Sending a Python Function to C++ and Calling Within C++

I thought I would share something I figured out how to do recently. In C++ I have a nice function that numerically integrates a given function. I wanted to be able to call this quadrature routine from Python while feeding it a Python function (callable). For the sake of simplicity I will show a much smaller example here. I realize there are quadrature routines in scipy.integrate but I wanted the flexibility of my C++ routine as well.

Suppose we have a very simply Python function called "myfunc":

def myfunc(x):
    return 2.0 * x;

I want to be able to send this function to C++ and use it within my C++ program. To do this I wrote the following C++ code:

#include "Python.h"

static PyObject *execMyPyFunc(PyObject *self, PyObject *args) {
 PyObject *Fx, *pyresult;
 double x, result;

 PyArg_ParseTuple(args, "dO", &x, &Fx);
 pyresult = PyObject_CallFunction(Fx, "d", x);
 result = PyFloat_AsDouble(pyresult);
 return Py_BuildValue("d", result);
}

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);
}

This creates a DLL (or .pyd for Python) module called "C_API_Test" that I can import into Python. This module contains the function "execMyPyFunc" that I can use within my Python scripts. The function "execMyPyfunc" takes two arguments:

1. a double precision scalar
2. a python function

I make use of PyObject_CallFunction. My Python script looks like this:

from C_API_Test import execMyPyFunc

def myfunc(x):
    return 2.0 * x;
    
fx = execMyPyFunc(1.28,myfunc)
print fx

And it returns 2.56! Voila! Note: I haven't read up on PY_INCREF & Py_DECREF. They may need to be added here. If someone knows, please let me know. Thanks!

This is a simple test to incorporate the SyntaxHighlighter in my blog. Quite handy!

import sys, copy
from xml.sax.drivers.drv_xmlproc_val import *
 
class myParser(SAX_XPValParser):
    """XML parser."""
 
    psl = None
 
    def handle_doctype(self, rootname, pub_id, sys_id):
        self.dtd_handler.doctypeDecl(rootname, pub_id, sys_id, self.psl)
 
 
class docHandler:
    """XML Document events handler."""
 
    def characters(self, ch, start, length):
        """Handle a character data event.
        The data are contained in the substring of ch
        starting at position start and of length length."""
 
        print ch[start:start+length],
 
    def endDocument(self):
        """Handle an event for the end of a document."""

I found the instructions here quite helpful!