Objects/typeobject.c

/* Type object implementation */

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

#include <ctype.h>


/* Support type attribute cache */

/* The cache can keep references to the names alive for longer than
   they normally would.  This is why the maximum size is limited to
   MCACHE_MAX_ATTR_SIZE, since it might be a problem if very large
   strings are used as attribute names. */
#define MCACHE_MAX_ATTR_SIZE    100
#define MCACHE_SIZE_EXP         10
#define MCACHE_HASH(version, name_hash)                                 \
        (((unsigned int)(version) * (unsigned int)(name_hash))          \
         >> (8*sizeof(unsigned int) - MCACHE_SIZE_EXP))
#define MCACHE_HASH_METHOD(type, name)                                  \
        MCACHE_HASH((type)->tp_version_tag,                     \
                    ((PyASCIIObject *)(name))->hash)
#define MCACHE_CACHEABLE_NAME(name)                                     \
        PyUnicode_CheckExact(name) &&                            \
        PyUnicode_READY(name) != -1 &&                      \
        PyUnicode_GET_LENGTH(name) <= MCACHE_MAX_ATTR_SIZE

struct method_cache_entry {
    unsigned int version;
    PyObject *name;             /* reference to exactly a str or None */
    PyObject *value;            /* borrowed */
};

static struct method_cache_entry method_cache[1 << MCACHE_SIZE_EXP];
static unsigned int next_version_tag = 0;

_Py_IDENTIFIER(__class__);
_Py_IDENTIFIER(__dict__);
_Py_IDENTIFIER(__doc__);
_Py_IDENTIFIER(__getitem__);
_Py_IDENTIFIER(__getattribute__);
_Py_IDENTIFIER(__hash__);
_Py_IDENTIFIER(__module__);
_Py_IDENTIFIER(__name__);
_Py_IDENTIFIER(__new__);

static PyObject *
_PyType_LookupId(PyTypeObject *type, struct _Py_Identifier *name);

unsigned int
PyType_ClearCache(void)
{
    Py_ssize_t i;
    unsigned int cur_version_tag = next_version_tag - 1;

    for (i = 0; i < (1 << MCACHE_SIZE_EXP); i++) {
        method_cache[i].version = 0;
        Py_CLEAR(method_cache[i].name);
        method_cache[i].value = NULL;
    }
    next_version_tag = 0;
    /* mark all version tags as invalid */
    PyType_Modified(&PyBaseObject_Type);
    return cur_version_tag;
}

void
PyType_Modified(PyTypeObject *type)
{
    /* Invalidate any cached data for the specified type and all
       subclasses.  This function is called after the base
       classes, mro, or attributes of the type are altered.

       Invariants:

       - Py_TPFLAGS_VALID_VERSION_TAG is never set if
         Py_TPFLAGS_HAVE_VERSION_TAG is not set (e.g. on type
         objects coming from non-recompiled extension modules)

       - before Py_TPFLAGS_VALID_VERSION_TAG can be set on a type,
         it must first be set on all super types.

       This function clears the Py_TPFLAGS_VALID_VERSION_TAG of a
       type (so it must first clear it on all subclasses).  The
       tp_version_tag value is meaningless unless this flag is set.
       We don't assign new version tags eagerly, but only as
       needed.
     */
    PyObject *raw, *ref;
    Py_ssize_t i, n;

    if (!PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG))
        return;

    raw = type->tp_subclasses;
    if (raw != NULL) {
        n = PyList_GET_SIZE(raw);
        for (i = 0; i < n; i++) {
            ref = PyList_GET_ITEM(raw, i);
            ref = PyWeakref_GET_OBJECT(ref);
            if (ref != Py_None) {
                PyType_Modified((PyTypeObject *)ref);
            }
        }
    }
    type->tp_flags &= ~Py_TPFLAGS_VALID_VERSION_TAG;
}

static void
type_mro_modified(PyTypeObject *type, PyObject *bases) {
    /*
       Check that all base classes or elements of the MRO of type are
       able to be cached.  This function is called after the base
       classes or mro of the type are altered.

       Unset HAVE_VERSION_TAG and VALID_VERSION_TAG if the type
       has a custom MRO that includes a type which is not officially
       super type.

       Called from mro_internal, which will subsequently be called on
       each subclass when their mro is recursively updated.
     */
    Py_ssize_t i, n;
    int clear = 0;

    if (!PyType_HasFeature(type, Py_TPFLAGS_HAVE_VERSION_TAG))
        return;

    n = PyTuple_GET_SIZE(bases);
    for (i = 0; i < n; i++) {
        PyObject *b = PyTuple_GET_ITEM(bases, i);
        PyTypeObject *cls;

        assert(PyType_Check(b));
        cls = (PyTypeObject *)b;

        if (!PyType_HasFeature(cls, Py_TPFLAGS_HAVE_VERSION_TAG) ||
            !PyType_IsSubtype(type, cls)) {
            clear = 1;
            break;
        }
    }

    if (clear)
        type->tp_flags &= ~(Py_TPFLAGS_HAVE_VERSION_TAG|
                            Py_TPFLAGS_VALID_VERSION_TAG);
}

static int
assign_version_tag(PyTypeObject *type)
{
    /* Ensure that the tp_version_tag is valid and set
       Py_TPFLAGS_VALID_VERSION_TAG.  To respect the invariant, this
       must first be done on all super classes.  Return 0 if this
       cannot be done, 1 if Py_TPFLAGS_VALID_VERSION_TAG.
    */
    Py_ssize_t i, n;
    PyObject *bases;

    if (PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG))
        return 1;
    if (!PyType_HasFeature(type, Py_TPFLAGS_HAVE_VERSION_TAG))
        return 0;
    if (!PyType_HasFeature(type, Py_TPFLAGS_READY))
        return 0;

    type->tp_version_tag = next_version_tag++;
    /* for stress-testing: next_version_tag &= 0xFF; */

    if (type->tp_version_tag == 0) {
        /* wrap-around or just starting Python - clear the whole
           cache by filling names with references to Py_None.
           Values are also set to NULL for added protection, as they
           are borrowed reference */
        for (i = 0; i < (1 << MCACHE_SIZE_EXP); i++) {
            method_cache[i].value = NULL;
            Py_XDECREF(method_cache[i].name);
            method_cache[i].name = Py_None;
            Py_INCREF(Py_None);
        }
        /* mark all version tags as invalid */
        PyType_Modified(&PyBaseObject_Type);
        return 1;
    }
    bases = type->tp_bases;
    n = PyTuple_GET_SIZE(bases);
    for (i = 0; i < n; i++) {
        PyObject *b = PyTuple_GET_ITEM(bases, i);
        assert(PyType_Check(b));
        if (!assign_version_tag((PyTypeObject *)b))
            return 0;
    }
    type->tp_flags |= Py_TPFLAGS_VALID_VERSION_TAG;
    return 1;
}


static PyMemberDef type_members[] = {
    {"__basicsize__", T_PYSSIZET, offsetof(PyTypeObject,tp_basicsize),READONLY},
    {"__itemsize__", T_PYSSIZET, offsetof(PyTypeObject, tp_itemsize), READONLY},
    {"__flags__", T_LONG, offsetof(PyTypeObject, tp_flags), READONLY},
    {"__weakrefoffset__", T_LONG,
     offsetof(PyTypeObject, tp_weaklistoffset), READONLY},
    {"__base__", T_OBJECT, offsetof(PyTypeObject, tp_base), READONLY},
    {"__dictoffset__", T_LONG,
     offsetof(PyTypeObject, tp_dictoffset), READONLY},
    {"__mro__", T_OBJECT, offsetof(PyTypeObject, tp_mro), READONLY},
    {0}
};

static int
check_set_special_type_attr(PyTypeObject *type, PyObject *value, const char *name)
{
    if (!(type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
        PyErr_Format(PyExc_TypeError,
                     "can't set %s.%s", type->tp_name, name);
        return 0;
    }
    if (!value) {
        PyErr_Format(PyExc_TypeError,
                     "can't delete %s.%s", type->tp_name, name);
        return 0;
    }
    return 1;
}

static PyObject *
type_name(PyTypeObject *type, void *context)
{
    const char *s;

    if (type->tp_flags & Py_TPFLAGS_HEAPTYPE) {
        PyHeapTypeObject* et = (PyHeapTypeObject*)type;

        Py_INCREF(et->ht_name);
        return et->ht_name;
    }
    else {
        s = strrchr(type->tp_name, '.');
        if (s == NULL)
            s = type->tp_name;
        else
            s++;
        return PyUnicode_FromString(s);
    }
}

static PyObject *
type_qualname(PyTypeObject *type, void *context)
{
    if (type->tp_flags & Py_TPFLAGS_HEAPTYPE) {
        PyHeapTypeObject* et = (PyHeapTypeObject*)type;
        Py_INCREF(et->ht_qualname);
        return et->ht_qualname;
    }
    else {
        return type_name(type, context);
    }
}

static int
type_set_name(PyTypeObject *type, PyObject *value, void *context)
{
    PyHeapTypeObject* et;
    char *tp_name;
    PyObject *tmp;

    if (!check_set_special_type_attr(type, value, "__name__"))
        return -1;
    if (!PyUnicode_Check(value)) {
        PyErr_Format(PyExc_TypeError,
                     "can only assign string to %s.__name__, not '%s'",
                     type->tp_name, Py_TYPE(value)->tp_name);
        return -1;
    }

    /* Check absence of null characters */
    tmp = PyUnicode_FromStringAndSize("\0", 1);
    if (tmp == NULL)
        return -1;
    if (PyUnicode_Contains(value, tmp) != 0) {
        Py_DECREF(tmp);
        PyErr_Format(PyExc_ValueError,
                     "__name__ must not contain null bytes");
        return -1;
    }
    Py_DECREF(tmp);

    tp_name = _PyUnicode_AsString(value);
    if (tp_name == NULL)
        return -1;

    et = (PyHeapTypeObject*)type;

    Py_INCREF(value);

    Py_DECREF(et->ht_name);
    et->ht_name = value;

    type->tp_name = tp_name;

    return 0;
}

static int
type_set_qualname(PyTypeObject *type, PyObject *value, void *context)
{
    PyHeapTypeObject* et;

    if (!PyUnicode_Check(value)) {
        PyErr_Format(PyExc_TypeError,
                     "can only assign string to %s.__qualname__, not '%s'",
                     type->tp_name, Py_TYPE(value)->tp_name);
        return -1;
    }

    et = (PyHeapTypeObject*)type;
    Py_INCREF(value);
    Py_DECREF(et->ht_qualname);
    et->ht_qualname = value;
    return 0;
}

static PyObject *
type_module(PyTypeObject *type, void *context)
{
    PyObject *mod;
    char *s;

    if (type->tp_flags & Py_TPFLAGS_HEAPTYPE) {
        mod = _PyDict_GetItemId(type->tp_dict, &PyId___module__);
        if (!mod) {
            PyErr_Format(PyExc_AttributeError, "__module__");
            return 0;
        }
        Py_XINCREF(mod);
        return mod;
    }
    else {
        s = strrchr(type->tp_name, '.');
        if (s != NULL)
            return PyUnicode_FromStringAndSize(
                type->tp_name, (Py_ssize_t)(s - type->tp_name));
        return PyUnicode_FromString("builtins");
    }
}

static int
type_set_module(PyTypeObject *type, PyObject *value, void *context)
{
    if (!check_set_special_type_attr(type, value, "__module__"))
        return -1;

    PyType_Modified(type);

    return _PyDict_SetItemId(type->tp_dict, &PyId___module__, value);
}

static PyObject *
type_abstractmethods(PyTypeObject *type, void *context)
{
    PyObject *mod = NULL;
    /* type itself has an __abstractmethods__ descriptor (this). Don't return
       that. */
    if (type != &PyType_Type)
        mod = PyDict_GetItemString(type->tp_dict, "__abstractmethods__");
    if (!mod) {
        PyErr_SetString(PyExc_AttributeError, "__abstractmethods__");
        return NULL;
    }
    Py_XINCREF(mod);
    return mod;
}

static int
type_set_abstractmethods(PyTypeObject *type, PyObject *value, void *context)
{
    /* __abstractmethods__ should only be set once on a type, in
       abc.ABCMeta.__new__, so this function doesn't do anything
       special to update subclasses.
    */
    int res;
    if (value != NULL) {
        res = PyDict_SetItemString(type->tp_dict, "__abstractmethods__", value);
    }
    else {
        res = PyDict_DelItemString(type->tp_dict, "__abstractmethods__");
        if (res && PyErr_ExceptionMatches(PyExc_KeyError)) {
            PyErr_SetString(PyExc_AttributeError, "__abstractmethods__");
            return -1;
        }
    }
    if (res == 0) {
        PyType_Modified(type);
        if (value && PyObject_IsTrue(value)) {
            type->tp_flags |= Py_TPFLAGS_IS_ABSTRACT;
        }
        else {
            type->tp_flags &= ~Py_TPFLAGS_IS_ABSTRACT;
        }
    }
    return res;
}

static PyObject *
type_get_bases(PyTypeObject *type, void *context)
{
    Py_INCREF(type->tp_bases);
    return type->tp_bases;
}

static PyTypeObject *best_base(PyObject *);
static int mro_internal(PyTypeObject *);
static int compatible_for_assignment(PyTypeObject *, PyTypeObject *, char *);
static int add_subclass(PyTypeObject*, PyTypeObject*);
static void remove_subclass(PyTypeObject *, PyTypeObject *);
static void update_all_slots(PyTypeObject *);

typedef int (*update_callback)(PyTypeObject *, void *);
static int update_subclasses(PyTypeObject *type, PyObject *name,
                             update_callback callback, void *data);
static int recurse_down_subclasses(PyTypeObject *type, PyObject *name,
                                   update_callback callback, void *data);

static int
mro_subclasses(PyTypeObject *type, PyObject* temp)
{
    PyTypeObject *subclass;
    PyObject *ref, *subclasses, *old_mro;
    Py_ssize_t i, n;

    subclasses = type->tp_subclasses;
    if (subclasses == NULL)
        return 0;
    assert(PyList_Check(subclasses));
    n = PyList_GET_SIZE(subclasses);
    for (i = 0; i < n; i++) {
        ref = PyList_GET_ITEM(subclasses, i);
        assert(PyWeakref_CheckRef(ref));
        subclass = (PyTypeObject *)PyWeakref_GET_OBJECT(ref);
        assert(subclass != NULL);
        if ((PyObject *)subclass == Py_None)
            continue;
        assert(PyType_Check(subclass));
        old_mro = subclass->tp_mro;
        if (mro_internal(subclass) < 0) {
            subclass->tp_mro = old_mro;
            return -1;
        }
        else {
            PyObject* tuple;
            tuple = PyTuple_Pack(2, subclass, old_mro);
            Py_DECREF(old_mro);
            if (!tuple)
                return -1;
            if (PyList_Append(temp, tuple) < 0)
                return -1;
            Py_DECREF(tuple);
        }
        if (mro_subclasses(subclass, temp) < 0)
            return -1;
    }
    return 0;
}

static int
type_set_bases(PyTypeObject *type, PyObject *value, void *context)
{
    Py_ssize_t i;
    int r = 0;
    PyObject *ob, *temp;
    PyTypeObject *new_base, *old_base;
    PyObject *old_bases, *old_mro;

    if (!check_set_special_type_attr(type, value, "__bases__"))
        return -1;
    if (!PyTuple_Check(value)) {
        PyErr_Format(PyExc_TypeError,
             "can only assign tuple to %s.__bases__, not %s",
                 type->tp_name, Py_TYPE(value)->tp_name);
        return -1;
    }
    if (PyTuple_GET_SIZE(value) == 0) {
        PyErr_Format(PyExc_TypeError,
             "can only assign non-empty tuple to %s.__bases__, not ()",
                 type->tp_name);
        return -1;
    }
    for (i = 0; i < PyTuple_GET_SIZE(value); i++) {
        ob = PyTuple_GET_ITEM(value, i);
        if (!PyType_Check(ob)) {
            PyErr_Format(PyExc_TypeError,
                         "%s.__bases__ must be tuple of classes, not '%s'",
                         type->tp_name, Py_TYPE(ob)->tp_name);
            return -1;
        }
        if (PyType_IsSubtype((PyTypeObject*)ob, type)) {
            PyErr_SetString(PyExc_TypeError,
                            "a __bases__ item causes an inheritance cycle");
            return -1;
        }
    }

    new_base = best_base(value);

    if (!new_base)
        return -1;

    if (!compatible_for_assignment(type->tp_base, new_base, "__bases__"))
        return -1;

