[capi-sig] How to manipulate C table as python list ?

Hrvoje Niksic hniksic at xemacs.org
Sun Jan 18 09:51:38 CET 2009

Marc <marcglec at free.fr> writes:

> I would like to use a C program which manipulates table
> as a python module acting on a list.
> My C code is the following:
> int sum(int *tab, int n)
> {
>   int i, s = 0;
>   for (i=0; i<n; i++)
>     {
>       s += tab[i];
>     }
>   return s;
> }
> Now, I would like to use it as a python module
> where tab would be a python list.
> I read some examples in the documentation but
> the size of the list is always assumed to be known
> such that one can use PyArg_ParseTuple
> with a format description for each argument.

In this case you only use PyArg_ParseTuple to get to the object that
represents the list.  In fact, since you only need one argument, you
don't need PyArg_ParseTuple at all, simply declare your function to
take one argument using the METH_O in the function description, and
code it like this:

PyObject *sum(PyObject *ignored, PyObject *lst)
    int i, s = 0;
    if (!PyList_Check(lst)) {
        PyErr_Format(PyExc_TypeError, "sum: expected list, got %s",
        return NULL;
    for (i = 0; i < PyList_GET_SIZE(lst); i++) {
        ... sum the list ...
    return PyInt_FromLong(s);

> My question is:
> which option should I use with PyArg_ParseTuple?

If your function is declared to take one argument with METH_O, you
don't need PyArg_ParseTuple at all.  If it's declared to take a
variable number of arguments, you'd use it like this:

PyObject *sum(PyObject *ignored, PyObject *args)
    int i, s = 0;
    PyObject *lst;
    if (!PyArg_ParseTuple(args, "O", &lst))
        return NULL;

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