[Python-Dev] Assignment to __class__

[Kevin Jacobs]

On Thu, 9 Jan 2003, Guido van Rossum wrote:
> You can only set __class__ when the old and new class instance have
> the same instance layout at the C level.  Changing this is impossible
> given the way objects are implemented in C.  This means you can never
> change a list into a dict or vice versa, because the C structs are
> different.
> 
> Or do I misunderstand you?  Can you give an example of something you
> think should be allowed but currently isn't?

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

Analogously, we want to transform native lists into AlgebraicLists, which of
course has list as a base class.

Hope this clears things up,
-Kevin

--
Kevin Jacobs
The OPAL Group - Enterprise Systems Architect
Voice: (216) 986-0710 x 19         E-mail: jacobs@theopalgroup.com
Fax:   (216) 986-0714              WWW:    http://www.theopalgroup.com