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

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

Derek Ribeiro · Accepted Answer · 2020-04-30 15:04:23Z

3

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!

edited Apr 30, 2020 at 15:04

answered Apr 30, 2020 at 4:57

Derek Ribeiro

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2 Comments

alehak Over a year ago

Thanks! The second way works well, the first way gives [[inf, inf], [3, 4]]

DavidW Over a year ago

The first way did be PyObject *tuple_with_list_and_divider = Py_BuildValue("(Od)", first_list, d); I think. @alehax if you had any error checking in your code you would probably get a more sensible message.

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