I am trying to write a python extension module where some of the functions are curried, but I am not quite sure how to go about doing so. The main difficulty being that I am not sure how to create and return a PyFunction object and how to then pass it the parse rules for its arguments. Is there a fairly efficient way to do this or is this insanity?
From the python side the desired semantics would be:
# given a function f(x, y)
f(a, b) -> result
f(a) -> f'
f'(b) -> result
Let's have a look at a possible Python implementation first.
def f(x, y=None):
if y is None:
return lambda y: f(x, y)
return 'result'
The only thing which needs to be done in C here is creating the lambda function somehow. Here we have the issue not knowing about the PyCFunction calling the C function itself. So we have to write wrapper around this and creates a new PyCFunction object.
static PyObject* curried (PyObject *old_args, PyObject *new_args);
static PyMethodDef curried_def = {"curried", curried, METH_VARARGS, "curried"};
static PyObject* f (PyObject *self, PyObject *args) {
PyObject *x = NULL, *y = NULL;
if(!PyArg_ParseTuple(args, "O|O", &x, &y))
return NULL;
// validate x
if (y == NULL)
return Py_INCREF(args), PyCFunction_New(&curried_def, args);
// validate y
// do something to obtain the result
return result;
}
static PyObject* curried (PyObject *old_args, PyObject *new_args) {
Py_ssize_t old_args_count = PyTuple_Size(old_args);
Py_ssize_t new_args_count = PyTuple_Size(new_args);
PyObject *all_args = PyTuple_New(old_args_count + new_args_count);
Py_ssize_t i;
PyObject *o;
for (i = 0; i < old_args_count; i++) {
o = PyTuple_GET_ITEM(old_args, i);
Py_INCREF(o);
PyTuple_SET_ITEM(all_args, i, o);
}
for (i = 0; i < new_args_count; i++) {
o = PyTuple_GET_ITEM(new_args, i);
Py_INCREF(o);
PyTuple_SET_ITEM(all_args, old_args_count + i, o);
}
return f(NULL, all_args);
}
This yields the desired semantics of
f(a, b) -> result
f(a) -> <built-in method curried of tuple object at 0x123456>
f(a)(b) -> result
Here we abuse the PyCFunction type a little and the second parameter passed to PyCFunction_New(&curried_def, args) is supposed to be the self object this function is bound to, hence we'll get a built-in method curried of tuple object. If you need the self parameter of the original function or use keyword arguments, you'd have to extend this hack a little and build a custom object to pass instead of the args. Also it's possible to create a type like of PyCFunction for curried functions. As far as I know, there isn't anything like that yet.