run.py

import tp_init_bug

i_was_called = False

class Foo(object):

    def __init__(self):
        global i_was_called
        i_was_called = True

x = object.__new__(Foo)
tp_init_bug.call_dunder_init(x)
assert i_was_called, "__init__ wasn't called :("

setup.py

from distutils.core import setup, Extension

setup(ext_modules=[Extension("tp_init_bug", ["tp_init_bug.c"])])

tp_init_bug.c

#include <Python.h>

static PyObject *call_dunder_init(PyObject *self, PyObject *args)
{
    PyObject *obj, *init_args, *init_kwargs;

    if (!PyArg_ParseTuple(args, "O", &obj))
        return NULL;

    init_args = PyTuple_New(0);
    init_kwargs = PyDict_New();

    if (Py_TYPE(obj)->tp_init(obj, init_args, init_kwargs) == -1)
        PyErr_Print();

    Py_DECREF (init_args);
    Py_DECREF (init_kwargs);

    Py_RETURN_NONE;
}

static PyMethodDef module_methods[] = {
    {"call_dunder_init", call_dunder_init, METH_VARARGS, NULL},
    {NULL, NULL, 0, NULL}
};

PyMODINIT_FUNC inittp_init_bug(void)
{
    PyObject *m = Py_InitModule3("tp_init_bug", module_methods, NULL);
    if (m == NULL)
        return;
}