[Python-Dev] Assignment to __class__

[Kevin Jacobs]

On Fri, 10 Jan 2003, Guido van Rossum wrote:
> > Sorry, I was not as clear as I should have been.  Here is what used to work,
> > and I hope can be made to work again:
> > 
> >   class AlgebraicDict(dict):
> >     def doReallyComplexThings(self, foo): ...
> >     def __add__(self, other): ...
> >     def __mul__(self, other): ...
> > 
> >   unsuspecting_dict = {1:[1,2],2:3}
> > 
> >   unsuspecting_dict.__class__ = AlgebraicDict
> >   > TypeError: __class__ assignment: only for heap types
> 
> But in this case, the instance layout of dict and AlgebraicDict is
> different anyway (AlgebraicDict has room for the __dict__ to contain
> instance variables) so you can't do that.
> 
> The layouts could be the same if you add __slots__ = [] to
> AlgebraicDict.

Again, I was not 100% clear.  AlgebraicDict and AlgebraicList do define
__slots__ to be an empty sequence.

> But the problem is that even then, AlgebraicDict may be allocated
> using a different free list than dict, and changing its __class__ to
> dict would free it using the wrong free list.
> 
> To work around, create a neutral dict subclass that has the same
> layout at AlgebraicDict.

The problem is that I do not control where the dict is being allocated.  I
suppose I can live with having to make copies, but it seems like a major
step backwards to me.

Thanks,
-Kevin

--
Kevin Jacobs
The OPAL Group - Enterprise Systems Architect
Voice: (216) 986-0710 x 19         E-mail: jacobs@theopalgroup.com
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