Question

I want to use an event based Python library in a C application. I use the offical C-API for embedding Python: http://docs.python.org/2/c-api/index.html#c-api-index

It is no problem to call methods from C and collect return values. However, I don’t know how to do the following:

Several of the python library functions take what I think is a python function pointer as an argument.

Is it possible, when calling these methods from C, to pass a C function pointer so the Python function uses a C function as callback?

If not, how to accomplish having Python use a C callback?

Answer 1

This is harder than one would expect, but it can be done.

If you have a single C function that you want to provide as a callback, you can use PyCFunction_New to convert it into a Python callable:

#include <python.h>

static PyObject *my_callback(PyObject *ignore, PyObject *args)
{
  /* ... */
}

static struct PyMethodDef callback_descr = {
  "function_name",
  (PyCFunction) my_callback,
  METH_VARARGS,                 /* or METH_O, METH_NOARGS, etc. */
  NULL
};

static PyObject *py_callback;

...
py_callback = PyCFunction_New(&callback_descr, NULL);

This approach won’t work if you want to choose different callbacks at run-time, e.g. to provide a generic c_to_python function that converts a C callback function to a Python callback. In that case, you’ll need to implement an extension type with its own tp_call.

typedef struct {
  PyObject_HEAD
  static PyObject (*callback)(PyObject *, PyObject *);
} CallbackObject;

static PyObject *
callback_call(CallbackObject *self, PyObject *args, PyObject *kwds)
{
  return self->callback(args, kwds);
}

static PyTypeObject CallbackType = {
    PyObject_HEAD_INIT(NULL)
    0,                          /*ob_size*/
    "Callback",                 /*tp_name*/
    sizeof(CallbackObject),     /*tp_basicsize*/
    0,                          /*tp_itemsize*/
    0,                          /*tp_dealloc*/
    0,                          /*tp_print*/
    0,                          /*tp_getattr*/
    0,                          /*tp_setattr*/
    0,                          /*tp_compare*/
    0,                          /*tp_repr*/
    0,                          /*tp_as_number*/
    0,                          /*tp_as_sequence*/
    0,                          /*tp_as_mapping*/
    0,                          /*tp_hash */
    (ternaryfunc) callback_call, /*tp_call*/
    0,                          /*tp_str*/
    0,                          /*tp_getattro*/
    0,                          /*tp_setattro*/
    0,                          /*tp_as_buffer*/
    Py_TPFLAGS_DEFAULT,         /*tp_flags*/
};

PyObject *
c_to_python(PyObject (*callback)(PyObject *, PyObject *))
{
  CallbackObject *pycallback = PyObject_New(CallbackObject, &CallbackType);
  if (pycallback)
    pycallback->callback = callback;
  return pycallback;
}

This code is trivially extended for the to also accept a user_data pointer; just store the user data in the CallbackObject struct.