Embedding Python into C - can't import method from python module

Question

I'm building C application which will be using Python plugins. When trying to call the method from another Python module, the function PyImport_ImportModule() seems to imports the module properly, then i try to get the function from this module using PyObject_GetAttrString() and all that I get is null.

I already tried using PyModule_GetDict() and PyDict_GetItemString() to get the method from the module, but the effect was the same.

main.c:

#include <stdio.h>
#include <python3.6/Python.h>

int main()
{
    PyObject *arg, *pModule, *ret, *pFunc, *pValue, *pMethod, *pDict;
    Py_Initialize();
    PyObject *sys = PyImport_ImportModule("sys");
    PyObject *path = PyObject_GetAttrString(sys, "path");
    PyList_Append(path, PyUnicode_FromString("."));

    pModule = PyImport_ImportModule("test");
    if(pModule == NULL)
    {
        perror("Can't open module");
    }
    pMethod = PyObject_GetAttrString(pModule, "myfun");
    if(pMethod == NULL)
    {
        perror("Can't find method");
    }

    ret = PyEval_CallObject(pMethod, NULL);
    if(ret == NULL)
    {
        perror("Couldn't call method");
    }

    PyArg_Parse(ret, "&d", pValue);
    printf("&d \n", pValue);

    Py_Finalize();
    return 0;
}

test.py:

def myfun():
  c = 123 + 123
  print('the result is: ', c)

myfun()

The result i got is:

Can't find method: Success
Segmentation fault (core dumped)

When I used the gdb debugger the output was:

pModule = (PyObject *) 0x7ffff5a96f48
pMethod = (PyObject *) 0x0

Your pMethod is NULL, and you then try to call it anyway, so that generates a segmentation fault and crashes your program. You should debug what happens before that. Also, do not use perror: the python C API does not set errno, so using perror will get useless messages (in fact in this case it says "Success"). — Marco Bonelli
– Marco Bonelli, Commented Jul 19, 2019 at 9:36
Thanks but seg fault is not a problem it's a consequence. The problem is, why pMethod is NULL? Before calling pMethod I import the module which gives me "pModule = (PyObject *) 0x7ffff5a96f48" so if the module imports correctly, why the pMethod is not there? --> pMethod = (PyObject *) 0x0 — Saeley
– Saeley, Commented Jul 19, 2019 at 12:08
what does PyObject_HasAttrString(pModule, "myfun") return? — Marco Bonelli
– Marco Bonelli, Commented Jul 19, 2019 at 12:11

Marco Bonelli · Accepted Answer · 2019-07-19 22:41:54Z

Your program is not wroking because the module being imported is the test built-in module, rather than your test.py script. This is because you are appending the current directory to sys.path, so it is checked after every other already existing path in the list. You should insert it at the beginning of the list instead, so that it is checked first.

This will work:

PyObject *sys = PyImport_ImportModule("sys");                                                                                                                                                                     
PyObject *path = PyObject_GetAttrString(sys, "path");                                                                                                                                                     
PyList_Insert(path, 0, PyUnicode_FromString("."));

By the way, you should #include the Python header before anything else, as stated in the documentation:

Note: Since Python may define some pre-processor definitions which affect the standard headers on some systems, you must include Python.h before any standard headers are included.

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