Embedding Python in C

Question

I've been trying to run a Python 3 script through C using code the documentation provided.

main.c:

#include <stdio.h>
#include <stdlib.h>
#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;
    }

    Py_Initialize();
    pName = PyUnicode_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 = PyLong_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", PyLong_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;
}

test.py:

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

I enter into the console:

./pytest test multiply 3 4

and I get:

AttributeError: 'module' object has no attribute 'multiply'
Cannot find function "multiply"

I'm using Ubuntu 13.10; I figured this was relevant because the documentation appeared to be addressing Windows. What am I missing?

Community · Accepted Answer · 2017-05-23 11:56:47Z

2

You've forgotten (or rather, were never told) that the current/script/executable directory is not added to sys.path when embedding the interpreter, so you've found some completely different module called "test" and are importing that instead.

edited May 23, 2017 at 11:56

CommunityBot

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answered Jan 9, 2014 at 2:06

Ignacio Vazquez-Abrams

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6 Comments

user1610810 Over a year ago

Thanks a bunch I'll keep this in mind in the future. Since I still have your attention, I've renamed the file to "qwe.py" and now I get ImportError: No module named 'qwe' Failed to load "qwe"

user1610810 Over a year ago

Never mind I'm headed towards the right direction now. Thanks again.

Ignacio Vazquez-Abrams Over a year ago

@user1610810: The problem isn't that it found some other module instead of yours, it's that it isn't capable of finding yours in the first place.

user1610810 Over a year ago

I figured that. I tried making it reachable using PySys_SetPath(PyBytes_FromString("/home/frosty/NetBeansProjects/pyTest/")); , but no luck. Is there documentation I can just dive into?

Ignacio Vazquez-Abrams Over a year ago

@user1610810: No, there is none. You can manipulate sys.path as you would any other list though.

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