exporting c++ function for python, pass several strings as parameters

Question

I'm trying to export some C++ functions into python, and it's working pretty well most of the time, but I'm having issues when a function takes more than on string in parameters, the first one is always ok, but the other ones are always gibberish.

C++ code:

#include <Python.h>

#include <iostream>

PyObject* wrap_one_string(PyObject *self, PyObject *args)
{
    char* str;
    if (!PyArg_ParseTuple(args,"s:wrap_one_string",&str))
    {
        return nullptr;
    }
    std::cout << str << std::endl;
    return Py_None;
}

PyObject* wrap_two_string(PyObject *self, PyObject *args)
{
    char* str1, str2;
    if (!PyArg_ParseTuple(args,"ss:wrap_one_string", &str1, &str2))
    {
        return nullptr;
    }
    std::cout << str1 << std::endl << str2 << std::endl;
    return Py_None;
}

static PyMethodDef exampleMethods[] = {
    { "one_string", wrap_one_string, METH_VARARGS, nullptr },
    { "two_string", wrap_two_string, METH_VARARGS, nullptr },
    { nullptr, nullptr, 0, nullptr}
};

extern "C"
{

    __declspec(dllexport)
    void initexample()
    {
         PyObject *m;
         m = Py_InitModule("example", exampleMethods);
    }

}

python code:

import example

example.one_string("ab")
example.two_string("abcd", "efgh")

And the result is:

ab
abcd
È

The second string parameter is always a weird character. Any idea where this might come from?

Thanks

did you try example.two_string(("abcd","efgh")) or example.two_string(["abcd","efgh"]) ? — Ashalynd
– Ashalynd, Commented Sep 9, 2015 at 13:28
Hmmm, still the same with a tuple, but it seems to work with a list, also I have to use a different method to parse the list... — Julien Lopez
– Julien Lopez, Commented Sep 9, 2015 at 13:40
That got me thinking, when I access the second string with macros for tuple object, I can get it properly: PyString_AsString(PyTuple_GET_ITEM(args, 1)) — Julien Lopez
– Julien Lopez, Commented Sep 9, 2015 at 13:44

Julien Lopez · Accepted Answer · 2015-09-09 13:50:16Z

3

Ah, nevermind, dumb mistake from my part

char* str1, str2;

should be

char* str1, *str2;

Too bad it compiled, though it did give an error using

str2 = PyString_AsString(PyTuple_GET_ITEM(args, 1))

to access the second string.

answered Sep 9, 2015 at 13:50

Julien Lopez

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1 Comment

Yuriosity Over a year ago

That's why some code conventions require * and & to be bound to variable name, not type :) Being written like char *str1, str2;, the mistake is much more obvious.

Fernando G. Testa · Accepted Answer · 2015-09-09 14:51:28Z

1

One small note, remember to Py_INCREF(Py_None); before returning Py_None, as stated in the reference [https://docs.python.org/2/extending/extending.html#back-to-the-example] Or use Py_RETURN_NONE macro [https://docs.python.org/2/c-api/none.html#c.Py_RETURN_NONE]

answered Sep 9, 2015 at 14:51

Fernando G. Testa

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