Py_BuildValue or PyList_SetItem()

[Jack Diederich]

On Wed, Jan 03, 2007 at 01:16:38PM -0800, Sheldon wrote:
> I have a function that creates python objects out of C arrays and
> returns them to Python. Below is a working example that I later want to
> expand to return 12 arrays back to Python. The problem is that when I
> print out the values in Python I get undesired reults. See below. Does
> anyone know what is going on here?
> The array values are good before the conversion.
> 
> **************************************
> int createPythonObject(void) {
>   int i,j,k;
>   PyObject *Rva=PyList_New(12);
> 
>   for (i = 0; i < 12; i++) {
>     PyObject *op = PyFloat_FromDouble((double)va[i]);
>     if (PyList_SetItem(Rva,i,op) !=0) {
>       fprintf(stderr,"Error in creating python va object\n");
>       exit(EXIT_FAILURE);
>     }
>     Py_DECREF(op);
>     op = 0;
>   return Py_BuildValue("N",Rva);
>  }
> 
> Results in Python:
> 
> <refcnt 0 at 0x80d885c>, <refcnt 0 at 0x80d885c>, <refcnt 0 at
> 0x80d885c>, <refcnt 0 at 0x80d885c>, <refcnt 0 at 0x80d885c>, <refcnt 0
> at 0x80d885c>, <refcnt 0 at 0x80d885c>, <refcnt 0 at 0x80d885c>,
> <refcnt 0 at 0x80d885c>, <refcnt 0 at 0x80d885c>, <refcnt 0 at
> 0x80d885c>, <refcnt 0 at 0x80d885c>
> 

PyList_SetItem steals a reference to "op" so DECREF'ing it reduces
the refcount to zero.

http://docs.python.org/api/listObjects.html

-Jack