Algorithm for finalizing cycles (Re: [Python-Dev] Garbage collecting closures)

[Martin v. Löwis]

Guido van Rossum <guido@python.org> writes:

> > > It could make a subclass, though...
> 
> > If the type is carefully designed, it can't...
> 
> I suppose you have something in mind like this (which is the only way
> I can come up with to implement something like a 'final' class in pure
> Python):

I was actually thinking about impure Python, i.e. by means of omitting
Py_TPFLAGS_BASETYPE.

> But how would you prevent this?
> 
>   >>> a = C()
>   >>> a.__class__ = D
>   >>>

For the issue at hand: Assigning __class__ won't change the object
layout, so if the object didn't have an __dict__ before, it won't have
an __dict__ afterwards. Of course, if there are writable slots,
the application could corrupt the underlying resource reference,
making __del__ meaningless, anyway. Here I need to bring up
Python's "we are all consenting adults" attitude...

Regards,
Martin