Python C API: problems with calling python c method in c - python-c-api

Question

In my module there is a method that takes python list and divides it by double
static PyObject *st_div_r(PyObject *self, PyObject *args){
PyObject *pList;
Py_ssize_t n;
double x;
int i;
if (!PyArg_ParseTuple(args, "O!d", &PyList_Type, &pList, &x)) {
PyErr_SetString(PyExc_TypeError, "parameters are wrong.");
return NULL;
}
n = PyList_Size(pList);
for(i=0;i<n;i++)
{
PyList_SetItem(pList,i,Py_BuildValue("d",PyFloat_AsDouble(PyList_GetItem(pList,i))/x));
};
Py_INCREF(pList);
return pList;
}
It works like this
my_mod.st_div_r([1,2,3],0.5)
[2,4,6]
I would like to use st_div_r in another method in my module, that takes list of lists, divides it's first element by double and returns it
static PyObject *det(PyObject *self, PyObject *args){
PyObject *pList;
double d = 2.0;
if (!PyArg_ParseTuple(args, "O!", &PyList_Type, &pList)) {
PyErr_SetString(PyExc_TypeError, "parameters are wrong.");
return NULL;
}
PyList_SetItem(pList,0,st_div_r(pList,(PyList_GetItem(pList,0),Py_BuildValue("d",d))));
Py_INCREF(pList);
return pList;
}
It should work like this
my_mod.det([[1,2],[3,4]])
[[0.5,1],[3,4]]
But this doesn't work
TypeError: parameters are wrong
So, the question is how to call st_div_r inside det and what to pass as self.
Would appreciate any help. Thank you

Answer 1

You must create a PyTuple as second argument of your own function(st_div_r)!
There are 2 main ways to do that!
First Way
PyObject *first_list = PyList_GetItem(pList, 0)
//PyObject *divider = Py_BuildValue("d", d)
//PyObject *tuple_with_list_and_divider = Py_BuildValue("(Od)", first_list, divider);
PyObject *tuple_with_list_and_divider = Py_BuildValue("(Od)", first_list, d);
PyObject *list_after_division = st_div_r(pList, tuple_with_list_and_divider)
PyList_SetItem(pList , 0, list_after_division);
Second Way
PyObject *first_list = PyList_GetItem(pList, 0)
PyObject *divider = Py_BuildValue("d", d)
PyObject *tuple_with_list_and_divider = PyTuple_New(2);
PyTuple_SetItem(tuple_with_list_and_divider, 0, first_list);
PyTuple_SetItem(tuple_with_list_and_divider, 1, divider);
PyObject *list_after_division = st_div_r(pList, tuple_with_list_and_divider)
PyList_SetItem(pList , 0, list_after_division);
In first example I forgot that Py_BuildValue converts d to Py_Double.
Sorry for my Bad English!