    Py_INCREF(new_base);
    Py_INCREF(value);

    old_bases = type->tp_bases;
    old_base = type->tp_base;
    old_mro = type->tp_mro;

    type->tp_bases = value;
    type->tp_base = new_base;

    if (mro_internal(type) < 0) {
        goto bail;
    }

    temp = PyList_New(0);
    if (!temp)
        goto bail;

    r = mro_subclasses(type, temp);

    if (r < 0) {
        for (i = 0; i < PyList_Size(temp); i++) {
            PyTypeObject* cls;
            PyObject* mro;
            PyArg_UnpackTuple(PyList_GET_ITEM(temp, i),
                             "", 2, 2, &cls, &mro);
            Py_INCREF(mro);
            ob = cls->tp_mro;
            cls->tp_mro = mro;
            Py_DECREF(ob);
        }
        Py_DECREF(temp);
        goto bail;
    }

    Py_DECREF(temp);

    /* any base that was in __bases__ but now isn't, we
       need to remove |type| from its tp_subclasses.
       conversely, any class now in __bases__ that wasn't
       needs to have |type| added to its subclasses. */

    /* for now, sod that: just remove from all old_bases,
       add to all new_bases */

    for (i = PyTuple_GET_SIZE(old_bases) - 1; i >= 0; i--) {
        ob = PyTuple_GET_ITEM(old_bases, i);
        if (PyType_Check(ob)) {
            remove_subclass(
                (PyTypeObject*)ob, type);
        }
    }

    for (i = PyTuple_GET_SIZE(value) - 1; i >= 0; i--) {
        ob = PyTuple_GET_ITEM(value, i);
        if (PyType_Check(ob)) {
            if (add_subclass((PyTypeObject*)ob, type) < 0)
                r = -1;
        }
    }

    update_all_slots(type);

    Py_DECREF(old_bases);
    Py_DECREF(old_base);
    Py_DECREF(old_mro);

    return r;

  bail:
    Py_DECREF(type->tp_bases);
    Py_DECREF(type->tp_base);
    if (type->tp_mro != old_mro) {
        Py_DECREF(type->tp_mro);
    }

    type->tp_bases = old_bases;
    type->tp_base = old_base;
    type->tp_mro = old_mro;

    return -1;
}

static PyObject *
type_dict(PyTypeObject *type, void *context)
{
    if (type->tp_dict == NULL) {
        Py_INCREF(Py_None);
        return Py_None;
    }
    return PyDictProxy_New(type->tp_dict);
}

static PyObject *
type_get_doc(PyTypeObject *type, void *context)
{
    PyObject *result;
    if (!(type->tp_flags & Py_TPFLAGS_HEAPTYPE) && type->tp_doc != NULL)
        return PyUnicode_FromString(type->tp_doc);
    result = _PyDict_GetItemId(type->tp_dict, &PyId___doc__);
    if (result == NULL) {
        result = Py_None;
        Py_INCREF(result);
    }
    else if (Py_TYPE(result)->tp_descr_get) {
        result = Py_TYPE(result)->tp_descr_get(result, NULL,
                                               (PyObject *)type);
    }
    else {
        Py_INCREF(result);
    }
    return result;
}

static int
type_set_doc(PyTypeObject *type, PyObject *value, void *context)
{
    if (!check_set_special_type_attr(type, value, "__doc__"))
        return -1;
    PyType_Modified(type);
    return _PyDict_SetItemId(type->tp_dict, &PyId___doc__, value);
}

static PyObject *
type___instancecheck__(PyObject *type, PyObject *inst)
{
    switch (_PyObject_RealIsInstance(inst, type)) {
    case -1:
        return NULL;
    case 0:
        Py_RETURN_FALSE;
    default:
        Py_RETURN_TRUE;
    }
}


static PyObject *
type___subclasscheck__(PyObject *type, PyObject *inst)
{
    switch (_PyObject_RealIsSubclass(inst, type)) {
    case -1:
        return NULL;
    case 0:
        Py_RETURN_FALSE;
    default:
        Py_RETURN_TRUE;
    }
}


static PyGetSetDef type_getsets[] = {
    {"__name__", (getter)type_name, (setter)type_set_name, NULL},
    {"__qualname__", (getter)type_qualname, (setter)type_set_qualname, NULL},
    {"__bases__", (getter)type_get_bases, (setter)type_set_bases, NULL},
    {"__module__", (getter)type_module, (setter)type_set_module, NULL},
    {"__abstractmethods__", (getter)type_abstractmethods,
     (setter)type_set_abstractmethods, NULL},
    {"__dict__",  (getter)type_dict,  NULL, NULL},
    {"__doc__", (getter)type_get_doc, (setter)type_set_doc, NULL},
    {0}
};

static PyObject *
type_repr(PyTypeObject *type)
{
    PyObject *mod, *name, *rtn;

    mod = type_module(type, NULL);
    if (mod == NULL)
        PyErr_Clear();
    else if (!PyUnicode_Check(mod)) {
        Py_DECREF(mod);
        mod = NULL;
    }
    name = type_qualname(type, NULL);
    if (name == NULL)
        return NULL;

    if (mod != NULL && PyUnicode_CompareWithASCIIString(mod, "builtins"))
        rtn = PyUnicode_FromFormat("<class '%U.%U'>", mod, name);
    else
        rtn = PyUnicode_FromFormat("<class '%s'>", type->tp_name);

    Py_XDECREF(mod);
    Py_DECREF(name);
    return rtn;
}

static PyObject *
type_call(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    PyObject *obj;

    if (type->tp_new == NULL) {
        PyErr_Format(PyExc_TypeError,
                     "cannot create '%.100s' instances",
                     type->tp_name);
        return NULL;
    }

    obj = type->tp_new(type, args, kwds);
    if (obj != NULL) {
        /* Ugly exception: when the call was type(something),
           don't call tp_init on the result. */
        if (type == &PyType_Type &&
            PyTuple_Check(args) && PyTuple_GET_SIZE(args) == 1 &&
            (kwds == NULL ||
             (PyDict_Check(kwds) && PyDict_Size(kwds) == 0)))
            return obj;
        /* If the returned object is not an instance of type,
           it won't be initialized. */
        if (!PyType_IsSubtype(Py_TYPE(obj), type))
            return obj;
        type = Py_TYPE(obj);
        if (type->tp_init != NULL &&
            type->tp_init(obj, args, kwds) < 0) {
            Py_DECREF(obj);
            obj = NULL;
        }
    }
    return obj;
}

PyObject *
PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)
{
    PyObject *obj;
    const size_t size = _PyObject_VAR_SIZE(type, nitems+1);
    /* note that we need to add one, for the sentinel */

    if (PyType_IS_GC(type))
        obj = _PyObject_GC_Malloc(size);
    else
        obj = (PyObject *)PyObject_MALLOC(size);

    if (obj == NULL)
        return PyErr_NoMemory();

    memset(obj, '\0', size);

    if (type->tp_flags & Py_TPFLAGS_HEAPTYPE)
        Py_INCREF(type);

    if (type->tp_itemsize == 0)
        PyObject_INIT(obj, type);
    else
        (void) PyObject_INIT_VAR((PyVarObject *)obj, type, nitems);

    if (PyType_IS_GC(type))
        _PyObject_GC_TRACK(obj);
    return obj;
}

PyObject *
PyType_GenericNew(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    return type->tp_alloc(type, 0);
}

/* Helpers for subtyping */

static int
traverse_slots(PyTypeObject *type, PyObject *self, visitproc visit, void *arg)
{
    Py_ssize_t i, n;
    PyMemberDef *mp;

    n = Py_SIZE(type);
    mp = PyHeapType_GET_MEMBERS((PyHeapTypeObject *)type);
    for (i = 0; i < n; i++, mp++) {
        if (mp->type == T_OBJECT_EX) {
            char *addr = (char *)self + mp->offset;
            PyObject *obj = *(PyObject **)addr;
            if (obj != NULL) {
                int err = visit(obj, arg);
                if (err)
                    return err;
            }
        }
    }
    return 0;
}

static int
subtype_traverse(PyObject *self, visitproc visit, void *arg)
{
    PyTypeObject *type, *base;
    traverseproc basetraverse;

    /* Find the nearest base with a different tp_traverse,
       and traverse slots while we're at it */
    type = Py_TYPE(self);
    base = type;
    while ((basetraverse = base->tp_traverse) == subtype_traverse) {
        if (Py_SIZE(base)) {
            int err = traverse_slots(base, self, visit, arg);
            if (err)
                return err;
        }
        base = base->tp_base;
        assert(base);
    }

    if (type->tp_dictoffset != base->tp_dictoffset) {
        PyObject **dictptr = _PyObject_GetDictPtr(self);
        if (dictptr && *dictptr)
            Py_VISIT(*dictptr);
    }

    if (type->tp_flags & Py_TPFLAGS_HEAPTYPE)
        /* For a heaptype, the instances count as references
           to the type.          Traverse the type so the collector
           can find cycles involving this link. */
        Py_VISIT(type);

    if (basetraverse)
        return basetraverse(self, visit, arg);
    return 0;
}

static void
clear_slots(PyTypeObject *type, PyObject *self)
{
    Py_ssize_t i, n;
    PyMemberDef *mp;

    n = Py_SIZE(type);
    mp = PyHeapType_GET_MEMBERS((PyHeapTypeObject *)type);
    for (i = 0; i < n; i++, mp++) {
        if (mp->type == T_OBJECT_EX && !(mp->flags & READONLY)) {
            char *addr = (char *)self + mp->offset;
            PyObject *obj = *(PyObject **)addr;
            if (obj != NULL) {
                *(PyObject **)addr = NULL;
                Py_DECREF(obj);
            }
        }
    }
}

static int
subtype_clear(PyObject *self)
{
    PyTypeObject *type, *base;
    inquiry baseclear;

    /* Find the nearest base with a different tp_clear
       and clear slots while we're at it */
    type = Py_TYPE(self);
    base = type;
    while ((baseclear = base->tp_clear) == subtype_clear) {
        if (Py_SIZE(base))
            clear_slots(base, self);
        base = base->tp_base;
        assert(base);
    }

    /* Clear the instance dict (if any), to break cycles involving only
       __dict__ slots (as in the case 'self.__dict__ is self'). */
    if (type->tp_dictoffset != base->tp_dictoffset) {
        PyObject **dictptr = _PyObject_GetDictPtr(self);
        if (dictptr && *dictptr)
            Py_CLEAR(*dictptr);
    }

    if (baseclear)
        return baseclear(self);
    return 0;
}

static void
subtype_dealloc(PyObject *self)
{
    PyTypeObject *type, *base;
    destructor basedealloc;

    /* Extract the type; we expect it to be a heap type */
    type = Py_TYPE(self);
    assert(type->tp_flags & Py_TPFLAGS_HEAPTYPE);

    /* Test whether the type has GC exactly once */

    if (!PyType_IS_GC(type)) {
        /* It's really rare to find a dynamic type that doesn't have
           GC; it can only happen when deriving from 'object' and not
           adding any slots or instance variables.  This allows
           certain simplifications: there's no need to call
           clear_slots(), or DECREF the dict, or clear weakrefs. */

        /* Maybe call finalizer; exit early if resurrected */
        if (type->tp_del) {
            type->tp_del(self);
            if (self->ob_refcnt > 0)
                return;
        }

        /* Find the nearest base with a different tp_dealloc */
        base = type;
        while ((basedealloc = base->tp_dealloc) == subtype_dealloc) {
            assert(Py_SIZE(base) == 0);
            base = base->tp_base;
            assert(base);
        }

        /* Extract the type again; tp_del may have changed it */
        type = Py_TYPE(self);

        /* Call the base tp_dealloc() */
        assert(basedealloc);
        basedealloc(self);

        /* Can't reference self beyond this point */
        Py_DECREF(type);

        /* Done */
        return;
    }

    /* We get here only if the type has GC */

    /* UnTrack and re-Track around the trashcan macro, alas */
    /* See explanation at end of function for full disclosure */
    PyObject_GC_UnTrack(self);
    ++_PyTrash_delete_nesting;
    Py_TRASHCAN_SAFE_BEGIN(self);
    --_PyTrash_delete_nesting;
    /* DO NOT restore GC tracking at this point.  weakref callbacks
     * (if any, and whether directly here or indirectly in something we
     * call) may trigger GC, and if self is tracked at that point, it
     * will look like trash to GC and GC will try to delete self again.
     */

    /* Find the nearest base with a different tp_dealloc */
    base = type;
    while ((/*basedealloc =*/ base->tp_dealloc) == subtype_dealloc) {
        base = base->tp_base;
        assert(base);
    }

    /* If we added a weaklist, we clear it.      Do this *before* calling
       the finalizer (__del__), clearing slots, or clearing the instance
       dict. */

    if (type->tp_weaklistoffset && !base->tp_weaklistoffset)
        PyObject_ClearWeakRefs(self);

    /* Maybe call finalizer; exit early if resurrected */
    if (type->tp_del) {
        _PyObject_GC_TRACK(self);
        type->tp_del(self);
        if (self->ob_refcnt > 0)
            goto endlabel;              /* resurrected */
        else
            _PyObject_GC_UNTRACK(self);
        /* New weakrefs could be created during the finalizer call.
            If this occurs, clear them out without calling their
            finalizers since they might rely on part of the object
            being finalized that has already been destroyed. */
        if (type->tp_weaklistoffset && !base->tp_weaklistoffset) {
            /* Modeled after GET_WEAKREFS_LISTPTR() */
            PyWeakReference **list = (PyWeakReference **) \
                PyObject_GET_WEAKREFS_LISTPTR(self);
            while (*list)
                _PyWeakref_ClearRef(*list);
        }
    }

    /*  Clear slots up to the nearest base with a different tp_dealloc */
    base = type;
    while ((basedealloc = base->tp_dealloc) == subtype_dealloc) {
        if (Py_SIZE(base))
            clear_slots(base, self);
        base = base->tp_base;
        assert(base);
    }

    /* If we added a dict, DECREF it */
    if (type->tp_dictoffset && !base->tp_dictoffset) {
        PyObject **dictptr = _PyObject_GetDictPtr(self);
        if (dictptr != NULL) {
            PyObject *dict = *dictptr;
            if (dict != NULL) {
                Py_DECREF(dict);
                *dictptr = NULL;
            }
        }
    }

    /* Extract the type again; tp_del may have changed it */
    type = Py_TYPE(self);

    /* Call the base tp_dealloc(); first retrack self if
     * basedealloc knows about gc.
     */
    if (PyType_IS_GC(base))
        _PyObject_GC_TRACK(self);
    assert(basedealloc);
    basedealloc(self);

    /* Can't reference self beyond this point */
    Py_DECREF(type);

  endlabel:
    ++_PyTrash_delete_nesting;
    Py_TRASHCAN_SAFE_END(self);
    --_PyTrash_delete_nesting;

    /* Explanation of the weirdness around the trashcan macros:

       Q. What do the trashcan macros do?

       A. Read the comment titled "Trashcan mechanism" in object.h.
          For one, this explains why there must be a call to GC-untrack
          before the trashcan begin macro.      Without understanding the
          trashcan code, the answers to the following questions don't make
          sense.

