Extending python with C: Pass a list to PyArg_ParseTuple

Question

I have been trying to get to grips with extending python with C, and so far, based on the documentation, I have had reasonable success in writing small C functions and extending it with Python.

However, I am now struck on a rather simple problem - to which I am not able to find a solution. So, what I'd like to do is pass a double list to my C function. For example, to pass an int, I do the following:

int squared(int n)
{
    if (n > 0)
        return n*n;
    else
        return 0;
}

static PyObject*
squaredfunc(PyObject* self, PyObject* args)
{
    int n;

    if (!PyArg_ParseTuple(args, "i", &n))
        return NULL;

    return Py_BuildValue("i", squared(n));
}

This passes the int n with no problems to my C function named squared.

But, how does one pass a list to the C function? I did try to google it and read the docs, and so far, I havent found anything useful on this.

Would really appreciate if someone could point me in the right direction.

Thanks.

Answer 1

PyArg_ParseTuple can only handle simple C types, complex numbers, char *, PyStringObject *, PyUnicodeObject *, and PyObject *. The only way to work with a PyListObject is by using some variant of "O" and extracting the object as a PyObject *. You can then use the List Object API to check that the object is indeed a list (PyList_Check). Then you can then use PyList_Size and PyList_GetItem to iterate over the list. Please note that when iterating, you will get PyObject * and will have to use the floating point API to access the actual values (by doing PyFloat_Check and PyFloat_AsDouble.) As an alternative to the List API, you can be more flexible and use the iterator protocol (in which case you should just use PyIter_Check). This will allow you to iterate over anything that supports the iterator protocol, like lists, tuples, sets, etc.

Finally, if you really want your function to accept double n[] and you want to avoid all of that manual conversion, then you should use something like boost::python. The learning curve and APIs are more complex, but boost::python will handle all of the conversions for you automatically.

Here is an example of looping using the iterator protocol (this is untested and you'd need to fill in the error handling code):

PyObject *obj;

if (!PyArg_ParseTuple(args, "O", &obj)) {
  // error
}

PyObject *iter = PyObject_GetIter(obj);
if (!iter) {
  // error not iterator
}

while (true) {
  PyObject *next = PyIter_Next(iter);
  if (!next) {
    // nothing left in the iterator
    break;
  }

  if (!PyFloat_Check(next)) {
    // error, we were expecting a floating point value
  }

  double foo = PyFloat_AsDouble(next);
  // do something with foo
}