Calling Python in C++

Question

I'm learning C++, and in particular C interface to Python. Right now, my focus is on calling or importing python objects from C++ main program.

I've been studying the following link but couldn't understand some concepts. (https://www.codeproject.com/Articles/820116/Embedding-Python-program-in-a-C-Cplusplus-code) Following is the sections of the tutorial that I can't understand fully.

My questions are:

calling of module: Is it correct for me to assume "CPyObject pModule = PyImport_Import(pName)" is doing this job?
importing of objects:

i. Is it correct for me to assume "CPyObject pFunc = PyObject_GetAttrString(pModule, "getInteger")" is doing this job?

ii.If I want to import a dataframe from python to C++ as a CPyObject, how can I manipulate this object in C++. I'm asking because there is no equivalent object to dataframe in C++. 3) Is there anything else I need to do to make sure my Python module file is visible and callable to C++ ? Such as saving them in the same folder?

Consider the following Python program, stored in pyemb3.py:

def getInteger():

print('Python function getInteger() called')

c = 100*50/30

return c

Now we want to call the function getInteger() from the following C++ code and print the value returned this function. This is the client C++ code:

#include <stdio.h>
#include <Python.h>
#include <pyhelper.hpp>
    
int main()
     {
    CPyInstance hInstance;

    CPyObject pName = PyUnicode_FromString("pyemb3");
    CPyObject pModule = PyImport_Import(pName);

    if(pModule)
    {
        CPyObject pFunc = PyObject_GetAttrString(pModule, "getInteger");
        if(pFunc && PyCallable_Check(pFunc))
        {
            CPyObject pValue = PyObject_CallObject(pFunc, NULL);

            printf_s("C: getInteger() = %ld\n", PyLong_AsLong(pValue));
        }
        else
        {
            printf("ERROR: function getInteger()\n");
        }

    }
    else
    {
        printf_s("ERROR: Module not imported\n");
    }

    return 0;
}

Szabolcs Dombi · Accepted Answer · 2020-08-17 05:33:28Z

1

The problem is that 100*50/30 is not an integer, it is a float.

to get an integer use integer division: 100*50//30

If you are not sure about the returned type, you can use the Py_TYPE macro on pValue or just simply check for the type you are looking for with: PyLong_Check or PyLong_CheckExact

1: if PyImport_Import does not return null then the import was successful and the module was already executed by the time the function returned.

2: The PyObject_GetAttrString or the PyObject_GetAttr is the right way to get the imported module's objects.

3: Use these flags to ensure Python is embedded. Use Py_SetPath before Py_Initialize to add your module's path to sys.path.

edited Aug 17, 2020 at 5:33

answered Aug 17, 2020 at 5:22

Szabolcs Dombi

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