PyListObject & C Modules

[Martin v. Loewis]

jggramlich at yahoo.com (Joshua Gramlich) writes:

> I am trying to find examples of this type of activity.  I've found the
> documentation at www.python.org (the C api), but I'm a bit green (new)
> and cannot read the List Objects page with any, well, usefulness. 

Here is a function that gets a list as an argument, and returns a list
that has the same first element:

PyObject*
get_first_elem(PyObject *unused, PyObject *args)
{
  PyObject *list;
  PyObject *first;
  PyObject *result;
  if(!PyArg_ParseTuple("O", &list))
    return NULL;

  if(!PyList_Check(list)) {
    PyErr_SetString(PyExc_TypeError, "get_first_elem expects a list");
    return NULL;
  }

  if(PyList_Size(list) == 0){
    PyErr_SetString(PyExc_ValueError, "empty lists not allowed");
    return NULL;
  }

  first = PyList_GetItem(list, 0);
  result = PyList_New(1);
  if (!result) return NULL;
  PyList_SetItem(result, 0, first);
  return result;
}

The Python equivalent of this is

def get_first_elem(_list):
  if type(_list) != list: raise TypeError, "expects a list"
  if len(_list) == 0: raise ValueError, "must not be empty"
  first = _list[0]
  return [first]

HTH,
Martin