Embedding Python does not throw an error, but the program crashes anyways

[lfreeman6490]

I am trying to embed a python interpreter into my c++ gui. I am including Python.h and then in my function

void pl::on_pb_connect_disconnect_clicked()
{
    PyObject* pName, *pModule, *pDict, *pFunc;

    Py_Initialize();

    std::string filename = "test_python_file.py";
    const char* cstr1 = filename.c_str();

    pName = PyUnicode_FromString(cstr1);

    pModule = PyImport_Import(pName);

    pDict = PyModule_GetDict(pModule);

    std::string function_name = "test_function";
    const char* cstr = function_name.c_str();

    pFunc = PyDict_GetItemString(pDict, cstr);

    if (PyCallable_Check(pFunc))
    {
        PyObject_CallObject(pFunc, NULL);
    }
    else
    {
        PyErr_Print();
    }

    Py_Finalize();
}

The basic idea is that once the 'connect' button is hit, it calls test_function in test_python_file.py. It builds and runs successfully, once it is launched and I hit the 'connect' button, the program crashes. I have cleaned, rebuild, ran qmake, etc. Running in debug mode gives me a division by 0 error. If I comment out the code line by line I see that the errors comes from

pDict = PyModule_GetDict(pModule);

Once that line is used again, the program crashes.

[SGaist]

Hi,

Since you are using a relative path, is your Python file in the same folder as the executable ?
You should check whether your objects are null just in case something goes wrong.

[lfreeman6490]

@SGaist Hi, Thanks for the response. The python files are in the same folder as my executable. How would I check for null objects? Am I not doing that already? I thought that

if (PyCallable_Check(pFunc))
    {
        PyObject_CallObject(pFunc, NULL);
    }
    else
    {
        PyErr_Print();
    }

Would print out the error message that is returned, is that not correct?

[eyllanesc]

@lfreeman6490 Have you verified that the pointers are not null?

[lfreeman6490]

@eyllanesc How would I check for that? Is my if loop at the end of my code not checking for error messages?

[eyllanesc]

@lfreeman6490 PyErr_Print only returns python errors, it does not check memory.

what is the output of:

qDebug() << pName << pModule  << pDict;

before PyDict_GetItemString(...)?

[lfreeman6490]

@eyllanesc There is no difference in the output when entering that line. I cleaned, reran qmake, rebuilt, then ran directly from the executable as well as from qtcreator. The output just says

The program has finished unexpectedly

[eyllanesc]

@lfreeman6490 I never said that was the solution, do not rush. I'm just asking for some debugging information to analyze the cause of the error.

[lfreeman6490]

@eyllanesc Okay, thanks. I just wanted to clearly show what the error message was and what I had done already

[eyllanesc]

@lfreeman6490 Comment on the line that generates the error and tell me what is printed on the console

[lfreeman6490]

@eyllanesc Okay, commenting out that line gives me hex code errors

0x67b0128 0x0 0x57290610 

[eyllanesc]

@lfreeman6490 From the print we can see that pModule is null, so you get this behavior.

Change to:

if(!(pModule = PyImport_Import(pName))){
    PyErr_Print();
    return;
}

[lfreeman6490]

@eyllanesc Where in the code should I add that? Should that be directly after creating pModule? Or at the very last spot in the code?

[eyllanesc]

@lfreeman6490 Change pModule = PyImport_Import(pName);

[lfreeman6490]

@eyllanesc Okay so by changing that it is no longer crashing, but it is not calling my function. The function is a simple test function that should return a number

def test_function():
    return 7

[eyllanesc]

@lfreeman6490 It seems that you have not understood the problem. Your code has not imported the module so pModule is null.

[JonB]

@lfreeman6490 said in Embedding Python does not throw an error, but the program crashes anyways:

std::string filename = "test_python_file.py";
const char* cstr1 = filename.c_str();

pName = PyUnicode_FromString(cstr1);

if(!(pModule = PyImport_Import(pName))){
    PyErr_Print();
    return;
}

From the 0x0 output you really should be able to understand that PyImport_Import(pName) returned nullptr. That means your test_python_file.py could not be loaded, Either the file could not be found, or there is some problem loading it. Hopefully the PyErr_Print(); gave some clue as to why, but you never said anything about what it output.

Start by changing your test_python_file.py to the full path to where that file is.