       Q. Why do we GC-untrack before the trashcan and then immediately
          GC-track again afterward?

       A. In the case that the base class is GC-aware, the base class
          probably GC-untracks the object.      If it does that using the
          UNTRACK macro, this will crash when the object is already
          untracked.  Because we don't know what the base class does, the
          only safe thing is to make sure the object is tracked when we
          call the base class dealloc.  But...  The trashcan begin macro
          requires that the object is *untracked* before it is called.  So
          the dance becomes:

         GC untrack
         trashcan begin
         GC track

       Q. Why did the last question say "immediately GC-track again"?
          It's nowhere near immediately.

       A. Because the code *used* to re-track immediately.      Bad Idea.
          self has a refcount of 0, and if gc ever gets its hands on it
          (which can happen if any weakref callback gets invoked), it
          looks like trash to gc too, and gc also tries to delete self
          then.  But we're already deleting self.  Double deallocation is
          a subtle disaster.

       Q. Why the bizarre (net-zero) manipulation of
          _PyTrash_delete_nesting around the trashcan macros?

       A. Some base classes (e.g. list) also use the trashcan mechanism.
          The following scenario used to be possible:

          - suppose the trashcan level is one below the trashcan limit

          - subtype_dealloc() is called

          - the trashcan limit is not yet reached, so the trashcan level
        is incremented and the code between trashcan begin and end is
        executed

          - this destroys much of the object's contents, including its
        slots and __dict__

          - basedealloc() is called; this is really list_dealloc(), or
        some other type which also uses the trashcan macros

          - the trashcan limit is now reached, so the object is put on the
        trashcan's to-be-deleted-later list

          - basedealloc() returns

          - subtype_dealloc() decrefs the object's type

          - subtype_dealloc() returns

          - later, the trashcan code starts deleting the objects from its
        to-be-deleted-later list

          - subtype_dealloc() is called *AGAIN* for the same object

          - at the very least (if the destroyed slots and __dict__ don't
        cause problems) the object's type gets decref'ed a second
        time, which is *BAD*!!!

          The remedy is to make sure that if the code between trashcan
          begin and end in subtype_dealloc() is called, the code between
          trashcan begin and end in basedealloc() will also be called.
          This is done by decrementing the level after passing into the
          trashcan block, and incrementing it just before leaving the
          block.

          But now it's possible that a chain of objects consisting solely
          of objects whose deallocator is subtype_dealloc() will defeat
          the trashcan mechanism completely: the decremented level means
          that the effective level never reaches the limit.      Therefore, we
          *increment* the level *before* entering the trashcan block, and
          matchingly decrement it after leaving.  This means the trashcan
          code will trigger a little early, but that's no big deal.

       Q. Are there any live examples of code in need of all this
          complexity?

       A. Yes.  See SF bug 668433 for code that crashed (when Python was
          compiled in debug mode) before the trashcan level manipulations
          were added.  For more discussion, see SF patches 581742, 575073
          and bug 574207.
    */
}

static PyTypeObject *solid_base(PyTypeObject *type);

/* type test with subclassing support */

int
PyType_IsSubtype(PyTypeObject *a, PyTypeObject *b)
{
    PyObject *mro;

    mro = a->tp_mro;
    if (mro != NULL) {
        /* Deal with multiple inheritance without recursion
           by walking the MRO tuple */
        Py_ssize_t i, n;
        assert(PyTuple_Check(mro));
        n = PyTuple_GET_SIZE(mro);
        for (i = 0; i < n; i++) {
            if (PyTuple_GET_ITEM(mro, i) == (PyObject *)b)
                return 1;
        }
        return 0;
    }
    else {
        /* a is not completely initilized yet; follow tp_base */
        do {
            if (a == b)
                return 1;
            a = a->tp_base;
        } while (a != NULL);
        return b == &PyBaseObject_Type;
    }
}

/* Internal routines to do a method lookup in the type
   without looking in the instance dictionary
   (so we can't use PyObject_GetAttr) but still binding
   it to the instance.  The arguments are the object,
   the method name as a C string, and the address of a
   static variable used to cache the interned Python string.

   Two variants:

   - lookup_maybe() returns NULL without raising an exception
     when the _PyType_Lookup() call fails;

   - lookup_method() always raises an exception upon errors.

   - _PyObject_LookupSpecial() exported for the benefit of other places.
*/

static PyObject *
lookup_maybe(PyObject *self, _Py_Identifier *attrid)
{
    PyObject *res;

    res = _PyType_LookupId(Py_TYPE(self), attrid);
    if (res != NULL) {
        descrgetfunc f;
        if ((f = Py_TYPE(res)->tp_descr_get) == NULL)
            Py_INCREF(res);
        else
            res = f(res, self, (PyObject *)(Py_TYPE(self)));
    }
    return res;
}

static PyObject *
lookup_method(PyObject *self, _Py_Identifier *attrid)
{
    PyObject *res = lookup_maybe(self, attrid);
    if (res == NULL && !PyErr_Occurred())
        PyErr_SetObject(PyExc_AttributeError, attrid->object);
    return res;
}

PyObject *
_PyObject_LookupSpecial(PyObject *self, _Py_Identifier *attrid)
{
    return lookup_maybe(self, attrid);
}

/* A variation of PyObject_CallMethod that uses lookup_method()
   instead of PyObject_GetAttrString().  This uses the same convention
   as lookup_method to cache the interned name string object. */

static PyObject *
call_method(PyObject *o, _Py_Identifier *nameid, char *format, ...)
{
    va_list va;
    PyObject *args, *func = 0, *retval;
    va_start(va, format);

    func = lookup_maybe(o, nameid);
    if (func == NULL) {
        va_end(va);
        if (!PyErr_Occurred())
            PyErr_SetObject(PyExc_AttributeError, nameid->object);
        return NULL;
    }

    if (format && *format)
        args = Py_VaBuildValue(format, va);
    else
        args = PyTuple_New(0);

    va_end(va);

    if (args == NULL)
        return NULL;

    assert(PyTuple_Check(args));
    retval = PyObject_Call(func, args, NULL);

    Py_DECREF(args);
    Py_DECREF(func);

    return retval;
}

/* Clone of call_method() that returns NotImplemented when the lookup fails. */

static PyObject *
call_maybe(PyObject *o, _Py_Identifier *nameid, char *format, ...)
{
    va_list va;
    PyObject *args, *func = 0, *retval;
    va_start(va, format);

    func = lookup_maybe(o, nameid);
    if (func == NULL) {
        va_end(va);
        if (!PyErr_Occurred())
            Py_RETURN_NOTIMPLEMENTED;
        return NULL;
    }

    if (format && *format)
        args = Py_VaBuildValue(format, va);
    else
        args = PyTuple_New(0);

    va_end(va);

    if (args == NULL)
        return NULL;

    assert(PyTuple_Check(args));
    retval = PyObject_Call(func, args, NULL);

    Py_DECREF(args);
    Py_DECREF(func);

    return retval;
}

/*
    Method resolution order algorithm C3 described in
    "A Monotonic Superclass Linearization for Dylan",
    by Kim Barrett, Bob Cassel, Paul Haahr,
    David A. Moon, Keith Playford, and P. Tucker Withington.
    (OOPSLA 1996)

    Some notes about the rules implied by C3:

    No duplicate bases.
    It isn't legal to repeat a class in a list of base classes.

    The next three properties are the 3 constraints in "C3".

    Local precendece order.
    If A precedes B in C's MRO, then A will precede B in the MRO of all
    subclasses of C.

    Monotonicity.
    The MRO of a class must be an extension without reordering of the
    MRO of each of its superclasses.

    Extended Precedence Graph (EPG).
    Linearization is consistent if there is a path in the EPG from
    each class to all its successors in the linearization.  See
    the paper for definition of EPG.
 */

static int
tail_contains(PyObject *list, int whence, PyObject *o) {
    Py_ssize_t j, size;
    size = PyList_GET_SIZE(list);

    for (j = whence+1; j < size; j++) {
        if (PyList_GET_ITEM(list, j) == o)
            return 1;
    }
    return 0;
}

static PyObject *
class_name(PyObject *cls)
{
    PyObject *name = _PyObject_GetAttrId(cls, &PyId___name__);
    if (name == NULL) {
        PyErr_Clear();
        Py_XDECREF(name);
        name = PyObject_Repr(cls);
    }
    if (name == NULL)
        return NULL;
    if (!PyUnicode_Check(name)) {
        Py_DECREF(name);
        return NULL;
    }
    return name;
}

static int
check_duplicates(PyObject *list)
{
    Py_ssize_t i, j, n;
    /* Let's use a quadratic time algorithm,
       assuming that the bases lists is short.
    */
    n = PyList_GET_SIZE(list);
    for (i = 0; i < n; i++) {
        PyObject *o = PyList_GET_ITEM(list, i);
        for (j = i + 1; j < n; j++) {
            if (PyList_GET_ITEM(list, j) == o) {
                o = class_name(o);
                if (o != NULL) {
                    PyErr_Format(PyExc_TypeError,
                                 "duplicate base class %U",
                                 o);
                    Py_DECREF(o);
                } else {
                    PyErr_SetString(PyExc_TypeError,
                                 "duplicate base class");
                }
                return -1;
            }
        }
    }
    return 0;
}

/* Raise a TypeError for an MRO order disagreement.

   It's hard to produce a good error message.  In the absence of better
   insight into error reporting, report the classes that were candidates
   to be put next into the MRO.  There is some conflict between the
   order in which they should be put in the MRO, but it's hard to
   diagnose what constraint can't be satisfied.
*/

static void
set_mro_error(PyObject *to_merge, int *remain)
{
    Py_ssize_t i, n, off, to_merge_size;
    char buf[1000];
    PyObject *k, *v;
    PyObject *set = PyDict_New();
    if (!set) return;

    to_merge_size = PyList_GET_SIZE(to_merge);
    for (i = 0; i < to_merge_size; i++) {
        PyObject *L = PyList_GET_ITEM(to_merge, i);
        if (remain[i] < PyList_GET_SIZE(L)) {
            PyObject *c = PyList_GET_ITEM(L, remain[i]);
            if (PyDict_SetItem(set, c, Py_None) < 0) {
                Py_DECREF(set);
                return;
            }
        }
    }
    n = PyDict_Size(set);

    off = PyOS_snprintf(buf, sizeof(buf), "Cannot create a \
consistent method resolution\norder (MRO) for bases");
    i = 0;
    while (PyDict_Next(set, &i, &k, &v) && (size_t)off < sizeof(buf)) {
        PyObject *name = class_name(k);
        char *name_str;
        if (name != NULL) {
            name_str = _PyUnicode_AsString(name);
            if (name_str == NULL)
                name_str = "?";
        } else
            name_str = "?";
        off += PyOS_snprintf(buf + off, sizeof(buf) - off, " %s", name_str);
        Py_XDECREF(name);
        if (--n && (size_t)(off+1) < sizeof(buf)) {
            buf[off++] = ',';
            buf[off] = '\0';
        }
    }
    PyErr_SetString(PyExc_TypeError, buf);
    Py_DECREF(set);
}

static int
pmerge(PyObject *acc, PyObject* to_merge) {
    Py_ssize_t i, j, to_merge_size, empty_cnt;
    int *remain;
    int ok;

    to_merge_size = PyList_GET_SIZE(to_merge);

    /* remain stores an index into each sublist of to_merge.
       remain[i] is the index of the next base in to_merge[i]
       that is not included in acc.
    */
    remain = (int *)PyMem_MALLOC(SIZEOF_INT*to_merge_size);
    if (remain == NULL)
        return -1;
    for (i = 0; i < to_merge_size; i++)
        remain[i] = 0;

  again:
    empty_cnt = 0;
    for (i = 0; i < to_merge_size; i++) {
        PyObject *candidate;

        PyObject *cur_list = PyList_GET_ITEM(to_merge, i);

        if (remain[i] >= PyList_GET_SIZE(cur_list)) {
            empty_cnt++;
            continue;
        }

        /* Choose next candidate for MRO.

