How to convert a python 2 C extension module to python 3

I am trying to compile an extension module where I define a custom type. However, I got hold of some python 2 code for this purpose, but I cannot for the life of me convert it into a python 3 module.

So far, I have tweaked the original code I found in the book 'Python in a nutshell' in order to attempt to compile it for python 3, by merging some bits from https://docs.python.org/3/extending/newtypes_tutorial.html,but I am still getting nowhere. I know the 3rd edition of this book has a python 3 version of the code I am trying to compile but I don't have access to that edition of the book.

So, here is the module source code test.c:

#include "Python.h"
#include "structmember.h"

/* per-instance data structure */
typedef struct {
    PyObject_HEAD
    int first, second;
} intpair;

static int
intpair_init(PyObject *self, PyObject *args, PyObject *kwds)
{
 static char* nams[] = {"first","second",NULL};
 int first, second;
 if(!PyArg_ParseTupleAndKeywords(args, kwds, "ii", nams, &first, &second))
 return -1;
 ((intpair*)self)->first = first;
 ((intpair*)self)->second = second;
 return 0;
}

static void
intpair_dealloc(PyObject *self)
{
 self->ob_type->tp_free(self);
}


static PyObject* nothing_method(PyObject* self, PyObject* args)
{
    printf("This does literally nothing!\n");

    Py_RETURN_NONE;
}

static PyMemberDef intpair_members[] = {
 {"first", T_INT, offsetof(intpair, first), 0, "first item" },
 {"second", T_INT, offsetof(intpair, second), 0, "second item" },
 {NULL}
};

static PyTypeObject t_intpair = {
 PyObject_HEAD_INIT(0) /* tp_head */
 0, /* tp_internal */
 "test.intpair", /* tp_name */
 sizeof(intpair), /* tp_basicsize */
 0, /* tp_itemsize */
 intpair_dealloc, /* tp_dealloc */
 0, /* tp_print */
 0, /* tp_getattr */
 0, /* tp_setattr */
 0, /* tp_compare */
 0, /* tp_as_number */
 0, /* tp_as_sequence */
 0, /* tp_as_mapping */
 0, /* tp_hash */
 0, /* tp_call */
 0, /* tp_str */
 PyObject_GenericGetAttr, /* tp_getattro */
 PyObject_GenericSetAttr, /* tp_setattro */
 0, /* tp_as_buffer */
 Py_TPFLAGS_DEFAULT,
 "two ints (first,second)",
 0, /* tp_traverse */
 0, /* tp_clear */
 0, /* tp_richcompare */
 0, /* tp_weaklistoffset */
 0, /* tp_iter */
 0, /* tp_iternext */
 0, /* tp_methods */
 intpair_members, /* tp_members */
 0, /* tp_getset */
  0, /* tp_base */
 0, /* tp_dict */
 0, /* tp_descr_get */
 0, /* tp_descr_set */
 0, /* tp_dictoffset */
 intpair_init, /* tp_init */
 PyType_GenericAlloc, /* tp_alloc */
 PyType_GenericNew, /* tp_new */
 PyObject_Del, /* tp_free */
};


static PyMemberDef Noddy_members[] = {
    {"PI", T_INT, 3, 0, "noddy number"},
    {NULL, NULL, 0, NULL}
};

static PyMethodDef CosMethods[] =
{
     {"does_nothing", nothing_method, METH_VARARGS, "This really does nothing"},
     {NULL, NULL, 0, NULL}
};

static PyModuleDef testmodule = {
    PyModuleDef_HEAD_INIT,
    .m_name = "test",
    .m_doc = "Example module that creates an extension type.",
    .m_size = -1,
    CosMethods,
};

PyMODINIT_FUNC
PyInit_test(void)
{
    PyObject *m;
    if (PyType_Ready(&t_intpair) < 0)
        return NULL;

    m = PyModule_Create(&testmodule);
    if (m == NULL)
        return NULL;

    Py_INCREF(&t_intpair);
    PyModule_AddObject(m, "Custom", (PyObject *) &t_intpair);
    return m;
}

And the file setup.py:

from distutils.core import setup, Extension
setup(name="test", version="1.0",
      ext_modules=[
         Extension("test", ["test.c"]),
         ])

I can successfully compile this module using python3 setup.py build_ext --inplace, however, when I import this module in python 3 I get the following error:

Python 3.7.2 (default, Jan 10 2019, 23:51:51) 
[GCC 8.2.1 20181127] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import test
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: Type does not define the tp_name field.

I do not understand this error message, since there is a tp_name field "test.intpair" defined in PyTypeObject. What am I missing here?

Also, what is necessary to change in this code to make it compile for python 3?

Thanks!

Solution

I'm pretty sure the flaw is the line 0, /* tp_internal */ in:

 PyObject_HEAD_INIT(0) /* tp_head */
 0, /* tp_internal */
 "test.intpair", /* tp_name */

If you search for this line the only references to it on the internet are this question and the book you're using - it's completely non-standard and I have no idea where it's come from. I half suspect that the Python 2 code uses PyVarObject and then uses tp_internal to fill in the size rather than PyVarObject_HEAD_INIT, but I can't see the relevant page of the book...

The current documentation recommends using C99 initialization like

static PyTypeObject t_intpair = {
    PyObject_HEAD_INIT(0)
    .tp_name = "test.intpair",
    // etc
 };

so that getting the order of the attributes right no longer matters (anything you don't specify is 0 initialized).

Building your code provides some pretty clear warnings about mismatched types:

warning: initialization of ‘long int’ from ‘char *’ makes integer from pointer without a cast [-Wint-conversion]"test_mod.intpair", /* tp_name */`

You should pay attention to these