Python Cookbook

Implementing C Function Callbacks to a Python Function

Credit: Swaminathan Narayanan

Problem

You must call a C function that takes a function callback as an argument, and you want to pass a Python function as the callback.

Solution

For this, we must wrap the Python function in a C function to be passed as the actual C-level callback. For example:

#include "python.h"

/* the C standard library qsort function, just as an example! */
extern void qsort(void *, size_t, size_t, int (*)(const void *, const void *));

/* static data (sigh), as we have no callback data in this (nasty) case */
static PyObject *py_compare_func = NULL;

static int
stub_compare_func(const void *cva, const void *cvb)
{
    int retvalue = 0;
    const PyObject **a = (const PyObject**)cva;
    const PyObject **b = (const PyObject**)cvb;

    // Build up the argument list...
    PyObject *arglist = Py_BuildValue("(OO)", *a, *b);

    // ...for calling the Python compare function
    PyObject *result = PyEval_CallObject(py_compare_func, arglist); if (result && PyInt_Check(result)) { retvalue = PyInt_AsLong(result); } Py_XDECREF(result); Py_DECREF(arglist); return retvalue; } static PyObject *pyqsort(PyObject *obj, PyObject *args) { PyObject *pycompobj; PyObject *list; if (!PyArg_ParseTuple(args, "OO", &list, &pycompobj)) return NULL; // Make sure second argument is a function if (!PyCallable_Check(pycompobj)) { PyErr_SetString(PyExc_TypeError, "Need a callable object!"); } else { // Save the compare function. This obviously won't work for multithreaded ...