Python Embedding in C++ : ImportError: No module named pyfunction

Question

Hi I'm trying to embed python (2.7) into C++ (g++ 4.8.2) and hence call a python function from C++. This is the basic code provided in python documentation for embedding:

This is my file call_function.cpp

#include <Python.h>

int main(int argc, char *argv[])
{
    PyObject *pName, *pModule, *pDict, *pFunc;
    PyObject *pArgs, *pValue;
    int i;

    if (argc < 3) {
        fprintf(stderr,"Usage: call pythonfile funcname [args]\n");
        return 1;
    }
   /* char pySearchPath[] = "/usr/include/python2.7";
    Py_SetPythonHome(pySearchPath);*/
    Py_Initialize();
    pName = PyString_FromString(argv[1]);
    /* Error checking of pName left out */

    pModule = PyImport_Import(pName);
    Py_DECREF(pName);

    if (pModule != NULL) {
        pFunc = PyObject_GetAttrString(pModule, argv[2]);
        /* pFunc is a new reference */

        if (pFunc && PyCallable_Check(pFunc)) {
            pArgs = PyTuple_New(argc - 3);
            for (i = 0; i < argc - 3; ++i) {
                pValue = PyInt_FromLong(atoi(argv[i + 3]));
                if (!pValue) {
                    Py_DECREF(pArgs);
                    Py_DECREF(pModule);
                    fprintf(stderr, "Cannot convert argument\n");
                    return 1;
                }
                /* pValue reference stolen here: */
                PyTuple_SetItem(pArgs, i, pValue);
            }
            pValue = PyObject_CallObject(pFunc, pArgs);
            Py_DECREF(pArgs);
            if (pValue != NULL) {
                printf("Result of call: %ld\n", PyInt_AsLong(pValue));
                Py_DECREF(pValue);
            }
            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 \"%s\"\n", argv[2]);
        }
        Py_XDECREF(pFunc);
        Py_DECREF(pModule);
    }
    else {
        PyErr_Print();
        fprintf(stderr, "Failed to load \"%s\"\n", argv[1]);
        return 1;
    }
    Py_Finalize();
    return 0;
}

Now my python script is saved as pyfunction.py placed in the same folder as call_function.cpp.

This is pyfunction.py :

def multiply(a,b):
    print "Will compute", a, "times", b
    c = 0
    for i in range(0, a):
        c = c + b
    return c

Now using the Terminal I'm calling :

$ g++ call_function.cpp -I/usr/include/python2.7 -lpython2.7 -o call_function

(Compiles successfully without any errors) (Running the program)

$ ./call_function pyfunction multiply 2 3

(I get this ERROR):

ImportError: No module named pyfunction    
Failed to load "pyfunction"

I don't understand how this is possible. I've followed the documentation and still I'm getting the error.

How can it not find pyfunction.py when it is placed in the same directory.

Answer 1

Put the following in the C/C++ code, just after Py_Initialize();

PyRun_SimpleString("import sys");
PyRun_SimpleString("sys.path.append(\".\")");

Answer 2

Try this one:

 $ PYTHONPATH=. ./call_function pyfunction multiply 2 3

if this won't work, try to make __init__.py file in this directory and try again.

UPDATE:

I think that PYTHONPATH is temporary solution, to test stuff. If you want to have a directory when all your embedded modules lives you have to put in your embedded interpreter something equilevant to this:

import sys
sys.path.insert(0, "./path/to/your/modules/")

You can do it probably in python in your interpreter or on C level. This will add search path in very similar manner as PYTHONPATH but it is more persistant and elegant (IMHO).

Answer 3

Hi to all those facing the same problem, I found the solution! setenv() is a function defined in stdlib.h which sets the environment variable. Just have to run it!

setenv("PYTHONPATH",".",1);

for more info on setenv:

$ man setenv

All the best :) Also, thanks to @spinus

Answer 4

You can also try to include these code to your c program

Py_Initialize();
PyObject *sys = PyImport_ImportModule("sys");
PyObject *path = PyObject_GetAttrString(sys, "path");
PyList_Append(path, PyUnicode_FromString("."));

Learn from Here

Answer 5

the solution provided by spinus works if the python file does not import any additional python-library.

However, if a python file imports an additional library, lets say numpy, the above code crashes as follows:

:~/programs/python$ ./a.out myModule multiply 4 3
Traceback (most recent call last):
  File "/home/a/programs/python/myModule.py", line 1, in <module>
    import numpy
ImportError: No module named 'numpy'
Failed to load "myModule"

As a remark, the import of the python-library from C does not work:

PyObject *pNumpy = PyUnicode_FromString("numpy");
PyObject *pModuleA = PyImport_Import(pNumpy); 

Does someone know how to call from C python-functions, which depend of some other python-libraries?

Answer 6

For anyone else having this problem:

Are you sure that your .py file lies in the same directory where C++ executable is?

I was programming in CLion and forgot that executable lies in cmake-build-debug. So I added .py file in project directory and no no surprise I was getting the same error ImportError. I placed .py file to cmake-build-debug (executable file by default lies there), used answers from this question and everything worked!