           The input sequences alone can determine the choice.
           If not, choose the class which appears in the MRO
           of the earliest direct superclass of the new class.
        */

        candidate = PyList_GET_ITEM(cur_list, remain[i]);
        for (j = 0; j < to_merge_size; j++) {
            PyObject *j_lst = PyList_GET_ITEM(to_merge, j);
            if (tail_contains(j_lst, remain[j], candidate)) {
                goto skip; /* continue outer loop */
            }
        }
        ok = PyList_Append(acc, candidate);
        if (ok < 0) {
            PyMem_Free(remain);
            return -1;
        }
        for (j = 0; j < to_merge_size; j++) {
            PyObject *j_lst = PyList_GET_ITEM(to_merge, j);
            if (remain[j] < PyList_GET_SIZE(j_lst) &&
                PyList_GET_ITEM(j_lst, remain[j]) == candidate) {
                remain[j]++;
            }
        }
        goto again;
      skip: ;
    }

    if (empty_cnt == to_merge_size) {
        PyMem_FREE(remain);
        return 0;
    }
    set_mro_error(to_merge, remain);
    PyMem_FREE(remain);
    return -1;
}

static PyObject *
mro_implementation(PyTypeObject *type)
{
    Py_ssize_t i, n;
    int ok;
    PyObject *bases, *result;
    PyObject *to_merge, *bases_aslist;

    if (type->tp_dict == NULL) {
        if (PyType_Ready(type) < 0)
            return NULL;
    }

    /* Find a superclass linearization that honors the constraints
       of the explicit lists of bases and the constraints implied by
       each base class.

       to_merge is a list of lists, where each list is a superclass
       linearization implied by a base class.  The last element of
       to_merge is the declared list of bases.
    */

    bases = type->tp_bases;
    n = PyTuple_GET_SIZE(bases);

    to_merge = PyList_New(n+1);
    if (to_merge == NULL)
        return NULL;

    for (i = 0; i < n; i++) {
        PyObject *base = PyTuple_GET_ITEM(bases, i);
        PyObject *parentMRO;
        parentMRO = PySequence_List(((PyTypeObject*)base)->tp_mro);
        if (parentMRO == NULL) {
            Py_DECREF(to_merge);
            return NULL;
        }

        PyList_SET_ITEM(to_merge, i, parentMRO);
    }

    bases_aslist = PySequence_List(bases);
    if (bases_aslist == NULL) {
        Py_DECREF(to_merge);
        return NULL;
    }
    /* This is just a basic sanity check. */
    if (check_duplicates(bases_aslist) < 0) {
        Py_DECREF(to_merge);
        Py_DECREF(bases_aslist);
        return NULL;
    }
    PyList_SET_ITEM(to_merge, n, bases_aslist);

    result = Py_BuildValue("[O]", (PyObject *)type);
    if (result == NULL) {
        Py_DECREF(to_merge);
        return NULL;
    }

    ok = pmerge(result, to_merge);
    Py_DECREF(to_merge);
    if (ok < 0) {
        Py_DECREF(result);
        return NULL;
    }

    return result;
}

static PyObject *
mro_external(PyObject *self)
{
    PyTypeObject *type = (PyTypeObject *)self;

    return mro_implementation(type);
}

static int
mro_internal(PyTypeObject *type)
{
    PyObject *mro, *result, *tuple;
    int checkit = 0;

    if (Py_TYPE(type) == &PyType_Type) {
        result = mro_implementation(type);
    }
    else {
        _Py_IDENTIFIER(mro);
        checkit = 1;
        mro = lookup_method((PyObject *)type, &PyId_mro);
        if (mro == NULL)
            return -1;
        result = PyObject_CallObject(mro, NULL);
        Py_DECREF(mro);
    }
    if (result == NULL)
        return -1;
    tuple = PySequence_Tuple(result);
    Py_DECREF(result);
    if (tuple == NULL)
        return -1;
    if (checkit) {
        Py_ssize_t i, len;
        PyObject *cls;
        PyTypeObject *solid;

        solid = solid_base(type);

        len = PyTuple_GET_SIZE(tuple);

        for (i = 0; i < len; i++) {
            PyTypeObject *t;
            cls = PyTuple_GET_ITEM(tuple, i);
            if (!PyType_Check(cls)) {
                PyErr_Format(PyExc_TypeError,
                 "mro() returned a non-class ('%.500s')",
                                 Py_TYPE(cls)->tp_name);
                Py_DECREF(tuple);
                return -1;
            }
            t = (PyTypeObject*)cls;
            if (!PyType_IsSubtype(solid, solid_base(t))) {
                PyErr_Format(PyExc_TypeError,
             "mro() returned base with unsuitable layout ('%.500s')",
                                     t->tp_name);
                        Py_DECREF(tuple);
                        return -1;
            }
        }
    }
    type->tp_mro = tuple;

    type_mro_modified(type, type->tp_mro);
    /* corner case: the super class might have been hidden
       from the custom MRO */
    type_mro_modified(type, type->tp_bases);

    PyType_Modified(type);

    return 0;
}


/* Calculate the best base amongst multiple base classes.
   This is the first one that's on the path to the "solid base". */

static PyTypeObject *
best_base(PyObject *bases)
{
    Py_ssize_t i, n;
    PyTypeObject *base, *winner, *candidate, *base_i;
    PyObject *base_proto;

    assert(PyTuple_Check(bases));
    n = PyTuple_GET_SIZE(bases);
    assert(n > 0);
    base = NULL;
    winner = NULL;
    for (i = 0; i < n; i++) {
        base_proto = PyTuple_GET_ITEM(bases, i);
        if (!PyType_Check(base_proto)) {
            PyErr_SetString(
                PyExc_TypeError,
                "bases must be types");
            return NULL;
        }
        base_i = (PyTypeObject *)base_proto;
        if (base_i->tp_dict == NULL) {
            if (PyType_Ready(base_i) < 0)
                return NULL;
        }
        candidate = solid_base(base_i);
        if (winner == NULL) {
            winner = candidate;
            base = base_i;
        }
        else if (PyType_IsSubtype(winner, candidate))
            ;
        else if (PyType_IsSubtype(candidate, winner)) {
            winner = candidate;
            base = base_i;
        }
        else {
            PyErr_SetString(
                PyExc_TypeError,
                "multiple bases have "
                "instance lay-out conflict");
            return NULL;
        }
    }
    assert (base != NULL);

    return base;
}

static int
extra_ivars(PyTypeObject *type, PyTypeObject *base)
{
    size_t t_size = type->tp_basicsize;
    size_t b_size = base->tp_basicsize;

    assert(t_size >= b_size); /* Else type smaller than base! */
    if (type->tp_itemsize || base->tp_itemsize) {
        /* If itemsize is involved, stricter rules */
        return t_size != b_size ||
            type->tp_itemsize != base->tp_itemsize;
    }
    if (type->tp_weaklistoffset && base->tp_weaklistoffset == 0 &&
        type->tp_weaklistoffset + sizeof(PyObject *) == t_size &&
        type->tp_flags & Py_TPFLAGS_HEAPTYPE)
        t_size -= sizeof(PyObject *);
    if (type->tp_dictoffset && base->tp_dictoffset == 0 &&
        type->tp_dictoffset + sizeof(PyObject *) == t_size &&
        type->tp_flags & Py_TPFLAGS_HEAPTYPE)
        t_size -= sizeof(PyObject *);

    return t_size != b_size;
}

static PyTypeObject *
solid_base(PyTypeObject *type)
{
    PyTypeObject *base;

    if (type->tp_base)
        base = solid_base(type->tp_base);
    else
        base = &PyBaseObject_Type;
    if (extra_ivars(type, base))
        return type;
    else
        return base;
}

static void object_dealloc(PyObject *);
static int object_init(PyObject *, PyObject *, PyObject *);
static int update_slot(PyTypeObject *, PyObject *);
static void fixup_slot_dispatchers(PyTypeObject *);

/*
 * Helpers for  __dict__ descriptor.  We don't want to expose the dicts
 * inherited from various builtin types.  The builtin base usually provides
 * its own __dict__ descriptor, so we use that when we can.
 */
static PyTypeObject *
get_builtin_base_with_dict(PyTypeObject *type)
{
    while (type->tp_base != NULL) {
        if (type->tp_dictoffset != 0 &&
            !(type->tp_flags & Py_TPFLAGS_HEAPTYPE))
            return type;
        type = type->tp_base;
    }
    return NULL;
}

static PyObject *
get_dict_descriptor(PyTypeObject *type)
{
    PyObject *descr;

    descr = _PyType_LookupId(type, &PyId___dict__);
    if (descr == NULL || !PyDescr_IsData(descr))
        return NULL;

    return descr;
}

static void
raise_dict_descr_error(PyObject *obj)
{
    PyErr_Format(PyExc_TypeError,
                 "this __dict__ descriptor does not support "
                 "'%.200s' objects", Py_TYPE(obj)->tp_name);
}

static PyObject *
subtype_dict(PyObject *obj, void *context)
{
    PyTypeObject *base;

    base = get_builtin_base_with_dict(Py_TYPE(obj));
    if (base != NULL) {
        descrgetfunc func;
        PyObject *descr = get_dict_descriptor(base);
        if (descr == NULL) {
            raise_dict_descr_error(obj);
            return NULL;
        }
        func = Py_TYPE(descr)->tp_descr_get;
        if (func == NULL) {
            raise_dict_descr_error(obj);
            return NULL;
        }
        return func(descr, obj, (PyObject *)(Py_TYPE(obj)));
    }
    return PyObject_GenericGetDict(obj, context);
}

static int
subtype_setdict(PyObject *obj, PyObject *value, void *context)
{
    PyObject *dict, **dictptr;
    PyTypeObject *base;

    base = get_builtin_base_with_dict(Py_TYPE(obj));
    if (base != NULL) {
        descrsetfunc func;
        PyObject *descr = get_dict_descriptor(base);
        if (descr == NULL) {
            raise_dict_descr_error(obj);
            return -1;
        }
        func = Py_TYPE(descr)->tp_descr_set;
        if (func == NULL) {
            raise_dict_descr_error(obj);
            return -1;
        }
        return func(descr, obj, value);
    }
    /* Almost like PyObject_GenericSetDict, but allow __dict__ to be deleted. */
    dictptr = _PyObject_GetDictPtr(obj);
    if (dictptr == NULL) {
        PyErr_SetString(PyExc_AttributeError,
                        "This object has no __dict__");
        return -1;
    }
    if (value != NULL && !PyDict_Check(value)) {
        PyErr_Format(PyExc_TypeError,
                     "__dict__ must be set to a dictionary, "
                     "not a '%.200s'", Py_TYPE(value)->tp_name);
        return -1;
    }
    dict = *dictptr;
    Py_XINCREF(value);
    *dictptr = value;
    Py_XDECREF(dict);
    return 0;
}

static PyObject *
subtype_getweakref(PyObject *obj, void *context)
{
    PyObject **weaklistptr;
    PyObject *result;

    if (Py_TYPE(obj)->tp_weaklistoffset == 0) {
        PyErr_SetString(PyExc_AttributeError,
                        "This object has no __weakref__");
        return NULL;
    }
    assert(Py_TYPE(obj)->tp_weaklistoffset > 0);
    assert(Py_TYPE(obj)->tp_weaklistoffset + sizeof(PyObject *) <=
           (size_t)(Py_TYPE(obj)->tp_basicsize));
    weaklistptr = (PyObject **)
        ((char *)obj + Py_TYPE(obj)->tp_weaklistoffset);
    if (*weaklistptr == NULL)
        result = Py_None;
    else
        result = *weaklistptr;
    Py_INCREF(result);
    return result;
}

/* Three variants on the subtype_getsets list. */

static PyGetSetDef subtype_getsets_full[] = {
    {"__dict__", subtype_dict, subtype_setdict,
     PyDoc_STR("dictionary for instance variables (if defined)")},
    {"__weakref__", subtype_getweakref, NULL,
     PyDoc_STR("list of weak references to the object (if defined)")},
    {0}
};

static PyGetSetDef subtype_getsets_dict_only[] = {
    {"__dict__", subtype_dict, subtype_setdict,
     PyDoc_STR("dictionary for instance variables (if defined)")},
    {0}
};

static PyGetSetDef subtype_getsets_weakref_only[] = {
    {"__weakref__", subtype_getweakref, NULL,
     PyDoc_STR("list of weak references to the object (if defined)")},
    {0}
};

static int
valid_identifier(PyObject *s)
{
    if (!PyUnicode_Check(s)) {
        PyErr_Format(PyExc_TypeError,
                     "__slots__ items must be strings, not '%.200s'",
                     Py_TYPE(s)->tp_name);
        return 0;
    }
    if (!PyUnicode_IsIdentifier(s)) {
        PyErr_SetString(PyExc_TypeError,
                        "__slots__ must be identifiers");
        return 0;
    }
    return 1;
}

/* Forward */
static int
object_init(PyObject *self, PyObject *args, PyObject *kwds);

static int
type_init(PyObject *cls, PyObject *args, PyObject *kwds)
{
    int res;

    assert(args != NULL && PyTuple_Check(args));
    assert(kwds == NULL || PyDict_Check(kwds));

    if (kwds != NULL && PyDict_Check(kwds) && PyDict_Size(kwds) != 0) {
        PyErr_SetString(PyExc_TypeError,
                        "type.__init__() takes no keyword arguments");
        return -1;
    }

    if (args != NULL && PyTuple_Check(args) &&
        (PyTuple_GET_SIZE(args) != 1 && PyTuple_GET_SIZE(args) != 3)) {
        PyErr_SetString(PyExc_TypeError,
                        "type.__init__() takes 1 or 3 arguments");
        return -1;
    }

    /* Call object.__init__(self) now. */
    /* XXX Could call super(type, cls).__init__() but what's the point? */
    args = PyTuple_GetSlice(args, 0, 0);
    res = object_init(cls, args, NULL);
    Py_DECREF(args);
    return res;
}

long
PyType_GetFlags(PyTypeObject *type)
{
    return type->tp_flags;
}

/* Determine the most derived metatype. */
PyTypeObject *
_PyType_CalculateMetaclass(PyTypeObject *metatype, PyObject *bases)
{
    Py_ssize_t i, nbases;
    PyTypeObject *winner;
    PyObject *tmp;
    PyTypeObject *tmptype;

    /* Determine the proper metatype to deal with this,
       and check for metatype conflicts while we're at it.
       Note that if some other metatype wins to contract,
       it's possible that its instances are not types. */

    nbases = PyTuple_GET_SIZE(bases);
    winner = metatype;
    for (i = 0; i < nbases; i++) {
        tmp = PyTuple_GET_ITEM(bases, i);
        tmptype = Py_TYPE(tmp);
        if (PyType_IsSubtype(winner, tmptype))
            continue;
        if (PyType_IsSubtype(tmptype, winner)) {
            winner = tmptype;
            continue;
        }
        /* else: */
        PyErr_SetString(PyExc_TypeError,
                        "metaclass conflict: "
                        "the metaclass of a derived class "
                        "must be a (non-strict) subclass "
                        "of the metaclasses of all its bases");
        return NULL;
    }
    return winner;
}

static PyObject *
type_new(PyTypeObject *metatype, PyObject *args, PyObject *kwds)
{
    PyObject *name, *bases = NULL, *orig_dict, *dict = NULL;
    static char *kwlist[] = {"name", "bases", "dict", 0};
    PyObject *qualname, *slots = NULL, *tmp, *newslots;
    PyTypeObject *type = NULL, *base, *tmptype, *winner;
    PyHeapTypeObject *et;
    PyMemberDef *mp;
    Py_ssize_t i, nbases, nslots, slotoffset, add_dict, add_weak;
    int j, may_add_dict, may_add_weak;
    _Py_IDENTIFIER(__qualname__);
    _Py_IDENTIFIER(__slots__);

    assert(args != NULL && PyTuple_Check(args));
    assert(kwds == NULL || PyDict_Check(kwds));

    /* Special case: type(x) should return x->ob_type */
    {
        const Py_ssize_t nargs = PyTuple_GET_SIZE(args);
        const Py_ssize_t nkwds = kwds == NULL ? 0 : PyDict_Size(kwds);

        if (PyType_CheckExact(metatype) && nargs == 1 && nkwds == 0) {
            PyObject *x = PyTuple_GET_ITEM(args, 0);
            Py_INCREF(Py_TYPE(x));
            return (PyObject *) Py_TYPE(x);
        }

        /* SF bug 475327 -- if that didn't trigger, we need 3
           arguments. but PyArg_ParseTupleAndKeywords below may give
           a msg saying type() needs exactly 3. */
        if (nargs + nkwds != 3) {
            PyErr_SetString(PyExc_TypeError,
                            "type() takes 1 or 3 arguments");
            return NULL;
        }
    }

