writing a C function to manipulate python lists

[Fredrik Lundh]

Michael Vanier wrote:
> I've been programming in python for many years but have never had to solve
> this particular problem.  I would like to write a short C function that will
> be part of a python-callable module.  The function will receive two python
> lists as arguments.  Currently I do this in python, but my code uses this
> function very heavily and it's the major speed bottleneck in the code.  The
> hope is that the C function will be much faster than iterating through the
> lists in python (which may not be true, but I'd like to see for myself).
>
> If anyone has some code lying around that does something like this, could
> they pass it on to me?

there's plenty of code in the Python source distribution...

searching for PyList and/or PySequence should help.  here's
one example, based on SRE's compile function:

static PyObject *
myfunc(PyObject* self_, PyObject* args)
{
    int i, n;

    /* accept a single list object */
    PyObject* code;
    if (!PyArg_ParseTuple(args, "O!", &PyList_Type, &code)
        return NULL;

    /* get the size */
    n = PyList_GET_SIZE(code);

    /* loop over the members */
    for (i = 0; i < n; i++) {
        int v;
        PyObject *o = PyList_GET_ITEM(code, i);
        v = PyInt_AsLong(o);
        if (v == -1 && PyErr_Occurred())
            goto oops; /* not a number */
        printf("%d\n", v);
    }

    ...

}

to handle any kind of sequence, use the PySequence abstract
API instead of the PyList API.

Cheers /F