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/%7Eguido/)