[Python-Dev] Afterthoughts: PEP 252, PEP 253 and jython issues

[Guido van Rossum]

> 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/)