Python func called from C++ not defined

Question

I am having a problem with calling a simple python script from C++. I have a simple testcase to illustrate the problem; In the python script - func fb should call fa which has previously defined.

However I get an error:

Called fb

Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "<string>", line 13, in <module>
    File "<string>", line 10, in fb
NameError: name 'fa' is not defined

The file test.py:

def fa() :
       print('Called fa')
       return 3
def fb() :
       print('Called fb')
       return fa() * 2
fb()
print('Finished test.py')

The C++ code:

#ifdef _DEBUG
#undef _DEBUG
#include <Python.h>
#define _DEBUG
#else
#include <Python.h>
#endif
#include <string>
int main() {

    // Initialize the python interpreter, setting path
    const char * app_path = "C:\\Python383-x64\\";
    size_t convertedChars = 0;
    size_t newsize = strlen(app_path);
    wchar_t* wcstring = new wchar_t[newsize];
    mbstowcs_s(&convertedChars, wcstring, newsize, app_path, _TRUNCATE);
    Py_SetPythonHome(wcstring);
    Py_Initialize();

    // Create dicts for globals and locals
    PyObject* globals = PyDict_New();
    PyObject* locals = PyDict_New();

    // Set the builtins
    PyDict_SetItemString(globals, "__builtins__", PyEval_GetBuiltins());

    // Evaluate python code and get the result
    PyObject* string_result = PyRun_String("exec(open(\"test.py\").read())",     Py_single_input, globals, locals);

    // check whether the python code caused any Exception
    if (PyErr_Occurred()) {
        PyErr_Print(); PyErr_Clear(); return 1;
    }
    else {
        // print the result
        PyObject_Print(string_result, stdout, Py_PRINT_RAW);
    }
    return 0;
}

It was missing a closing bracket during cut&paste, sorry. That should have been obvious. Why the '#undef _DEBUG', it's because otherwise its looking for the debug version of python which wasn't present in the python library. — keith969
– keith969, Commented Feb 25 at 11:09
I'd also point out that using PyRun_SimpleString("exec(open(\"test.py\").read())") works, i.e. prints 'Called fb' then 'Called fa'. So I'm guessing its something to do with setting globals/locals. — keith969
– keith969, Commented Feb 25 at 11:19
Look at PyRun_SimpleString source code, you will see that it is setting globals/locals differently. — n. m. could be an AI
– n. m. could be an AI, Commented Feb 25 at 11:30
Thanks! Yes adding: PyObject* main_module = PyImport_AddModule("main"); PyObject* dict = PyModule_GetDict(main_module); and using dict for globals/locals works. — keith969
– keith969, Commented Feb 25 at 11:38

Yksisarvinen · Accepted Answer · 2026-02-25 11:49:30Z

So the answer was to add __main__ to the dicts.

#ifdef _DEBUG
#undef _DEBUG
#include <Python.h>
#define _DEBUG
#else
#include <Python.h>
#endif
#include <string>
int main() {

    // Initialize the python interpreter, setting path
    const char * app_path = "C:\\Python383-x64\\";
    size_t convertedChars = 0;
    size_t newsize = strlen(app_path);
    wchar_t* wcstring = new wchar_t[newsize];
    mbstowcs_s(&convertedChars, wcstring, newsize, app_path, _TRUNCATE);
    Py_SetPythonHome(wcstring);
    Py_Initialize();

    // Create dicts for globals and locals
    PyObject* main_module = PyImport_AddModule("__main__");
    PyObject* locals = PyModule_GetDict(main_module);
    PyObject* globals = PyModule_GetDict(main_module);
    PyDict_SetItemString(globals, "__builtins__", PyEval_GetBuiltins());

    // Evaluate python code and get the result
    PyObject* string_result = PyRun_String("exec(open(\"test.py\").read())", Py_file_input, globals, locals);

    // check whether the python code caused any Exception
    if (PyErr_Occurred()) {
        PyErr_Print(); PyErr_Clear(); return 1;
    }
    return 0;
}

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