P.K. P.K. - 2 months ago 25
C++ Question

Python C API and C++ functions

I'm trying to extend the Python interpreter in my C++ program, my problem is as follows.

When I'm trying to call a function, explained in the code below, I get a

NameError
, from the Python interpreter.
The error is



Traceback (most recent call last):

File "", line 3, in module

NameError: name 'func' is not defined


I used the following code to bind it, according to the Python wiki for version 3.3.2, which I'm using here


double func( int a )
{
return a*a-0.5;
}

static PyObject *TestError;
static PyObject * func_test(PyObject * self, PyObject *args)
{
const int * command;
double sts;
if( !PyArg_ParseTuple(args, "i", &command) )
return NULL;
sts = func( *command );
return PyFloat_FromDouble(sts);
}

static PyMethodDef TestMethods[] = {
{"func", func_test, METH_VARARGS,
"Thing."},
{NULL, NULL, 0, NULL} /* Sentinel */
};

static struct PyModuleDef testmodule = {
PyModuleDef_HEAD_INIT,
"test", /* name of module */
NULL, /* module documentation, may be NULL */
-1, /* size of per-interpreter state of the module,
or -1 if the module keeps state in global variables. */
TestMethods
};

PyMODINIT_FUNC PyInit_test()
{
PyObject *m;
m = PyModule_Create(&testmodule);
if (m == NULL)
return NULL;
TestError = PyErr_NewException("test.error", NULL, NULL);
Py_INCREF(TestError);
PyModule_AddObject(m, "error", TestError);
return m;
}




Then I'm calling
PyImport_AppendInittab("test", PyInit_test);


Py_Initialize();
, and then I'm trying to run a simple test, with

    
PyRun_SimpleString("import test\n"
"print('Hi!')\n"
"b = func(5)\n"
"print(b)\n");


Yet, I keep getting the error. Can someone please explain, what am I doing wrong here?

Answer
PyRun_SimpleString("import test\n"
                   "print('Hi!')\n"
                   "b = test.func(5)\n"   # <--
                   "print(b)\n");

EDIT: Another problem:

int command;   // not "int *"
double sts;
if( !PyArg_ParseTuple(args, "i", &command) )

Note that I'd recommend using CFFI if you're not familiar yet with how to write a CPython C extension module.