Python 3: Python/C API String Problems

Question

I am trying to use the Python/C API to run a Python function which returns a string. I want to store that returned string to a C++ variable, but I can't get it to work. I am using Python 3 and it seems like the PyString_FromString() method doesn't work anymore. Here is my code:

int main(int argc, char *argv[])
{
    PyObject *pName, *pModule, *pDict, *pFunc, *pValue;

    Py_Initialize();

    pName = PyUnicode_FromString("ocr");

    pModule = PyImport_Import(pName);

    pDict = PyModule_GetDict(pModule);

    pFunc = PyDict_GetItemString(pDict, "get_text");

    pValue = PyUnicode_FromString("ocr_noise.png");
    pValue = PyObject_CallObject(pFunc, pValue);

    std::string result = PyUnicode_FromObject(pValue);

    Py_DECREF(pModule);
    Py_DECREF(pName);
    Py_DECREF(pValue);

    Py_Finalize();

    return 0;
}

The python file is called ocr.py, and the function I am trying to call is called get_text(value). I am trying to pass in "ocr_noise.png" as an argument. Any ideas what I should do?

EDIT 2: I don't need to use std::string as in the code. What else can I use to store the string returned from the function?

What do you mean with "it seems like the PyUnicode_FromString() method doesn't work anymore"? Are you getting an exception? A segfault? Something else? — Andrea Corbellini
– Andrea Corbellini, Commented Nov 15, 2015 at 17:59
Have you considered using an existing wrapper library like Boost.Python instead of the low-level C API? It should make these things much simpler. That said, please read the guidelines here, your question is by definition off-topic, as explained there. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Nov 16, 2015 at 9:13
and there's also PyUnicode_AsUTF8 if you really want a string — Andrea Corbellini
– Andrea Corbellini, Commented Nov 16, 2015 at 9:34

tynn · Accepted Answer · 2015-11-15 20:49:55Z

3

It's essential to check the return values of all Python functions. Python returns a nullptr if there was an error. So PyUnicode_FromString() works fine in your code. It just segfaults because pValue is a nullptr you got from PyObject_CallObject(). Putting PyErr_Print() just after this call prints:

TypeError: argument list must be a tuple

You need to pass a tuple of objects as argument, not a single str. Instead you might want to use PyObject_CallFunction(pFunc, "O", pValue) or PyObject_CallFunction(pFunc, "s", "ocr_noise.png").

Additionally have a look into PyImport_ImportModule(). Furthermore

std::string result = PyUnicode_FromObject(pValue);

should not even compile since it returns a PyObject * not a string.

answered Nov 15, 2015 at 20:49

tynn

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2 Comments

MarcioPorto Over a year ago

What should I do to store the returned string then?

tynn Over a year ago

PyUnicode_FromObject() might not be useful at all. pValue should be a unicode object already. See Clean Way to Convert Python 3 Unicode to std::string for more detail on how to get a c string from the Python unicode object.

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