Python C++ API - overloading functions with variable number of arguments

Question

I am trying to overload the following sayHi function, that is meant to receive a char* as input argument, or, alternatively, a char* and an integer.

It is part of class Box (which itself is used to define the python object PyBox):

class Box {
   public:

   Box();

   void sayHi(char *name);

   void sayHi(char *name, int number);
   };

In my wrapper, I have defined the following methods:

static PyObject *pyBox_sayHi_char(PyBox *self, char *Name)
{
    self->bx->sayHi(Name);

    Py_RETURN_NONE;
}

static PyObject *pyBox_sayHi_char_int(PyBox *self, char *Name, int number)
{
        self->bx->sayHi(Name, number);

    Py_RETURN_NONE;
}

static PyObject *Hi_overload_switch(PyBox *self,  PyObject *args)
{
    PyObject *x = NULL;
    PyObject *y = NULL;

    if (!PyArg_ParseTuple(args, "|OO", &x, &y))
        return NULL;

    if (PyUnicode_Check(x) && PyLong_Check(y) && y != NULL)
    {
        printf("A!\n\n");

        const char* s = PyBytes_AsString(PyUnicode_AsUTF8String(x)); 

        Py_DECREF(x);

        return pyBox_sayHi_char_int(self, s, PyLong_AsLong(y));

    }
    else if (PyUnicode_Check(x) && y == NULL)
    {   
        printf("B!\n\n");

        const char* s = PyBytes_AsString(PyUnicode_AsUTF8String(x)); 

        Py_DECREF(x);

        return pyBox_sayHi_char(self, s);
    }
    else
    {
        Py_RETURN_NOTIMPLEMENTED;
    }

    Py_RETURN_NOTIMPLEMENTED;
}

static PyMethodDef pyBox_methods[] = {
    {"Hi", (PyCFunction)Hi_overload_switch, METH_VARARGS, "Hi"},
    {NULL, NULL, 0, NULL}
};

However, when I go to python 3.7 and I run:

bo.Hi("John", 52364)

bo.Hi("Steve")

it prints the first statement but segfaults on Steve. Any suggestions why this is the case?

Thanks!

In your else if (PyLong_Check(x)) you're checking for long and then converting x to string... is it right? — João Paulo
– João Paulo, Commented May 21, 2019 at 16:57
well spotted! I have corrected in the question, however the result is the same! — PintoDoido
– PintoDoido, Commented May 21, 2019 at 17:12

gct · Accepted Answer · 2019-05-21 16:57:02Z

2

Problem is here:

if (!PyArg_ParseTuple(args, "|OO", &x, &y))
    return NULL;

if (PyUnicode_Check(x) && PyLong_Check(y)) ...

You're taking two optional parameters (PyArg_ParseTuple does not modify the passed variables if no parameter is given). So bo.Hi("Steve") only populates x, but then you use y without checking it, thus the segfault. You must have been getting lucky with previous versions on the initial value of y.

answered May 21, 2019 at 16:57

gct

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3 Comments

PintoDoido Over a year ago

Thanks! So how do I check for the number of arguments that were passed to Hi function?

gct Over a year ago

In this case I'd set them to nullptr explicitly and then check that they're not null after calling PyArg_ParseTuple

gct Over a year ago

Note that PyArg_ParseTuple does not increment the reference count for you in the event you need to retain the objects you have to do it yourself.

PintoDoido · Accepted Answer · 2019-05-21 17:26:19Z

This fixes the issue (see answer by @gct to understand why):

static PyObject *Hi_overload_switch(PyBox *self,  PyObject *args)
{
PyObject *x = Py_None;
PyObject *y = Py_None;

if (!PyArg_ParseTuple(args, "|OO", &x, &y))
    {return NULL;}

if (PyUnicode_Check(x) && PyLong_Check(y) && y != Py_None)
{
    printf("A!\n\n");

    char* s = PyBytes_AsString(PyUnicode_AsUTF8String(x));

    Py_DECREF(x);

    return pyBox_sayHi_char_int(self, s, PyLong_AsLong(y));

}
else if (PyUnicode_Check(x) && y == Py_None)
{   
    printf("B!\n\n");

    char* s = PyBytes_AsString(PyUnicode_AsUTF8String(x)); 

    Py_DECREF(x);

    return pyBox_sayHi_char(self, s);
}
else
{
    Py_RETURN_NOTIMPLEMENTED;
}
Py_RETURN_NOTIMPLEMENTED;
}

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