Passing a Python list to C function using the Python/C API

Question

I\'ve recently started using the Python/C API to build modules for Python using C code. I\'ve been trying to pass a Python list of numbers to a C function without success:

Answer 1

This example will show you how to

1. Get the length of the list
2. Allocate storage for an array of double
3. Get each element of the list
4. Convert each element to a double
5. Store each converted element in the array

Here is the code:

#include "Python.h"

int _asdf(double pr[], int length) {
    for (int index = 0; index < length; index++)
        printf("pr[%d] = %f\n", index, pr[index]);
    return 0;
}

static PyObject *asdf(PyObject *self, PyObject *args)
{
    PyObject *float_list;
    int pr_length;
    double *pr;

    if (!PyArg_ParseTuple(args, "O", &float_list))
        return NULL;
    pr_length = PyObject_Length(float_list);
    if (pr_length < 0)
        return NULL;
    pr = (double *) malloc(sizeof(double *) * pr_length);
    if (pr == NULL)
        return NULL;
    for (int index = 0; index < pr_length; index++) {
        PyObject *item;
        item = PyList_GetItem(float_list, index);
        if (!PyFloat_Check(item))
            pr[index] = 0.0;
        pr[index] = PyFloat_AsDouble(item);
    }
    return Py_BuildValue("i", _asdf(pr, pr_length));
}

NOTE: White space and braces removed to keep code from scrolling.

Test program

import asdf
print asdf.asdf([0.7, 0.0, 0.1, 0.0, 0.0, 0.2])

Output

pr[0] = 0.700000
pr[1] = 0.000000
pr[2] = 0.100000
pr[3] = 0.000000
pr[4] = 0.000000
pr[5] = 0.200000
0

Answer 2

Would this do to properly free memory?

PyObject *res;
res = Py_BuildValue("i", _asdf(pr, pr_length));
free(pr);
return res;