Calling a python method from C/C++, and extracting its return value

Question

I'd like to call a custom function that is defined in a Python module from C. I have some preliminary code to do that, but it just prints the output to stdout.

mytest.py

import math  def myabs(x):     return math.fabs(x) 

test.cpp

#include <Python.h>  int main() {     Py_Initialize();     PyRun_SimpleString("import sys; sys.path.append('.')");     PyRun_SimpleString("import mytest;");     PyRun_SimpleString("print mytest.myabs(2.0)");     Py_Finalize();      return 0; } 

How can I extract the return value into a C double and use it in C?

Answer 1

As explained before, using PyRun_SimpleString seems to be a bad idea.

You should definitely use the methods provided by the C-API (http://docs.python.org/c-api/).

Reading the introduction is the first thing to do to understand the way it works.

First, you have to learn about PyObject that is the basic object for the C API. It can represent any kind of python basic types (string, float, int,...).

Many functions exist to convert for example python string to char* or PyFloat to double.

First, import your module :

PyObject* myModuleString = PyString_FromString((char*)"mytest"); PyObject* myModule = PyImport_Import(myModuleString); 

Then getting a reference to your function :

PyObject* myFunction = PyObject_GetAttrString(myModule,(char*)"myabs"); PyObject* args = PyTuple_Pack(1,PyFloat_FromDouble(2.0)); 

Then getting your result :

PyObject* myResult = PyObject_CallObject(myFunction, args) 

And getting back to a double :

double result = PyFloat_AsDouble(myResult); 

You should obviously check the errors (cf. link given by Mark Tolonen).

If you have any question, don't hesitate. Good luck.