    /* Check arguments: (name, bases, dict) */
    if (!PyArg_ParseTupleAndKeywords(args, kwds, "UO!O!:type", kwlist,
                                     &name,
                                     &PyTuple_Type, &bases,
                                     &PyDict_Type, &orig_dict))
        return NULL;

    /* Determine the proper metatype to deal with this: */
    winner = _PyType_CalculateMetaclass(metatype, bases);
    if (winner == NULL) {
        return NULL;
    }

    if (winner != metatype) {
        if (winner->tp_new != type_new) /* Pass it to the winner */
            return winner->tp_new(winner, args, kwds);
        metatype = winner;
    }

    /* Adjust for empty tuple bases */
    nbases = PyTuple_GET_SIZE(bases);
    if (nbases == 0) {
        bases = PyTuple_Pack(1, &PyBaseObject_Type);
        if (bases == NULL)
            goto error;
        nbases = 1;
    }
    else
        Py_INCREF(bases);

    /* Calculate best base, and check that all bases are type objects */
    base = best_base(bases);
    if (base == NULL) {
        goto error;
    }
    if (!PyType_HasFeature(base, Py_TPFLAGS_BASETYPE)) {
        PyErr_Format(PyExc_TypeError,
                     "type '%.100s' is not an acceptable base type",
                     base->tp_name);
        goto error;
    }

    dict = PyDict_Copy(orig_dict);
    if (dict == NULL)
        goto error;

    /* Check for a __slots__ sequence variable in dict, and count it */
    slots = _PyDict_GetItemId(dict, &PyId___slots__);
    nslots = 0;
    add_dict = 0;
    add_weak = 0;
    may_add_dict = base->tp_dictoffset == 0;
    may_add_weak = base->tp_weaklistoffset == 0 && base->tp_itemsize == 0;
    if (slots == NULL) {
        if (may_add_dict) {
            add_dict++;
        }
        if (may_add_weak) {
            add_weak++;
        }
    }
    else {
        /* Have slots */

        /* Make it into a tuple */
        if (PyUnicode_Check(slots))
            slots = PyTuple_Pack(1, slots);
        else
            slots = PySequence_Tuple(slots);
        if (slots == NULL)
            goto error;
        assert(PyTuple_Check(slots));

        /* Are slots allowed? */
        nslots = PyTuple_GET_SIZE(slots);
        if (nslots > 0 && base->tp_itemsize != 0) {
            PyErr_Format(PyExc_TypeError,
                         "nonempty __slots__ "
                         "not supported for subtype of '%s'",
                         base->tp_name);
            goto error;
        }

        /* Check for valid slot names and two special cases */
        for (i = 0; i < nslots; i++) {
            PyObject *tmp = PyTuple_GET_ITEM(slots, i);
            if (!valid_identifier(tmp))
                goto error;
            assert(PyUnicode_Check(tmp));
            if (PyUnicode_CompareWithASCIIString(tmp, "__dict__") == 0) {
                if (!may_add_dict || add_dict) {
                    PyErr_SetString(PyExc_TypeError,
                        "__dict__ slot disallowed: "
                        "we already got one");
                    goto error;
                }
                add_dict++;
            }
            if (PyUnicode_CompareWithASCIIString(tmp, "__weakref__") == 0) {
                if (!may_add_weak || add_weak) {
                    PyErr_SetString(PyExc_TypeError,
                        "__weakref__ slot disallowed: "
                        "either we already got one, "
                        "or __itemsize__ != 0");
                    goto error;
                }
                add_weak++;
            }
        }

        /* Copy slots into a list, mangle names and sort them.
           Sorted names are needed for __class__ assignment.
           Convert them back to tuple at the end.
        */
        newslots = PyList_New(nslots - add_dict - add_weak);
        if (newslots == NULL)
            goto error;
        for (i = j = 0; i < nslots; i++) {
            tmp = PyTuple_GET_ITEM(slots, i);
            if ((add_dict &&
                 PyUnicode_CompareWithASCIIString(tmp, "__dict__") == 0) ||
                (add_weak &&
                 PyUnicode_CompareWithASCIIString(tmp, "__weakref__") == 0))
                continue;
            tmp =_Py_Mangle(name, tmp);
            if (!tmp) {
                Py_DECREF(newslots);
                goto error;
            }
            PyList_SET_ITEM(newslots, j, tmp);
            if (PyDict_GetItem(dict, tmp)) {
                PyErr_Format(PyExc_ValueError,
                             "%R in __slots__ conflicts with class variable",
                             tmp);
                Py_DECREF(newslots);
                goto error;
            }
            j++;
        }
        assert(j == nslots - add_dict - add_weak);
        nslots = j;
        Py_CLEAR(slots);
        if (PyList_Sort(newslots) == -1) {
            Py_DECREF(newslots);
            goto error;
        }
        slots = PyList_AsTuple(newslots);
        Py_DECREF(newslots);
        if (slots == NULL)
            goto error;

        /* Secondary bases may provide weakrefs or dict */
        if (nbases > 1 &&
            ((may_add_dict && !add_dict) ||
             (may_add_weak && !add_weak))) {
            for (i = 0; i < nbases; i++) {
                tmp = PyTuple_GET_ITEM(bases, i);
                if (tmp == (PyObject *)base)
                    continue; /* Skip primary base */
                assert(PyType_Check(tmp));
                tmptype = (PyTypeObject *)tmp;
                if (may_add_dict && !add_dict &&
                    tmptype->tp_dictoffset != 0)
                    add_dict++;
                if (may_add_weak && !add_weak &&
                    tmptype->tp_weaklistoffset != 0)
                    add_weak++;
                if (may_add_dict && !add_dict)
                    continue;
                if (may_add_weak && !add_weak)
                    continue;
                /* Nothing more to check */
                break;
            }
        }
    }

    /* Allocate the type object */
    type = (PyTypeObject *)metatype->tp_alloc(metatype, nslots);
    if (type == NULL)
        goto error;

    /* Keep name and slots alive in the extended type object */
    et = (PyHeapTypeObject *)type;
    Py_INCREF(name);
    et->ht_name = name;
    et->ht_slots = slots;
    slots = NULL;

    /* Initialize tp_flags */
    type->tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HEAPTYPE |
        Py_TPFLAGS_BASETYPE;
    if (base->tp_flags & Py_TPFLAGS_HAVE_GC)
        type->tp_flags |= Py_TPFLAGS_HAVE_GC;

    /* Initialize essential fields */
    type->tp_as_number = &et->as_number;
    type->tp_as_sequence = &et->as_sequence;
    type->tp_as_mapping = &et->as_mapping;
    type->tp_as_buffer = &et->as_buffer;
    type->tp_name = _PyUnicode_AsString(name);
    if (!type->tp_name)
        goto error;

    /* Set tp_base and tp_bases */
    type->tp_bases = bases;
    bases = NULL;
    Py_INCREF(base);
    type->tp_base = base;

    /* Initialize tp_dict from passed-in dict */
    Py_INCREF(dict);
    type->tp_dict = dict;

    /* Set __module__ in the dict */
    if (_PyDict_GetItemId(dict, &PyId___module__) == NULL) {
        tmp = PyEval_GetGlobals();
        if (tmp != NULL) {
            tmp = _PyDict_GetItemId(tmp, &PyId___name__);
            if (tmp != NULL) {
                if (_PyDict_SetItemId(dict, &PyId___module__,
                                      tmp) < 0)
                    goto error;
            }
        }
    }

    /* Set ht_qualname to dict['__qualname__'] if available, else to
       __name__.  The __qualname__ accessor will look for ht_qualname.
    */
    qualname = _PyDict_GetItemId(dict, &PyId___qualname__);
    if (qualname != NULL) {
        if (!PyUnicode_Check(qualname)) {
            PyErr_Format(PyExc_TypeError,
                         "type __qualname__ must be a str, not %s",
                         Py_TYPE(qualname)->tp_name);
            goto error;
        }
    }
    else {
        qualname = et->ht_name;
    }
    Py_INCREF(qualname);
    et->ht_qualname = qualname;

    /* Set tp_doc to a copy of dict['__doc__'], if the latter is there
       and is a string.  The __doc__ accessor will first look for tp_doc;
       if that fails, it will still look into __dict__.
    */
    {
        PyObject *doc = _PyDict_GetItemId(dict, &PyId___doc__);
        if (doc != NULL && PyUnicode_Check(doc)) {
            Py_ssize_t len;
            char *doc_str;
            char *tp_doc;

            doc_str = _PyUnicode_AsString(doc);
            if (doc_str == NULL)
                goto error;
            /* Silently truncate the docstring if it contains null bytes. */
            len = strlen(doc_str);
            tp_doc = (char *)PyObject_MALLOC(len + 1);
            if (tp_doc == NULL)
                goto error;
            memcpy(tp_doc, doc_str, len + 1);
            type->tp_doc = tp_doc;
        }
    }

    /* Special-case __new__: if it's a plain function,
       make it a static function */
    tmp = _PyDict_GetItemId(dict, &PyId___new__);
    if (tmp != NULL && PyFunction_Check(tmp)) {
        tmp = PyStaticMethod_New(tmp);
        if (tmp == NULL)
            goto error;
        if (_PyDict_SetItemId(dict, &PyId___new__, tmp) < 0)
            goto error;
        Py_DECREF(tmp);
    }

    /* Add descriptors for custom slots from __slots__, or for __dict__ */
    mp = PyHeapType_GET_MEMBERS(et);
    slotoffset = base->tp_basicsize;
    if (et->ht_slots != NULL) {
        for (i = 0; i < nslots; i++, mp++) {
            mp->name = _PyUnicode_AsString(
                PyTuple_GET_ITEM(et->ht_slots, i));
            if (mp->name == NULL)
                goto error;
            mp->type = T_OBJECT_EX;
            mp->offset = slotoffset;

            /* __dict__ and __weakref__ are already filtered out */
            assert(strcmp(mp->name, "__dict__") != 0);
            assert(strcmp(mp->name, "__weakref__") != 0);

            slotoffset += sizeof(PyObject *);
        }
    }
    if (add_dict) {
        if (base->tp_itemsize)
            type->tp_dictoffset = -(long)sizeof(PyObject *);
        else
            type->tp_dictoffset = slotoffset;
        slotoffset += sizeof(PyObject *);
    }
    if (add_weak) {
        assert(!base->tp_itemsize);
        type->tp_weaklistoffset = slotoffset;
        slotoffset += sizeof(PyObject *);
    }
    type->tp_basicsize = slotoffset;
    type->tp_itemsize = base->tp_itemsize;
    type->tp_members = PyHeapType_GET_MEMBERS(et);

    if (type->tp_weaklistoffset && type->tp_dictoffset)
        type->tp_getset = subtype_getsets_full;
    else if (type->tp_weaklistoffset && !type->tp_dictoffset)
        type->tp_getset = subtype_getsets_weakref_only;
    else if (!type->tp_weaklistoffset && type->tp_dictoffset)
        type->tp_getset = subtype_getsets_dict_only;
    else
        type->tp_getset = NULL;

    /* Special case some slots */
    if (type->tp_dictoffset != 0 || nslots > 0) {
        if (base->tp_getattr == NULL && base->tp_getattro == NULL)
            type->tp_getattro = PyObject_GenericGetAttr;
        if (base->tp_setattr == NULL && base->tp_setattro == NULL)
            type->tp_setattro = PyObject_GenericSetAttr;
    }
    type->tp_dealloc = subtype_dealloc;

    /* Enable GC unless there are really no instance variables possible */
    if (!(type->tp_basicsize == sizeof(PyObject) &&
          type->tp_itemsize == 0))
        type->tp_flags |= Py_TPFLAGS_HAVE_GC;

    /* Always override allocation strategy to use regular heap */
    type->tp_alloc = PyType_GenericAlloc;
    if (type->tp_flags & Py_TPFLAGS_HAVE_GC) {
        type->tp_free = PyObject_GC_Del;
        type->tp_traverse = subtype_traverse;
        type->tp_clear = subtype_clear;
    }
    else
        type->tp_free = PyObject_Del;

    /* Initialize the rest */
    if (PyType_Ready(type) < 0)
        goto error;

    /* Put the proper slots in place */
    fixup_slot_dispatchers(type);

    Py_DECREF(dict);
    return (PyObject *)type;

error:
    Py_XDECREF(dict);
    Py_XDECREF(bases);
    Py_XDECREF(slots);
    Py_XDECREF(type);
    return NULL;
}

static short slotoffsets[] = {
    -1, /* invalid slot */
#include "typeslots.inc"
};

PyObject *
PyType_FromSpec(PyType_Spec *spec)
{
    PyHeapTypeObject *res = (PyHeapTypeObject*)PyType_GenericAlloc(&PyType_Type, 0);
    char *res_start = (char*)res;
    PyType_Slot *slot;

    if (res == NULL)
      return NULL;
    res->ht_name = PyUnicode_FromString(spec->name);
    if (!res->ht_name)
        goto fail;
    res->ht_qualname = res->ht_name;
    Py_INCREF(res->ht_qualname);
    res->ht_type.tp_name = _PyUnicode_AsString(res->ht_name);
    if (!res->ht_type.tp_name)
        goto fail;

    res->ht_type.tp_basicsize = spec->basicsize;
    res->ht_type.tp_itemsize = spec->itemsize;
    res->ht_type.tp_flags = spec->flags | Py_TPFLAGS_HEAPTYPE;

    for (slot = spec->slots; slot->slot; slot++) {
        if (slot->slot >= Py_ARRAY_LENGTH(slotoffsets)) {
            PyErr_SetString(PyExc_RuntimeError, "invalid slot offset");
            goto fail;
        }
        *(void**)(res_start + slotoffsets[slot->slot]) = slot->pfunc;

        /* need to make a copy of the docstring slot, which usually
           points to a static string literal */
        if (slot->slot == Py_tp_doc) {
            ssize_t len = strlen(slot->pfunc)+1;
            char *tp_doc = PyObject_MALLOC(len);
            if (tp_doc == NULL)
                goto fail;
            memcpy(tp_doc, slot->pfunc, len);
            res->ht_type.tp_doc = tp_doc;
        }
    }

    if (PyType_Ready(&res->ht_type) < 0)
        goto fail;

    return (PyObject*)res;

 fail:
    Py_DECREF(res);
    return NULL;
}


/* Internal API to look for a name through the MRO.
   This returns a borrowed reference, and doesn't set an exception! */
PyObject *
_PyType_Lookup(PyTypeObject *type, PyObject *name)
{
    Py_ssize_t i, n;
    PyObject *mro, *res, *base, *dict;
    unsigned int h;

    if (MCACHE_CACHEABLE_NAME(name) &&
        PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG)) {
        /* fast path */
        h = MCACHE_HASH_METHOD(type, name);
        if (method_cache[h].version == type->tp_version_tag &&
            method_cache[h].name == name)
            return method_cache[h].value;
    }

    /* Look in tp_dict of types in MRO */
    mro = type->tp_mro;

