Inheritance in extension types

[Martin von Loewis]

Adrien Hernot <amh at BSD-DK.dk> writes:

> I want to export several C++ classes to python. I succeded for most
> of the functionalities I want, but for one: inheritance. That is, I
> would like the wrapper to class A to be able to access function in
> class B if B is a base of A. Also, I would like a function accepting
> B as argument to be able to accept A. For that I need inheritance
> information for my wrappers to use.

I don't believe that statement is true. Python wrappers are not
classes, so they don't inheritance. OTOH, you don't need inheritance
in Python to get polymorphism: two objects can be processed by the
same routine even if they don't share a common base type, as long as
they offer the same attributes.

If, by "classes", you mean "Python types", and by "function", you mean
"Python functions" (as opposed to C++ functions), then I can see no
reason why the types should inherit.

>   I can see easy ways to do that, but I guess there is a 'supported'
> way of doing thoses things.

In Python 2.1 and before, the clear answer is "no". Types don't
inherit.

> I saw in the PyTypeObject structure a zone reserved to 'Attribute
> descriptor and subclassing stuff' (as the comment says,
> Include/object.h l.275).  To be precise, the fields tp_base and
> tp_bases seems to be what I need.

Note that this will be released with Python 2.2 only. Also, I very
much doubt that this is what you need. I guess you can easily do
without it.

>   Is it possible to set A's tp_base (tp_bases) to point to B so that
> magically if an attribute is not found by A's getattr(), python will
> call B's getattr() ?

I think so. However, you might need to go through type_new to create
such a type, plus you have to use PyObject_GenericGetAttr.

>   Is it possible to set A's tp_base (tp_bases?) to point to B so
> that my hand written getattr() for A will also call any object found
> in A's tp_base ?

No. You'll have to use PyObject_GenericGetAttr.

>   What are those fields used for ?

They are still under developement and not documented, so you best
inspect the code for their usage.

>   Also, looking through the python sources, I found a PyClass_Type
> definition.  As of now, my wrappers declare new instances of
> PyTypeObject (one per wrapped class) and in the init function of the
> module, I explicitly set the ob_type field to PyType_Type.  Could
> not I set it to PyClass_Type and benefit from new python
> functionalities ?

No. PyClass_Type are for old-style classes only; those don't allow
C-defined methods.

>   Please keep in mind that I don't master python at all and that the
> python internals are even more obscure to me.  (I am not part of the
> mailing list, so please CC me in replies)

Then I strongly advise to restrict yourself to what is available in
Python 2.1. It should be easy enough to solve your problem without
type inheritance.

Regards,
Martin