[Python-Dev] Advanced C Types
Michael Hudson
mwh at python.net
Fri Jul 2 13:00:07 CEST 2004
Pete Shinners <pete at shinners.org> writes:
> 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.
Um. What am I missing that makes this not totally pointless? The
"instance PyObject*" *is* the "base type's PyObject* data". Cast it.
> 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?
The same way _randommodule.c does it :-)
> Especially when my C type is derived from other C type?
Hmm, not sure about that. Have you looked at xxsubtype in the Python
source? (Mind you, almost all C types derive from another C type:
PyBaseObjectType, so maybe this isn't a big deal).
> I feel wrong digging into my PyTypeObject for various tp_
> pointers.
Why?
> I was comfortable with the PyObject_New() type functions, but from
> what I can see those are illegal for newstyle types?
I think so, yes.
Cheers,
mwh
--
Considering that this thread is completely on-topic in the way only
c.l.py threads can be, I think I can say that you should replace
"Oblivion" with "Gravity", and increase your Radiohead quotient.
-- Ben Wolfson, comp.lang.python
More information about the Python-Dev
mailing list