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> I'm confused: the rule for negative slot offsets appear to be
> different to the one for tp_dictoffset
Yes. I think this is actually necessary.
Consider:
class S(str):
 __slots__ = ['a']
you'd except S.__dict__['a'].__offset__ (well, if the attribute existed) to be 
-4.
Then
class T(S):
 __slots__ = ['b']
then using the 'from the end of the object' rule for T().a would actually find 
T.b. (IOW, T.__dict__['b'].__offset__ == T.__dict__['a'].__offset__ == -4). 
The alternative would be to somehow override all the slots in S when T is 
defined -- and this doesn't seem wise.
__dict__ indeed works differently, because 
instance.__class__.__dictoffset__ is updated on subclassing. You could 
make __dict__ work like the slots mentioned above, but then you'd have to 
find the '__dict__' descriptor every time you wanted to access an 
instance's dictionary, and that would be slow (and might even not really 
work, but I don't want to risk brain-explosion by thinking about it too hard)
> which only increases the amount of obscurity around here.
Yeah, sorry about that.
I think something I've realised over the past few days is that __dict__ 
really is special. I'm not sure __weakref__ is (though I guess it's special in 
that you want to be able to access it from C without any risk of executing 
Python level code, i.e. replacing Py_GETWEAKREFLIST(ob) with 
PyOjbect_GetAttrString(ob, "__weakref__") would be unfortunate).
> This should be resolved one way or the other 
See above -- don't think you can.
> -- and I think that
> a clear picture of the various parts of the object and how they
> are measured would be a good start.
No kidding here!
> That's also related to your proposed change to extra_ivars(),
> which would become slightly more permissive; I strongly suspect
> that it would allow more strange segfaulting cases to sneak in
> undetected...
Almost certainly!