> 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.
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.
--Guido van Rossum (home page: http://www.python.org/~guido/)
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