Hrvoje Niksic <[EMAIL PROTECTED]> writes:
> Chris <[EMAIL PROTECTED]> writes:
>
>>> descr = GetAttrString(cls,"varname");
>>> offset = descr->d_member->offset;
>>> slotvar = (PyObject*)(((char*)obj)+offset)
>>
>> Unfortunately, I am inexperienced at this kind of thing, so I wasn't
>> able to get something working. Maybe someone could tell me what's
>> wrong with the code below (it gives the error "'struct _object' has no
>> member named 'd_member'")?
>
> You are getting that error because Carl forgot to cast the descriptor
> to the appropriate C type, in this case PyMemberDescrObject. The last
> line is also incorrect, I think. Try something like this:
>
> PyObject *descr = PyObject_GetAttrString(x,"attr_one");
Also note that x should be your object's type, i.e. o->ob_type cast to
PyObject *. The complete incantation would be:
// outside the loop
PyObject *descr = PyObject_GetAttrString((PyObject *) obj->ob_type,
"attr_one");
if (!descr)
return NULL;
int offset = ((PyMemberDescrObject *) descr)->d_member->offset;
Py_DECREF(descr);
// inside the loop
PyObject *value = *(PyObject **)((char *)obj + offset);
if (!value) {
PyErr_SetString(PyExc_AttributeError, "attribute missing");
return NULL;
}
// Remember to Py_INCREF(value) before using it and Py_DECREF it after
// using it. Otherwise if the object changes its slot to something
// else, you might be using a dead object.
With this code, accessing the value is practically free once the
offset is calculated. Here is a bench3 function updated to use this
strategy:
static PyObject *
bench3(PyObject *ignored, PyObject *obj)
{
PyObject *descr = PyObject_GetAttrString((PyObject *) obj->ob_type,
"abcdef");
if (!descr)
return NULL;
// Here you should also assert that PyObject_TypeCheck(descr,
// PyMemberDescr_Type) holds true. Since PyMemberDescr_Type is static,
// you'll have to get it by stashing away ob_type of a value known to be
// the descriptor. This is left as excercise to the reader.
int offset = ((PyMemberDescrObject *) descr)->d_member->offset;
Py_DECREF(descr);
int i;
PyObject *volatile attr; // 'volatile' for benchmarking, prevents gcc
// from optimizing away the loop
for (i = 0; i < 1000000; i++) {
attr = *(PyObject **)((char *)obj + offset);
}
Py_INCREF(attr);
return attr; // to prove that we retrieved the correct value
}
>>> t0 = time.time(); attr.bench3(o); t1 = time.time()
1
>>> t1-t0
0.00071597099304199219 # for 1,000,000 iterations, as cheap as it gets
--
http://mail.python.org/mailman/listinfo/python-list
