately, if you want to walk the object graph to measure memory usage
> per object type, you're probably going to be using gc.get_referents already
> anyway, so this is just confirmation that you're getting what you want in
> one corner case.)
>
> -- Devin
>



-- 
--Guido van Rossum (python.org/~guido)
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