    /* If mro is NULL, the type is either not yet initialized
       by PyType_Ready(), or already cleared by type_clear().
       Either way the safest thing to do is to return NULL. */
    if (mro == NULL)
        return NULL;

    res = NULL;
    /* keep a strong reference to mro because type->tp_mro can be replaced
       during PyDict_GetItem(dict, name)  */
    Py_INCREF(mro);
    assert(PyTuple_Check(mro));
    n = PyTuple_GET_SIZE(mro);
    for (i = 0; i < n; i++) {
        base = PyTuple_GET_ITEM(mro, i);
        assert(PyType_Check(base));
        dict = ((PyTypeObject *)base)->tp_dict;
        assert(dict && PyDict_Check(dict));
        res = PyDict_GetItem(dict, name);
        if (res != NULL)
            break;
    }
    Py_DECREF(mro);

    if (MCACHE_CACHEABLE_NAME(name) && assign_version_tag(type)) {
        h = MCACHE_HASH_METHOD(type, name);
        method_cache[h].version = type->tp_version_tag;
        method_cache[h].value = res;  /* borrowed */
        Py_INCREF(name);
        Py_DECREF(method_cache[h].name);
        method_cache[h].name = name;
    }
    return res;
}

static PyObject *
_PyType_LookupId(PyTypeObject *type, struct _Py_Identifier *name)
{
    PyObject *oname;
    oname = _PyUnicode_FromId(name);   /* borrowed */
    if (oname == NULL)
        return NULL;
    return _PyType_Lookup(type, oname);
}

/* This is similar to PyObject_GenericGetAttr(),
   but uses _PyType_Lookup() instead of just looking in type->tp_dict. */
static PyObject *
type_getattro(PyTypeObject *type, PyObject *name)
{
    PyTypeObject *metatype = Py_TYPE(type);
    PyObject *meta_attribute, *attribute;
    descrgetfunc meta_get;

    if (!PyUnicode_Check(name)) {
        PyErr_Format(PyExc_TypeError,
                     "attribute name must be string, not '%.200s'",
                     name->ob_type->tp_name);
        return NULL;
    }

    /* Initialize this type (we'll assume the metatype is initialized) */
    if (type->tp_dict == NULL) {
        if (PyType_Ready(type) < 0)
            return NULL;
    }

    /* No readable descriptor found yet */
    meta_get = NULL;

    /* Look for the attribute in the metatype */
    meta_attribute = _PyType_Lookup(metatype, name);

    if (meta_attribute != NULL) {
        meta_get = Py_TYPE(meta_attribute)->tp_descr_get;

        if (meta_get != NULL && PyDescr_IsData(meta_attribute)) {
            /* Data descriptors implement tp_descr_set to intercept
             * writes. Assume the attribute is not overridden in
             * type's tp_dict (and bases): call the descriptor now.
             */
            return meta_get(meta_attribute, (PyObject *)type,
                            (PyObject *)metatype);
        }
        Py_INCREF(meta_attribute);
    }

    /* No data descriptor found on metatype. Look in tp_dict of this
     * type and its bases */
    attribute = _PyType_Lookup(type, name);
    if (attribute != NULL) {
        /* Implement descriptor functionality, if any */
        descrgetfunc local_get = Py_TYPE(attribute)->tp_descr_get;

        Py_XDECREF(meta_attribute);

        if (local_get != NULL) {
            /* NULL 2nd argument indicates the descriptor was
             * found on the target object itself (or a base)  */
            return local_get(attribute, (PyObject *)NULL,
                             (PyObject *)type);
        }

        Py_INCREF(attribute);
        return attribute;
    }

    /* No attribute found in local __dict__ (or bases): use the
     * descriptor from the metatype, if any */
    if (meta_get != NULL) {
        PyObject *res;
        res = meta_get(meta_attribute, (PyObject *)type,
                       (PyObject *)metatype);
        Py_DECREF(meta_attribute);
        return res;
    }

    /* If an ordinary attribute was found on the metatype, return it now */
    if (meta_attribute != NULL) {
        return meta_attribute;
    }

    /* Give up */
    PyErr_Format(PyExc_AttributeError,
                 "type object '%.50s' has no attribute '%U'",
                 type->tp_name, name);
    return NULL;
}

static int
type_setattro(PyTypeObject *type, PyObject *name, PyObject *value)
{
    if (!(type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
        PyErr_Format(
            PyExc_TypeError,
            "can't set attributes of built-in/extension type '%s'",
            type->tp_name);
        return -1;
    }
    if (PyObject_GenericSetAttr((PyObject *)type, name, value) < 0)
        return -1;
    return update_slot(type, name);
}

static void
type_dealloc(PyTypeObject *type)
{
    PyHeapTypeObject *et;

    /* Assert this is a heap-allocated type object */
    assert(type->tp_flags & Py_TPFLAGS_HEAPTYPE);
    _PyObject_GC_UNTRACK(type);
    PyObject_ClearWeakRefs((PyObject *)type);
    et = (PyHeapTypeObject *)type;
    Py_XDECREF(type->tp_base);
    Py_XDECREF(type->tp_dict);
    Py_XDECREF(type->tp_bases);
    Py_XDECREF(type->tp_mro);
    Py_XDECREF(type->tp_cache);
    Py_XDECREF(type->tp_subclasses);
    /* A type's tp_doc is heap allocated, unlike the tp_doc slots
     * of most other objects.  It's okay to cast it to char *.
     */
    PyObject_Free((char *)type->tp_doc);
    Py_XDECREF(et->ht_name);
    Py_XDECREF(et->ht_qualname);
    Py_XDECREF(et->ht_slots);
    Py_TYPE(type)->tp_free((PyObject *)type);
}

static PyObject *
type_subclasses(PyTypeObject *type, PyObject *args_ignored)
{
    PyObject *list, *raw, *ref;
    Py_ssize_t i, n;

    list = PyList_New(0);
    if (list == NULL)
        return NULL;
    raw = type->tp_subclasses;
    if (raw == NULL)
        return list;
    assert(PyList_Check(raw));
    n = PyList_GET_SIZE(raw);
    for (i = 0; i < n; i++) {
        ref = PyList_GET_ITEM(raw, i);
        assert(PyWeakref_CheckRef(ref));
        ref = PyWeakref_GET_OBJECT(ref);
        if (ref != Py_None) {
            if (PyList_Append(list, ref) < 0) {
                Py_DECREF(list);
                return NULL;
            }
        }
    }
    return list;
}

static PyObject *
type_prepare(PyObject *self, PyObject *args, PyObject *kwds)
{
    return PyDict_New();
}

/*
   Merge the __dict__ of aclass into dict, and recursively also all
   the __dict__s of aclass's base classes.  The order of merging isn't
   defined, as it's expected that only the final set of dict keys is
   interesting.
   Return 0 on success, -1 on error.
*/

static int
merge_class_dict(PyObject *dict, PyObject *aclass)
{
    PyObject *classdict;
    PyObject *bases;
    _Py_IDENTIFIER(__bases__);

    assert(PyDict_Check(dict));
    assert(aclass);

    /* Merge in the type's dict (if any). */
    classdict = _PyObject_GetAttrId(aclass, &PyId___dict__);
    if (classdict == NULL)
        PyErr_Clear();
    else {
        int status = PyDict_Update(dict, classdict);
        Py_DECREF(classdict);
        if (status < 0)
            return -1;
    }

    /* Recursively merge in the base types' (if any) dicts. */
    bases = _PyObject_GetAttrId(aclass, &PyId___bases__);
    if (bases == NULL)
        PyErr_Clear();
    else {
        /* We have no guarantee that bases is a real tuple */
        Py_ssize_t i, n;
        n = PySequence_Size(bases); /* This better be right */
        if (n < 0)
            PyErr_Clear();
        else {
            for (i = 0; i < n; i++) {
                int status;
                PyObject *base = PySequence_GetItem(bases, i);
                if (base == NULL) {
                    Py_DECREF(bases);
                    return -1;
                }
                status = merge_class_dict(dict, base);
                Py_DECREF(base);
                if (status < 0) {
                    Py_DECREF(bases);
                    return -1;
                }
            }
        }
        Py_DECREF(bases);
    }
    return 0;
}

/* __dir__ for type objects: returns __dict__ and __bases__.
   We deliberately don't suck up its __class__, as methods belonging to the
   metaclass would probably be more confusing than helpful.
*/
static PyObject *
type_dir(PyObject *self, PyObject *args)
{
    PyObject *result = NULL;
    PyObject *dict = PyDict_New();

    if (dict != NULL && merge_class_dict(dict, self) == 0)
        result = PyDict_Keys(dict);

    Py_XDECREF(dict);
    return result;
}

static PyMethodDef type_methods[] = {
    {"mro", (PyCFunction)mro_external, METH_NOARGS,
     PyDoc_STR("mro() -> list\nreturn a type's method resolution order")},
    {"__subclasses__", (PyCFunction)type_subclasses, METH_NOARGS,
     PyDoc_STR("__subclasses__() -> list of immediate subclasses")},
    {"__prepare__", (PyCFunction)type_prepare,
     METH_VARARGS | METH_KEYWORDS | METH_CLASS,
     PyDoc_STR("__prepare__() -> dict\n"
               "used to create the namespace for the class statement")},
    {"__instancecheck__", type___instancecheck__, METH_O,
     PyDoc_STR("__instancecheck__() -> bool\ncheck if an object is an instance")},
    {"__subclasscheck__", type___subclasscheck__, METH_O,
     PyDoc_STR("__subclasscheck__() -> bool\ncheck if a class is a subclass")},
    {"__dir__", type_dir, METH_NOARGS,
     PyDoc_STR("__dir__() -> list\nspecialized __dir__ implementation for types")},
    {0}
};

PyDoc_STRVAR(type_doc,
"type(object) -> the object's type\n"
"type(name, bases, dict) -> a new type");

static int
type_traverse(PyTypeObject *type, visitproc visit, void *arg)
{
    /* Because of type_is_gc(), the collector only calls this
       for heaptypes. */
    assert(type->tp_flags & Py_TPFLAGS_HEAPTYPE);

    Py_VISIT(type->tp_dict);
    Py_VISIT(type->tp_cache);
    Py_VISIT(type->tp_mro);
    Py_VISIT(type->tp_bases);
    Py_VISIT(type->tp_base);

    /* There's no need to visit type->tp_subclasses or
       ((PyHeapTypeObject *)type)->ht_slots, because they can't be involved
       in cycles; tp_subclasses is a list of weak references,
       and slots is a tuple of strings. */

    return 0;
}

static int
type_clear(PyTypeObject *type)
{
    /* Because of type_is_gc(), the collector only calls this
       for heaptypes. */
    assert(type->tp_flags & Py_TPFLAGS_HEAPTYPE);

    /* We need to invalidate the method cache carefully before clearing
       the dict, so that other objects caught in a reference cycle
       don't start calling destroyed methods.

       Otherwise, the only field we need to clear is tp_mro, which is
       part of a hard cycle (its first element is the class itself) that
       won't be broken otherwise (it's a tuple and tuples don't have a
       tp_clear handler).  None of the other fields need to be
       cleared, and here's why:

       tp_cache:
           Not used; if it were, it would be a dict.

       tp_bases, tp_base:
           If these are involved in a cycle, there must be at least
           one other, mutable object in the cycle, e.g. a base
           class's dict; the cycle will be broken that way.

       tp_subclasses:
           A list of weak references can't be part of a cycle; and
           lists have their own tp_clear.

       slots (in PyHeapTypeObject):
           A tuple of strings can't be part of a cycle.
    */

    PyType_Modified(type);
    if (type->tp_dict)
        PyDict_Clear(type->tp_dict);
    Py_CLEAR(type->tp_mro);

    return 0;
}

static int
type_is_gc(PyTypeObject *type)
{
    return type->tp_flags & Py_TPFLAGS_HEAPTYPE;
}

PyTypeObject PyType_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "type",                                     /* tp_name */
    sizeof(PyHeapTypeObject),                   /* tp_basicsize */
    sizeof(PyMemberDef),                        /* tp_itemsize */
    (destructor)type_dealloc,                   /* tp_dealloc */
    0,                                          /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_reserved */
    (reprfunc)type_repr,                        /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    (ternaryfunc)type_call,                     /* tp_call */
    0,                                          /* tp_str */
    (getattrofunc)type_getattro,                /* tp_getattro */
    (setattrofunc)type_setattro,                /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
        Py_TPFLAGS_BASETYPE | Py_TPFLAGS_TYPE_SUBCLASS,         /* tp_flags */
    type_doc,                                   /* tp_doc */
    (traverseproc)type_traverse,                /* tp_traverse */
    (inquiry)type_clear,                        /* tp_clear */
    0,                                          /* tp_richcompare */
    offsetof(PyTypeObject, tp_weaklist),        /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    type_methods,                               /* tp_methods */
    type_members,                               /* tp_members */
    type_getsets,                               /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    offsetof(PyTypeObject, tp_dict),            /* tp_dictoffset */
    type_init,                                  /* tp_init */
    0,                                          /* tp_alloc */
    type_new,                                   /* tp_new */
    PyObject_GC_Del,                            /* tp_free */
    (inquiry)type_is_gc,                        /* tp_is_gc */
};


/* The base type of all types (eventually)... except itself. */

/* You may wonder why object.__new__() only complains about arguments
   when object.__init__() is not overridden, and vice versa.

   Consider the use cases:

   1. When neither is overridden, we want to hear complaints about
      excess (i.e., any) arguments, since their presence could
      indicate there's a bug.

   2. When defining an Immutable type, we are likely to override only
      __new__(), since __init__() is called too late to initialize an
      Immutable object.  Since __new__() defines the signature for the
      type, it would be a pain to have to override __init__() just to
      stop it from complaining about excess arguments.

   3. When defining a Mutable type, we are likely to override only
      __init__().  So here the converse reasoning applies: we don't
      want to have to override __new__() just to stop it from
      complaining.

   4. When __init__() is overridden, and the subclass __init__() calls
      object.__init__(), the latter should complain about excess
      arguments; ditto for __new__().

   Use cases 2 and 3 make it unattractive to unconditionally check for
   excess arguments.  The best solution that addresses all four use
   cases is as follows: __init__() complains about excess arguments
   unless __new__() is overridden and __init__() is not overridden
   (IOW, if __init__() is overridden or __new__() is not overridden);
   symmetrically, __new__() complains about excess arguments unless
   __init__() is overridden and __new__() is not overridden
   (IOW, if __new__() is overridden or __init__() is not overridden).

   However, for backwards compatibility, this breaks too much code.
   Therefore, in 2.6, we'll *warn* about excess arguments when both
   methods are overridden; for all other cases we'll use the above
   rules.

