[Python-Dev] Extending types in C - help needed

Guido van Rossum guido@python.org
Wed, 06 Feb 2002 09:36:27 -0500

> > I wish I had time to explain this, but I don't.  For now, you'll have
> > to read how types are initialized in typeobject.c -- maybe there's a
> > way, maybe there isn't.
> > 
> > > Any tips about the route to take?
> > 
> > It can be done easily dynamically.
> I'm still struggling with this. How can it be done dynamically?
> My idea would be to realloc() the object after creation, adding
> a few bytes at the end. The problem is that I don't know how to
> find out about the object size without knowledge about the internals.
> The formula given in PEP 253
>   type->tp_basicsize  +  nitems * type->tp_itemsize
> seems not to be valid any more (at least with CYCLE GC).

I have thought about this a little more and come to the conclusion
that you cannot define a metaclass that creates type objects that have
more C slots than the standard type object lay-out.  It would be the
same as trying to add a C slot to the instances of a string subtype:
there's variable-length data at the end, and you cannot place anything
*before* that variable-length data because all the C code that works
with the base type knows where the variable length data start; you
cannot place anything *after* that variable-lenth data because there's
no way to address it from C.

The only way around this would be to duplicate *all* the code of
type_new(), which I don't recommend because it will probably have to
be changed for each Python version (even for bugfix releases).

A better solution is to store additional information in the __dict__.

--Guido van Rossum (home page: http://www.python.org/~guido/)