Calling a Python function within a C++ program

Question

I have a real simple Python function:

def myfunc(x): 
    return 2.0 * x

I want to send this function to a C++ program and call it so I have done this:

#include "Python.h"
static PyObject *execMyPyFunc(PyObject *self, PyObject *args) {
    PyObject *Fx, *pyresult;
    double x;
    PyArg_ParseTuple(args, "dO", &x, &Fx);
    pyresult = PyObject_CallFunction(Fx, "d", x);
    return pyresult;
}
static PyMethodDef C_API_TestMethods[] = {
    {"execMyPyFunc", execMyPyFunc, METH_VARARGS, "Add documentation here.."},
    {NULL, NULL}
};
PyMODINIT_FUNC initC_API_Test(void) {
    Py_InitModule("C_API_Test", C_API_TestMethods);
}

My Python program works correctly:

from C_API_Test import execMyPyFunc
def myfunc(x): 
    return 2.0 * x
fx = execMyPyFunc(1.28,myfunc)
print fx

What I would like to do though is to somehow get the pointer from my Python function (PyObject *Fx) and pass this to a C++ function expecting: double(*fx)(double). Does anyone know how to do this (if possible)? I tried to initialize double(*cFx)(double) and cast my Python function as cFx = (double(*)(double))Fx but this does not work. Any ideas?

Did you look at Boost.Python ?

P0W
– P0W

2013-08-26 17:26:39 +00:00
Commented Aug 26, 2013 at 17:26 — P0W
– P0W, Commented Aug 26, 2013 at 17:26
Yes, but how would one do this without Boost, SWIG, etc.

Joel Vroom
– Joel Vroom

2013-08-26 17:30:59 +00:00
Commented Aug 26, 2013 at 17:30 — Joel Vroom
– Joel Vroom, Commented Aug 26, 2013 at 17:30

Jared · Accepted Answer · 2013-08-26 18:31:08Z

You aren't going to be able to simply cast a Python function to C like that.

Instead pass the PyObject function pointer, call the function, and convert to C double. This code will return -1 on failure.

static double cFx(PyObject *fx, double x){
    PyObject *pyresult = PyObject_CallFunction(fx, "d", x);
    if (pyresult == NULL) return -1;
    double cppresult = PyFloat_AsDouble(pyresult);
    Py_DECREF(pyresult);
    if (PyErr_Occurred()) return -1;
    return cppresult;
}

Important part of this is to decrement the reference count to the return value of PyObject_CallFunction since you aren't passing it off to the Python interpreter to deal with.

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