Yes, I'm thinking furiously.

See if I can keep up with you. :-)

GvR > A safe rule would be to require that all Python classes in a GvR > given inheritance graph should derive from the same C/Java class. I

...

That would mean that all C/Java behaviour is always only at the top of the mro, right?

I had the same idea because this would make my life easier.

A problem: some of the Zope porters reported that there are classes that inherits from more than a single ExtensionClass in Zope :-( don't know if that is true. (**)

Oops, this is already a problem with the actual best_base rule, I don't know if my reinterpretation is OK, but doesn't it basically states that all C types subclassed by a new C level class should be in a subtyping relationship that means:

with A isa list isa object B isa special_list isa list isa object C isa object

class D(A,B,C): ... is OK

but with

Z isa Cbadboy isa object

then class D(Z,A) isn't allowed.

Correct. You can't multiply inherit from two types that have a different structure layout. The C code must be able to safely cast a PyObject* to a PyListObject* or a Cbadboy* and if those don't have the same structure it's not safe. The implementation restricts this pretty effectively I believe. (You can't multiply inherit from list and dictionary, but you can from list and xxsubtype.spamlist.)

but if that is already *not* *a* *problem* then I think the rule could be not that bad ...

What do you mean by "already *not* *a* *problem*"? I hope "it is already forbidden". And "the rule" refers back to my proposal quoted above with "GvR >", right? --Guido van Rossum (home page: http://www.python.org/~guido/)