[Python-Dev] Advanced C Types

[Pete Shinners]

Hello Python developers, I have been learning my way through new style 
type in Python, but it hasn't been exactly easy. I have narrowed my 
remaining troubles down to two specific questions for this list. Thanks 
for pointers.

First, I have a base type written in C. Python classes are inheriting 
from these and those instances are being passed to another C function. 
This function needs access to data from the original type's PyObject 
structure. How do I get from the instance PyObject* to my base type's 
PyObject* data? My workaround is a hackish, the base has a method like this:
     PyObject *dirtydirty(PyObject *self, PyObject *args) {
          Py_INCREF(self);
          return self;
     }
Now my external C code call something like this:
     PyObject *GetBaseObjectFromInstance(PyObject *o) {
         PyObject *meth = PyObject_GetAttrString(o, "dirtydirty");
         PyObject *baseobj = PyObject_CallObject(meth, NULL);
	Py_DECREF(meth);
         return baseobj;
     }
How can I recreate this without requiring the 'dirtydirty' method? 
Obviously Python is internally keeping this data around, as it is passed 
to the C methods on my type. I cannot find a more direct route.


Second, I am very uncomfortable with the newstyle type allocation 
mechanisms. I understand the use of separate tp_init, tp_alloc, and 
tp_new. I do not understand how to use these from C. This is more a 
matter of clarification, my question is this; What is the proper way for 
one of my C functions to create my C type? Especially when my C type is 
derived from other C type? I feel wrong digging into my PyTypeObject for 
various tp_ pointers. I was comfortable with the PyObject_New() type 
functions, but from what I can see those are illegal for newstyle types?


Hmm, it appears my questions have expanded from simple and consise. 
Thanks again.