How to define class methods in Python C API?

Question

I was trying to create a Python class definition in C++ code and access it in Python. However, the function is called but the parameters are not received correctly. Please help me in doing this properly.

#include <iostream>
#include <Python.h>

using namespace std;

#include <Python.h>

static PyObject* MyClass__Init(PyObject *self, PyObject *args)
{
    cout << "MyClass__Init Called" << endl;
    Py_INCREF(Py_None);
    return Py_None;
};

static PyObject* MyModule__Start(PyObject *self, PyObject *args)
{
    const char* zBuff;

    if (PyArg_ParseTuple(args, "s", &zBuff))
        cout << "MyModule Start Called with parameter " << zBuff << endl;
    else
        cout << "MyModule Start ERROR" << endl;

    Py_INCREF(Py_None);
    return Py_None;
};

static PyObject* MyClass__Start(PyObject *self, PyObject *args)
{
    const char* zBuff;

    if (PyArg_ParseTuple(args, "s", &zBuff))
        cout << "MyClass Start Called with parameter" << zBuff << endl;
    else
        cout << "MyClass Start ERROR" << endl;

    Py_INCREF(Py_None);
    return Py_None;
};

static PyMethodDef pModuleMethods[] =
{
    {"Start", MyModule__Start, METH_VARARGS, ""},
    {NULL, NULL, 0, NULL}
};

static PyMethodDef pClassMethods[] = 
{
    {"__init__", MyClass__Init, METH_VARARGS, ""},
    {"Start", MyClass__Start, METH_VARARGS, ""},
    {NULL, NULL, 0, NULL}
};

void Start()
{
    Py_Initialize();

    /* create a new module and class */
    PyObject *pClassDic = PyDict_New();
    PyObject *pClassName = PyString_FromString("MyClass");
    PyObject *pClass = PyClass_New(NULL, pClassDic, pClassName);

    PyObject *pModule = Py_InitModule("MyModule", pModuleMethods);
    PyObject *pModuleDic = PyModule_GetDict(pModule);

    /* add methods to class */
    for (PyMethodDef* pDef = pClassMethods; pDef->ml_name != NULL; pDef++)
    {
        PyObject *pFunc = PyCFunction_New(pDef, NULL);
        PyObject *pMethod = PyMethod_New(pFunc, NULL, pClass);
        PyDict_SetItemString(pClassDic, pDef->ml_name, pMethod);
    }

    PyDict_SetItemString(pModuleDic, "MyClass", pClass);

    PyRun_SimpleString("import MyModule\n"
        "MyModule.Start('Hello Module')\n"
        "myObj = MyModule.MyClass()\n"
        "myObj.Start('Hello Class')\n");

    Py_Finalize();
};

int main()
{
    Start();
};

Output is,

MyModule Start Called with parameter Hello Module
MyClass__Init Called
MyClass Start ERROR

The Module function is called without any issue, but the class method is called without the proper input variable.

bbastu · Accepted Answer · 2018-02-20 15:10:10Z

It appears that the parameter to self is always NULL and instead - for class methods - the reference to self is passed within the argument list. So, to parse the arguments of a class method you need to parse the reference to self, too.

Since Python 2.6 you can provide a list of format specifiers to PyArg_ParseTuple. The number of format specifiers has to fit the number of arguments that are passed to the function (see https://docs.python.org/2/c-api/arg.html under (items) (tuple) [matching-items]).

By modifying your MyClass__Start function to parse an additional parameter, you are able to parse and print both arguments to inspect them. For me, the following code

static PyObject* MyClass__Start(PyObject *self, PyObject *args)
{
    PyObject* argListSelf;
    const char* zBuff;

    if (PyArg_ParseTuple(args, "Os", &argListSelf, &zBuff)) {
        cout << "MyClass Start Called with parameters " <<  
        cout << PyString_AsString(PyObject_Str(argListSelf)) <<
        cout << " and " << zBuff << endl;

        cout << "self " << PyString_AsString(PyObject_Str(self)) << endl;
    }   
    else {
        if(PyErr_Occurred())
            PyErr_Print();

        cout << "MyClass Start ERROR" << endl;
    }   

    Py_INCREF(Py_None);
    return Py_None;
};

results in

MyModule Start Called with parameter Hello Module
MyClass__Init Called
MyClass Start Called with parameters \
    0x602428<?.MyClass instance at 0x7f484a333200>0x602428 and HelloClass
self <NULL>

Note that I printed the pointer value of self, which is NULL. I also added

if(PyErr_Occurred())
    PyErr_Print();

which I would always add for debugging purposes.

"It appears that the parameter to self is always NULL" is this a bug? Anyway issue resolved. Thanks bbastu.
I tried to figure out if it is a bug. All documentations and examples on the web that I found explicitly state, that self will be a pointer to the class instance. but none of them shows and example which uses the pointer. Is there anyone for whom self is not NULL?

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