The class of list (or dict) can no longer be changed in
Python 2.3a2 ; this was possible with Python 2.2.2 (as
long as the new class extend list and has no slot).
When doing so ([].__class__ = ...), i get this error :

TypeError: __class__ assignment: only for heap types


Not being able to change the class of non-mutable
object (e.g. int, float or tuple) is AMHO not a great
loss, but list and dict ARE mutable, and so changing
their class does have a sens !

Typically it can be used to observed a list that you
have not created yourself (see atteched script); i was
relying a lot on such observation features.

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IMHO, changing the class of an immutable object is, was and
always will be Right Out.

Changing the class of a mutable is dicier.  I personally
don't think it's worth the effort, but assigning to Guido
for pronouncement.

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The reason for the restriction is not (im)mutability, but
the fact that many built-in types have (or can have)
non-standard (de)allocators. A subclass of a built-in type
doesn't necessarily have the same deallocator. If someone
can come up with a patch *AND* a proof that the patch is
correct, I'll consider it, but I'd rather be safe than
sorry. Until then, I'll close this as "won't fix".

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I've submitted a patch for it (#722462), as well as a PyUnit
test case.