Add C object to Python List & return the list of C objects to python

Question

I am writing C extensions for python.

I want to know how to add a C object to python list using PyList_SetItem. For example,

I have a C object.

Atom *a = (Atom *)(PyTuple_GET_ITEM(tmp2, 0));

I made a list:

PyObject* MyList = PyList_New(3);

I am not sure what the third argument must be in the following statement.

PyObject_SetItem(MyList, 0, a);

Also how do I return this list of C objects to python.

Answer 1

The third argument to PyList_SetItem is the Python Object to be added to the list, which is usually converted from a C type, as in this simple example:

/* This adds one to each item in a list.  For example:
        alist = [1,2,3,4,5]
        RefArgs.MyFunc(alist)
*/
static PyObject * MyFunc(PyObject *self, PyObject *args)
{
   PyObject * ArgList;
   int i;

   PyArg_ParseTuple(args, "O!", &PyList_Type, &ArgList));

   for (i = 0; i < PyList_Size(ArgList); i++)
   {
      PyObject * PyValue;
      long iValue;

      PyValue = PyList_GetItem(ArgList, i);

      /* Add 1 to each item in the list (trivial, I know) */
      iValue = PyLong_AsLong(PyValue) + 1;

      /* SETTING THE ITEM */
      iRetn = PyList_SetItem(ArgList, i, PyLong_FromLong(iValue));

      if (iRetn == -1) Py_RETURN_FALSE;
   }

   Py_RETURN_TRUE;
}

PyObject_SetItem is similar. The difference is that PyList_SetItem steals the reference, but PyObject_SetItem only borrows it. PyObject_SetItem cannot be used with an immutable object, like a tuple.