Question

While working on a C++ project, I was looking for a third party library for something that is not my core business. I found a really good library, doing exactly what’s needed, but it is written in Python. I decided to experiment with embedding Python code in C++, using the Boost.Python library.

The C++ code looks something like this:

#include <string> #include <iostream> #include <boost/python.hpp>  using namespace boost::python;  int main(int, char **)  {     Py_Initialize();      try      {         object module((handle<>(borrowed(PyImport_AddModule('__main__')))));          object name_space = module.attr('__dict__');         object ignored = exec('from myModule import MyFunc\n'                           'MyFunc(\'some_arg\')\n',                           name_space);          std::string res = extract<std::string>(name_space['result']);     }      catch (error_already_set)      {         PyErr_Print();     }      Py_Finalize();     return 0; } 

A (very) simplified version of the Python code looks like this:

import thirdparty  def MyFunc(some_arg):     result = thirdparty.go()     print result 

Now the problem is this: ‘MyFunc’ executes fine, i can see the print of ‘result’. What i cannot do is read ‘result’ back from the C++ code. The extract command never finds ‘result’ in any namespace. I tried defining ‘result’ as a global, i even tried returning a tuple, but i cannot get it to work.

Answer 1

First of all, change your function to return the value. printing it will complicate things since you want to get the value back. Suppose your MyModule.py looks like this:

import thirdparty  def MyFunc(some_arg):     result = thirdparty.go()     return result 

Now, to do what you want, you have to go beyond basic embedding, as the documentation says. Here is the full code to run your function:

#include <Python.h>  int main(int argc, char *argv[]) {     PyObject *pName, *pModule, *pFunc;     PyObject *pArgs, *pArg, *pResult;     int i;      Py_Initialize();     pName = PyString_FromString('MyModule.py');     /* Error checking of pName left out as exercise */      pModule = PyImport_Import(pName);     Py_DECREF(pName);      if (pModule != NULL) {         pFunc = PyObject_GetAttrString(pModule, 'MyFunc');         /* pFunc is a new reference */          if (pFunc) {             pArgs = PyTuple_New(0);             pArg = PyString_FromString('some parameter')             /* pArg reference stolen here: */             PyTuple_SetItem(pArgs, 0, pArg);             pResult = PyObject_CallObject(pFunc, pArgs);             Py_DECREF(pArgs);             if (pResult != NULL) {                 printf('Result of call: %s\n', PyString_AsString(pResult));                 Py_DECREF(pResult);             }             else {                 Py_DECREF(pFunc);                 Py_DECREF(pModule);                 PyErr_Print();                 fprintf(stderr,'Call failed\n');                 return 1;             }         }         else {             if (PyErr_Occurred())                 PyErr_Print();             fprintf(stderr, 'Cannot find function');         }         Py_XDECREF(pFunc);         Py_DECREF(pModule);     }     else {         PyErr_Print();         fprintf(stderr, 'Failed to load module');         return 1;     }     Py_Finalize();     return 0; }