*/

/* Forward */
static PyObject *
object_new(PyTypeObject *type, PyObject *args, PyObject *kwds);

static int
excess_args(PyObject *args, PyObject *kwds)
{
    return PyTuple_GET_SIZE(args) ||
        (kwds && PyDict_Check(kwds) && PyDict_Size(kwds));
}

static int
object_init(PyObject *self, PyObject *args, PyObject *kwds)
{
    int err = 0;
    PyTypeObject *type = Py_TYPE(self);
    if (excess_args(args, kwds) &&
        (type->tp_new == object_new || type->tp_init != object_init)) {
        PyErr_SetString(PyExc_TypeError, "object.__init__() takes no parameters");
        err = -1;
    }
    return err;
}

static PyObject *
object_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    if (excess_args(args, kwds) &&
        (type->tp_init == object_init || type->tp_new != object_new)) {
        PyErr_SetString(PyExc_TypeError, "object.__new__() takes no parameters");
        return NULL;
    }

    if (type->tp_flags & Py_TPFLAGS_IS_ABSTRACT) {
        PyObject *abstract_methods = NULL;
        PyObject *builtins;
        PyObject *sorted;
        PyObject *sorted_methods = NULL;
        PyObject *joined = NULL;
        PyObject *comma;
        _Py_static_string(comma_id, ", ");
        _Py_IDENTIFIER(sorted);

        /* Compute ", ".join(sorted(type.__abstractmethods__))
           into joined. */
        abstract_methods = type_abstractmethods(type, NULL);
        if (abstract_methods == NULL)
            goto error;
        builtins = PyEval_GetBuiltins();
        if (builtins == NULL)
            goto error;
        sorted = _PyDict_GetItemId(builtins, &PyId_sorted);
        if (sorted == NULL)
            goto error;
        sorted_methods = PyObject_CallFunctionObjArgs(sorted,
                                                      abstract_methods,
                                                      NULL);
        if (sorted_methods == NULL)
            goto error;
        comma = _PyUnicode_FromId(&comma_id);
        if (comma == NULL)
            goto error;
        joined = PyUnicode_Join(comma, sorted_methods);
        if (joined == NULL)
            goto error;

        PyErr_Format(PyExc_TypeError,
                     "Can't instantiate abstract class %s "
                     "with abstract methods %U",
                     type->tp_name,
                     joined);
    error:
        Py_XDECREF(joined);
        Py_XDECREF(sorted_methods);
        Py_XDECREF(abstract_methods);
        return NULL;
    }
    return type->tp_alloc(type, 0);
}

static void
object_dealloc(PyObject *self)
{
    Py_TYPE(self)->tp_free(self);
}

static PyObject *
object_repr(PyObject *self)
{
    PyTypeObject *type;
    PyObject *mod, *name, *rtn;

    type = Py_TYPE(self);
    mod = type_module(type, NULL);
    if (mod == NULL)
        PyErr_Clear();
    else if (!PyUnicode_Check(mod)) {
        Py_DECREF(mod);
        mod = NULL;
    }
    name = type_qualname(type, NULL);
    if (name == NULL)
        return NULL;
    if (mod != NULL && PyUnicode_CompareWithASCIIString(mod, "builtins"))
        rtn = PyUnicode_FromFormat("<%U.%U object at %p>", mod, name, self);
    else
        rtn = PyUnicode_FromFormat("<%s object at %p>",
                                  type->tp_name, self);
    Py_XDECREF(mod);
    Py_DECREF(name);
    return rtn;
}

static PyObject *
object_str(PyObject *self)
{
    unaryfunc f;

    f = Py_TYPE(self)->tp_repr;
    if (f == NULL || f == object_str)
        f = object_repr;
    return f(self);
}

static PyObject *
object_richcompare(PyObject *self, PyObject *other, int op)
{
    PyObject *res;

    switch (op) {

    case Py_EQ:
        /* Return NotImplemented instead of False, so if two
           objects are compared, both get a chance at the
           comparison.  See issue #1393. */
        res = (self == other) ? Py_True : Py_NotImplemented;
        Py_INCREF(res);
        break;

    case Py_NE:
        /* By default, != returns the opposite of ==,
           unless the latter returns NotImplemented. */
        res = PyObject_RichCompare(self, other, Py_EQ);
        if (res != NULL && res != Py_NotImplemented) {
            int ok = PyObject_IsTrue(res);
            Py_DECREF(res);
            if (ok < 0)
                res = NULL;
            else {
                if (ok)
                    res = Py_False;
                else
                    res = Py_True;
                Py_INCREF(res);
            }
        }
        break;

    default:
        res = Py_NotImplemented;
        Py_INCREF(res);
        break;
    }

    return res;
}

static PyObject *
object_get_class(PyObject *self, void *closure)
{
    Py_INCREF(Py_TYPE(self));
    return (PyObject *)(Py_TYPE(self));
}

static int
equiv_structs(PyTypeObject *a, PyTypeObject *b)
{
    return a == b ||
           (a != NULL &&
        b != NULL &&
        a->tp_basicsize == b->tp_basicsize &&
        a->tp_itemsize == b->tp_itemsize &&
        a->tp_dictoffset == b->tp_dictoffset &&
        a->tp_weaklistoffset == b->tp_weaklistoffset &&
        ((a->tp_flags & Py_TPFLAGS_HAVE_GC) ==
         (b->tp_flags & Py_TPFLAGS_HAVE_GC)));
}

static int
same_slots_added(PyTypeObject *a, PyTypeObject *b)
{
    PyTypeObject *base = a->tp_base;
    Py_ssize_t size;
    PyObject *slots_a, *slots_b;

    assert(base == b->tp_base);
    size = base->tp_basicsize;
    if (a->tp_dictoffset == size && b->tp_dictoffset == size)
        size += sizeof(PyObject *);
    if (a->tp_weaklistoffset == size && b->tp_weaklistoffset == size)
        size += sizeof(PyObject *);

    /* Check slots compliance */
    slots_a = ((PyHeapTypeObject *)a)->ht_slots;
    slots_b = ((PyHeapTypeObject *)b)->ht_slots;
    if (slots_a && slots_b) {
        if (PyObject_RichCompareBool(slots_a, slots_b, Py_EQ) != 1)
            return 0;
        size += sizeof(PyObject *) * PyTuple_GET_SIZE(slots_a);
    }
    return size == a->tp_basicsize && size == b->tp_basicsize;
}

static int
compatible_for_assignment(PyTypeObject* oldto, PyTypeObject* newto, char* attr)
{
    PyTypeObject *newbase, *oldbase;

    if (newto->tp_dealloc != oldto->tp_dealloc ||
        newto->tp_free != oldto->tp_free)
    {
        PyErr_Format(PyExc_TypeError,
                     "%s assignment: "
                     "'%s' deallocator differs from '%s'",
                     attr,
                     newto->tp_name,
                     oldto->tp_name);
        return 0;
    }
    newbase = newto;
    oldbase = oldto;
    while (equiv_structs(newbase, newbase->tp_base))
        newbase = newbase->tp_base;
    while (equiv_structs(oldbase, oldbase->tp_base))
        oldbase = oldbase->tp_base;
    if (newbase != oldbase &&
        (newbase->tp_base != oldbase->tp_base ||
         !same_slots_added(newbase, oldbase))) {
        PyErr_Format(PyExc_TypeError,
                     "%s assignment: "
                     "'%s' object layout differs from '%s'",
                     attr,
                     newto->tp_name,
                     oldto->tp_name);
        return 0;
    }

    return 1;
}

static int
object_set_class(PyObject *self, PyObject *value, void *closure)
{
    PyTypeObject *oldto = Py_TYPE(self);
    PyTypeObject *newto;

    if (value == NULL) {
        PyErr_SetString(PyExc_TypeError,
                        "can't delete __class__ attribute");
        return -1;
    }
    if (!PyType_Check(value)) {
        PyErr_Format(PyExc_TypeError,
          "__class__ must be set to a class, not '%s' object",
          Py_TYPE(value)->tp_name);
        return -1;
    }
    newto = (PyTypeObject *)value;
    if (!(newto->tp_flags & Py_TPFLAGS_HEAPTYPE) ||
        !(oldto->tp_flags & Py_TPFLAGS_HEAPTYPE))
    {
        PyErr_Format(PyExc_TypeError,
                     "__class__ assignment: only for heap types");
        return -1;
    }
    if (compatible_for_assignment(newto, oldto, "__class__")) {
        Py_INCREF(newto);
        Py_TYPE(self) = newto;
        Py_DECREF(oldto);
        return 0;
    }
    else {
        return -1;
    }
}

static PyGetSetDef object_getsets[] = {
    {"__class__", object_get_class, object_set_class,
     PyDoc_STR("the object's class")},
    {0}
};


/* Stuff to implement __reduce_ex__ for pickle protocols >= 2.
   We fall back to helpers in copyreg for:
   - pickle protocols < 2
   - calculating the list of slot names (done only once per class)
   - the __newobj__ function (which is used as a token but never called)
*/

static PyObject *
import_copyreg(void)
{
    static PyObject *copyreg_str;
    static PyObject *mod_copyreg = NULL;

    if (!copyreg_str) {
        copyreg_str = PyUnicode_InternFromString("copyreg");
        if (copyreg_str == NULL)
            return NULL;
    }
    if (!mod_copyreg) {
        mod_copyreg = PyImport_Import(copyreg_str);
    }

    Py_XINCREF(mod_copyreg);
    return mod_copyreg;
}

static PyObject *
slotnames(PyObject *cls)
{
    PyObject *clsdict;
    PyObject *copyreg;
    PyObject *slotnames;
    _Py_IDENTIFIER(__slotnames__);
    _Py_IDENTIFIER(_slotnames);

    clsdict = ((PyTypeObject *)cls)->tp_dict;
    slotnames = _PyDict_GetItemId(clsdict, &PyId___slotnames__);
    if (slotnames != NULL && PyList_Check(slotnames)) {
        Py_INCREF(slotnames);
        return slotnames;
    }

    copyreg = import_copyreg();
    if (copyreg == NULL)
        return NULL;

    slotnames = _PyObject_CallMethodId(copyreg, &PyId__slotnames, "O", cls);
    Py_DECREF(copyreg);
    if (slotnames != NULL &&
        slotnames != Py_None &&
        !PyList_Check(slotnames))
    {
        PyErr_SetString(PyExc_TypeError,
            "copyreg._slotnames didn't return a list or None");
        Py_DECREF(slotnames);
        slotnames = NULL;
    }

    return slotnames;
}

static PyObject *
reduce_2(PyObject *obj)
{
    PyObject *cls, *getnewargs;
    PyObject *args = NULL, *args2 = NULL;
    PyObject *getstate = NULL, *state = NULL, *names = NULL;
    PyObject *slots = NULL, *listitems = NULL, *dictitems = NULL;
    PyObject *copyreg = NULL, *newobj = NULL, *res = NULL;
    Py_ssize_t i, n;
    _Py_IDENTIFIER(__getnewargs__);
    _Py_IDENTIFIER(__getstate__);
    _Py_IDENTIFIER(__newobj__);

    cls = (PyObject *) Py_TYPE(obj);

    getnewargs = _PyObject_GetAttrId(obj, &PyId___getnewargs__);
    if (getnewargs != NULL) {
        args = PyObject_CallObject(getnewargs, NULL);
        Py_DECREF(getnewargs);
        if (args != NULL && !PyTuple_Check(args)) {
            PyErr_Format(PyExc_TypeError,
                "__getnewargs__ should return a tuple, "
                "not '%.200s'", Py_TYPE(args)->tp_name);
            goto end;
        }
    }
    else {
        PyErr_Clear();
        args = PyTuple_New(0);
    }
    if (args == NULL)
        goto end;

    getstate = _PyObject_GetAttrId(obj, &PyId___getstate__);
    if (getstate != NULL) {
        state = PyObject_CallObject(getstate, NULL);
        Py_DECREF(getstate);
        if (state == NULL)
            goto end;
    }
    else {
        PyObject **dict;
        PyErr_Clear();
        dict = _PyObject_GetDictPtr(obj);
        if (dict && *dict)
            state = *dict;
        else
            state = Py_None;
        Py_INCREF(state);
        names = slotnames(cls);
        if (names == NULL)
            goto end;
        if (names != Py_None && PyList_GET_SIZE(names) > 0) {
            assert(PyList_Check(names));
            slots = PyDict_New();
            if (slots == NULL)
                goto end;
            n = 0;
            /* Can't pre-compute the list size; the list
               is stored on the class so accessible to other
               threads, which may be run by DECREF */
            for (i = 0; i < PyList_GET_SIZE(names); i++) {
                PyObject *name, *value;
                name = PyList_GET_ITEM(names, i);
                value = PyObject_GetAttr(obj, name);
                if (value == NULL)
                    PyErr_Clear();
                else {
                    int err = PyDict_SetItem(slots, name,
                                             value);
                    Py_DECREF(value);
                    if (err)
                        goto end;
                    n++;
                }
            }
            if (n) {
                state = Py_BuildValue("(NO)", state, slots);
                if (state == NULL)
                    goto end;
            }
        }
    }

    if (!PyList_Check(obj)) {
        listitems = Py_None;
        Py_INCREF(listitems);
    }
    else {
        listitems = PyObject_GetIter(obj);
        if (listitems == NULL)
            goto end;
    }

    if (!PyDict_Check(obj)) {
        dictitems = Py_None;
        Py_INCREF(dictitems);
    }
    else {
        _Py_IDENTIFIER(items);
        PyObject *items = _PyObject_CallMethodId(obj, &PyId_items, "");
        if (items == NULL)
            goto end;
        dictitems = PyObject_GetIter(items);
        Py_DECREF(items);
        if (dictitems == NULL)
            goto end;
    }

    copyreg = import_copyreg();
    if (copyreg == NULL)
        goto end;
    newobj = _PyObject_GetAttrId(copyreg, &PyId___newobj__);
    if (newobj == NULL)
        goto end;

    n = PyTuple_GET_SIZE(args);
    args2 = PyTuple_New(n+1);
    if (args2 == NULL)
        goto end;
    Py_INCREF(cls);
    PyTuple_SET_ITEM(args2, 0, cls);
    for (i = 0; i < n; i++) {
        PyObject *v = PyTuple_GET_ITEM(args, i);
        Py_INCREF(v);
        PyTuple_SET_ITEM(args2, i+1, v);
    }

    res = PyTuple_Pack(5, newobj, args2, state, listitems, dictitems);

  end:
    Py_XDECREF(args);
    Py_XDECREF(args2);
    Py_XDECREF(slots);
    Py_XDECREF(state);
    Py_XDECREF(names);
    Py_XDECREF(listitems);
    Py_XDECREF(dictitems);
    Py_XDECREF(copyreg);
    Py_XDECREF(newobj);
    return res;
}

