Callbacks

Sometimes you need to run a Python code segment from C in your module. There's a lot of good reasons to do this. C doesn't have much support for writing regular expressions, while Python is pretty robust. You can take a PyObject with a C string and send it to a Python parsing function. When the Python code is done manipulating the PyObject, you have it back safe and sound in C.

Writing callback functions is pretty easy.

static PyObject *callback = NULL;

static PyObject *
set_callback(PyObject *dummy, PyObject *args)
{
PyObject *result = NULL;
PyObject *temp;

if (PyArg_ParseTuple(args, "O:set_callback", &temp))
{
if (!PyCallable_Check(temp))
{
PyErr_SetString(PyExc_TypeError, "parameter must be callable");
return NULL;
}
// Reference to new callback
Py_XINCREF(temp);
// Dispose of previous callback
Py_XDECREF(callback);
// Remember new callback
callback = temp;
// Boilerplate to return "None"
Py_INCREF(Py_None);
result = Py_None;
}
return result;
}

This function takes a dummy object and some arguments. It stores a callable function into a global PyObject named callback. You can later use this global PyObject with the callback function stored in it like this:

result = PyEval_CallObject(callback, arglist);

Result is a PyObject (pointer) with the result of the callback function stored in it. Here's a pretty simple example:

def add_ftn(a,b):
return a + b

parsedates.set_callback(add_ftn)

We set the callback PyObject to store the callable Python function add_ftn. We can test the callback by running the code above with the PyEval_CallObject(callback, arglist). This C method will take arguments (for add_ftn, we need two) and send them to the callback function stored in the global PyObject variable callback.