GeezerVison

#define PY_SSIZE_T_CLEAN
#include "Python.h"

static PyTypeObject CustomBaseType1 = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Base1",
    .tp_basicsize = sizeof(PyObject),
};

static PyTypeObject CustomBaseType2 = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Base2",
    .tp_basicsize = sizeof(PyObject),
};

static PyTypeObject CustomDerivedType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Derived",
    .tp_basicsize = sizeof(PyObject),
};

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

PyMODINIT_FUNC
PyInit_custom(void)
{
    PyObject *m;

    if (PyType_Ready(&CustomBaseType1) < 0)
        return NULL;

    if (PyType_Ready(&CustomBaseType2) < 0)
        return NULL;

    CustomDerivedType.tp_base = &CustomBaseType2;
    CustomDerivedType.tp_bases =
        Py_BuildValue("(OO)", &CustomBaseType2, &CustomBaseType1);
    CustomDerivedType.tp_hash = CustomBaseType1.tp_hash;
    if (PyType_Ready(&CustomDerivedType) < 0)
        return NULL;

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

    Py_INCREF(&CustomDerivedType);
    if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomDerivedType) < 0) {
        Py_DECREF(&CustomDerivedType);
        Py_DECREF(m);
        return NULL;
    }

    return m;
}