/*
 * There were two problems when object.__reduce__ and object.__reduce_ex__
 * were implemented in the same function:
 *  - trying to pickle an object with a custom __reduce__ method that
 *    fell back to object.__reduce__ in certain circumstances led to
 *    infinite recursion at Python level and eventual RuntimeError.
 *  - Pickling objects that lied about their type by overwriting the
 *    __class__ descriptor could lead to infinite recursion at C level
 *    and eventual segfault.
 *
 * Because of backwards compatibility, the two methods still have to
 * behave in the same way, even if this is not required by the pickle
 * protocol. This common functionality was moved to the _common_reduce
 * function.
 */
static PyObject *
_common_reduce(PyObject *self, int proto)
{
    PyObject *copyreg, *res;

    if (proto >= 2)
        return reduce_2(self);

    copyreg = import_copyreg();
    if (!copyreg)
        return NULL;

    res = PyEval_CallMethod(copyreg, "_reduce_ex", "(Oi)", self, proto);
    Py_DECREF(copyreg);

    return res;
}

static PyObject *
object_reduce(PyObject *self, PyObject *args)
{
    int proto = 0;

    if (!PyArg_ParseTuple(args, "|i:__reduce__", &proto))
        return NULL;

    return _common_reduce(self, proto);
}

static PyObject *
object_reduce_ex(PyObject *self, PyObject *args)
{
    static PyObject *objreduce;
    PyObject *reduce, *res;
    int proto = 0;
    _Py_IDENTIFIER(__reduce__);

    if (!PyArg_ParseTuple(args, "|i:__reduce_ex__", &proto))
        return NULL;

    if (objreduce == NULL) {
        objreduce = _PyDict_GetItemId(PyBaseObject_Type.tp_dict,
                                      &PyId___reduce__);
        if (objreduce == NULL)
            return NULL;
    }

    reduce = _PyObject_GetAttrId(self, &PyId___reduce__);
    if (reduce == NULL)
        PyErr_Clear();
    else {
        PyObject *cls, *clsreduce;
        int override;

        cls = (PyObject *) Py_TYPE(self);
        clsreduce = _PyObject_GetAttrId(cls, &PyId___reduce__);
        if (clsreduce == NULL) {
            Py_DECREF(reduce);
            return NULL;
        }
        override = (clsreduce != objreduce);
        Py_DECREF(clsreduce);
        if (override) {
            res = PyObject_CallObject(reduce, NULL);
            Py_DECREF(reduce);
            return res;
        }
        else
            Py_DECREF(reduce);
    }

    return _common_reduce(self, proto);
}

static PyObject *
object_subclasshook(PyObject *cls, PyObject *args)
{
    Py_RETURN_NOTIMPLEMENTED;
}

PyDoc_STRVAR(object_subclasshook_doc,
"Abstract classes can override this to customize issubclass().\n"
"\n"
"This is invoked early on by abc.ABCMeta.__subclasscheck__().\n"
"It should return True, False or NotImplemented.  If it returns\n"
"NotImplemented, the normal algorithm is used.  Otherwise, it\n"
"overrides the normal algorithm (and the outcome is cached).\n");

/*
   from PEP 3101, this code implements:

   class object:
       def __format__(self, format_spec):
       return format(str(self), format_spec)
*/
static PyObject *
object_format(PyObject *self, PyObject *args)
{
    PyObject *format_spec;
    PyObject *self_as_str = NULL;
    PyObject *result = NULL;

    if (!PyArg_ParseTuple(args, "U:__format__", &format_spec))
        return NULL;

    self_as_str = PyObject_Str(self);
    if (self_as_str != NULL) {
        /* Issue 7994: If we're converting to a string, we
           should reject format specifications */
        if (PyUnicode_GET_LENGTH(format_spec) > 0) {
            if (PyErr_WarnEx(PyExc_DeprecationWarning,
                             "object.__format__ with a non-empty format "
                             "string is deprecated", 1) < 0) {
              goto done;
            }
            /* Eventually this will become an error:
               PyErr_Format(PyExc_TypeError,
               "non-empty format string passed to object.__format__");
               goto done;
            */
        }

        result = PyObject_Format(self_as_str, format_spec);
    }

done:
    Py_XDECREF(self_as_str);

    return result;
}

static PyObject *
object_sizeof(PyObject *self, PyObject *args)
{
    Py_ssize_t res, isize;

    res = 0;
    isize = self->ob_type->tp_itemsize;
    if (isize > 0)
        res = Py_SIZE(self->ob_type) * isize;
    res += self->ob_type->tp_basicsize;

    return PyLong_FromSsize_t(res);
}

/* __dir__ for generic objects: returns __dict__, __class__,
   and recursively up the __class__.__bases__ chain.
*/
static PyObject *
object_dir(PyObject *self, PyObject *args)
{
    PyObject *result = NULL;
    PyObject *dict = NULL;
    PyObject *itsclass = NULL;

    /* Get __dict__ (which may or may not be a real dict...) */
    dict = _PyObject_GetAttrId(self, &PyId___dict__);
    if (dict == NULL) {
        PyErr_Clear();
        dict = PyDict_New();
    }
    else if (!PyDict_Check(dict)) {
        Py_DECREF(dict);
        dict = PyDict_New();
    }
    else {
        /* Copy __dict__ to avoid mutating it. */
        PyObject *temp = PyDict_Copy(dict);
        Py_DECREF(dict);
        dict = temp;
    }

    if (dict == NULL)
        goto error;

    /* Merge in attrs reachable from its class. */
    itsclass = _PyObject_GetAttrId(self, &PyId___class__);
    if (itsclass == NULL)
        /* XXX(tomer): Perhaps fall back to obj->ob_type if no
                       __class__ exists? */
        PyErr_Clear();
    else if (merge_class_dict(dict, itsclass) != 0)
        goto error;

    result = PyDict_Keys(dict);
    /* fall through */
error:
    Py_XDECREF(itsclass);
    Py_XDECREF(dict);
    return result;
}

static PyMethodDef object_methods[] = {
    {"__reduce_ex__", object_reduce_ex, METH_VARARGS,
     PyDoc_STR("helper for pickle")},
    {"__reduce__", object_reduce, METH_VARARGS,
     PyDoc_STR("helper for pickle")},
    {"__subclasshook__", object_subclasshook, METH_CLASS | METH_VARARGS,
     object_subclasshook_doc},
    {"__format__", object_format, METH_VARARGS,
     PyDoc_STR("default object formatter")},
    {"__sizeof__", object_sizeof, METH_NOARGS,
     PyDoc_STR("__sizeof__() -> int\nsize of object in memory, in bytes")},
    {"__dir__", object_dir, METH_NOARGS,
     PyDoc_STR("__dir__() -> list\ndefault dir() implementation")},
    {0}
};


PyTypeObject PyBaseObject_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "object",                                   /* tp_name */
    sizeof(PyObject),                           /* tp_basicsize */
    0,                                          /* tp_itemsize */
    object_dealloc,                             /* tp_dealloc */
    0,                                          /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_reserved */
    object_repr,                                /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    (hashfunc)_Py_HashPointer,                  /* tp_hash */
    0,                                          /* tp_call */
    object_str,                                 /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    PyObject_GenericSetAttr,                    /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /* tp_flags */
    PyDoc_STR("The most base type"),            /* tp_doc */
    0,                                          /* tp_traverse */
    0,                                          /* tp_clear */
    object_richcompare,                         /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    object_methods,                             /* tp_methods */
    0,                                          /* tp_members */
    object_getsets,                             /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    object_init,                                /* tp_init */
    PyType_GenericAlloc,                        /* tp_alloc */
    object_new,                                 /* tp_new */
    PyObject_Del,                               /* tp_free */
};


/* Add the methods from tp_methods to the __dict__ in a type object */

static int
add_methods(PyTypeObject *type, PyMethodDef *meth)
{
    PyObject *dict = type->tp_dict;

    for (; meth->ml_name != NULL; meth++) {
        PyObject *descr;
        if (PyDict_GetItemString(dict, meth->ml_name) &&
            !(meth->ml_flags & METH_COEXIST))
                continue;
        if (meth->ml_flags & METH_CLASS) {
            if (meth->ml_flags & METH_STATIC) {
                PyErr_SetString(PyExc_ValueError,
                     "method cannot be both class and static");
                return -1;
            }
            descr = PyDescr_NewClassMethod(type, meth);
        }
        else if (meth->ml_flags & METH_STATIC) {
            PyObject *cfunc = PyCFunction_New(meth, (PyObject*)type);
            if (cfunc == NULL)
                return -1;
            descr = PyStaticMethod_New(cfunc);
            Py_DECREF(cfunc);
        }
        else {
            descr = PyDescr_NewMethod(type, meth);
        }
        if (descr == NULL)
            return -1;
        if (PyDict_SetItemString(dict, meth->ml_name, descr) < 0)
            return -1;
        Py_DECREF(descr);
    }
    return 0;
}

static int
add_members(PyTypeObject *type, PyMemberDef *memb)
{
    PyObject *dict = type->tp_dict;

    for (; memb->name != NULL; memb++) {
        PyObject *descr;
        if (PyDict_GetItemString(dict, memb->name))
            continue;
        descr = PyDescr_NewMember(type, memb);
        if (descr == NULL)
            return -1;
        if (PyDict_SetItemString(dict, memb->name, descr) < 0)
            return -1;
        Py_DECREF(descr);
    }
    return 0;
}

static int
add_getset(PyTypeObject *type, PyGetSetDef *gsp)
{
    PyObject *dict = type->tp_dict;

    for (; gsp->name != NULL; gsp++) {
        PyObject *descr;
        if (PyDict_GetItemString(dict, gsp->name))
            continue;
        descr = PyDescr_NewGetSet(type, gsp);

        if (descr == NULL)
            return -1;
        if (PyDict_SetItemString(dict, gsp->name, descr) < 0)
            return -1;
        Py_DECREF(descr);
    }
    return 0;
}

static void
inherit_special(PyTypeObject *type, PyTypeObject *base)
{

    /* Copying basicsize is connected to the GC flags */
    if (!(type->tp_flags & Py_TPFLAGS_HAVE_GC) &&
        (base->tp_flags & Py_TPFLAGS_HAVE_GC) &&
        (!type->tp_traverse && !type->tp_clear)) {
        type->tp_flags |= Py_TPFLAGS_HAVE_GC;
        if (type->tp_traverse == NULL)
            type->tp_traverse = base->tp_traverse;
        if (type->tp_clear == NULL)
            type->tp_clear = base->tp_clear;
    }
    {
        /* The condition below could use some explanation.
           It appears that tp_new is not inherited for static types
           whose base class is 'object'; this seems to be a precaution
           so that old extension types don't suddenly become
           callable (object.__new__ wouldn't insure the invariants
           that the extension type's own factory function ensures).
           Heap types, of course, are under our control, so they do
           inherit tp_new; static extension types that specify some
           other built-in type as the default also
           inherit object.__new__. */
        if (base != &PyBaseObject_Type ||
            (type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
            if (type->tp_new == NULL)
                type->tp_new = base->tp_new;
        }
    }
    if (type->tp_basicsize == 0)
        type->tp_basicsize = base->tp_basicsize;

    /* Copy other non-function slots */

#undef COPYVAL
#define COPYVAL(SLOT) \
    if (type->SLOT == 0) type->SLOT = base->SLOT

    COPYVAL(tp_itemsize);
    COPYVAL(tp_weaklistoffset);
    COPYVAL(tp_dictoffset);

    /* Setup fast subclass flags */
    if (PyType_IsSubtype(base, (PyTypeObject*)PyExc_BaseException))
        type->tp_flags |= Py_TPFLAGS_BASE_EXC_SUBCLASS;
    else if (PyType_IsSubtype(base, &PyType_Type))
        type->tp_flags |= Py_TPFLAGS_TYPE_SUBCLASS;
    else if (PyType_IsSubtype(base, &PyLong_Type))
        type->tp_flags |= Py_TPFLAGS_LONG_SUBCLASS;
    else if (PyType_IsSubtype(base, &PyBytes_Type))
        type->tp_flags |= Py_TPFLAGS_BYTES_SUBCLASS;
    else if (PyType_IsSubtype(base, &PyUnicode_Type))
        type->tp_flags |= Py_TPFLAGS_UNICODE_SUBCLASS;
    else if (PyType_IsSubtype(base, &PyTuple_Type))
        type->tp_flags |= Py_TPFLAGS_TUPLE_SUBCLASS;
    else if (PyType_IsSubtype(base, &PyList_Type))
        type->tp_flags |= Py_TPFLAGS_LIST_SUBCLASS;
    else if (PyType_IsSubtype(base, &PyDict_Type))
        type->tp_flags |= Py_TPFLAGS_DICT_SUBCLASS;
}

static int
overrides_hash(PyTypeObject *type)
{
    PyObject *dict = type->tp_dict;
    _Py_IDENTIFIER(__eq__);

    assert(dict != NULL);
    if (_PyDict_GetItemId(dict, &PyId___eq__) != NULL)
        return 1;
    if (_PyDict_GetItemId(dict, &PyId___hash__) != NULL)
        return 1;
    return 0;
}

static void
inherit_slots(PyTypeObject *type, PyTypeObject *base)
{
    PyTypeObject *basebase;

#undef SLOTDEFINED
#undef COPYSLOT
#undef COPYNUM
#undef COPYSEQ
#undef COPYMAP
#undef COPYBUF

#define SLOTDEFINED(SLOT) \
    (base->SLOT != 0 && \
     (basebase == NULL || base->SLOT != basebase->SLOT))

#define COPYSLOT(SLOT) \
    if (!type->SLOT && SLOTDEFINED(SLOT)) type->SLOT = base->SLOT

#define COPYNUM(SLOT) COPYSLOT(tp_as_number->SLOT)
#define COPYSEQ(SLOT) COPYSLOT(tp_as_sequence->SLOT)
#define COPYMAP(SLOT) COPYSLOT(tp_as_mapping->SLOT)
#define COPYBUF(SLOT) COPYSLOT(tp_as_buffer->SLOT)

    /* This won't inherit indirect slots (from tp_as_number etc.)
       if type doesn't provide the space. */

    if (type->tp_as_number != NULL && base->tp_as_number != NULL) {
        basebase = base->tp_base;
        if (basebase->tp_as_number == NULL)
            basebase = NULL;
        COPYNUM(nb_add);
        COPYNUM(nb_subtract);
        COPYNUM(nb_multiply);
        COPYNUM(nb_remainder);
        COPYNUM(nb_divmod);
        COPYNUM(nb_power);
        COPYNUM(nb_negative);
        COPYNUM(nb_positive);
        COPYNUM(nb_absolute);
        COPYNUM(nb_bool);
        COPYNUM(nb_invert);
        COPYNUM(nb_lshift);
        COPYNUM(nb_rshift);
        COPYNUM(nb_and);
        COPYNUM(nb_xor);
        COPYNUM(nb_or);
        COPYNUM(nb_int);
        COPYNUM(nb_float);
        COPYNUM(nb_inplace_add);
        COPYNUM(nb_inplace_subtract);
        COPYNUM(nb_inplace_multiply);
        COPYNUM(nb_inplace_remainder);
        COPYNUM(nb_inplace_power);
        COPYNUM(nb_inplace_lshift);
        COPYNUM(nb_inplace_rshift);
        COPYNUM(nb_inplace_and);
        COPYNUM(nb_inplace_xor);
        COPYNUM(nb_inplace_or);
        COPYNUM(nb_true_divide);
        COPYNUM(nb_floor_divide);
        COPYNUM(nb_inplace_true_divide);
        COPYNUM(nb_inplace_floor_divide);
        COPYNUM(nb_index);
    }

    if (type->tp_as_sequence != NULL && base->tp_as_sequence != NULL) {
        basebase = base->tp_base;
        if (basebase->tp_as_sequence == NULL)
            basebase = NULL;
        COPYSEQ(sq_length);
        COPYSEQ(sq_concat);
        COPYSEQ(sq_repeat);
        COPYSEQ(sq_item);
        COPYSEQ(sq_ass_item);
        COPYSEQ(sq_contains);
        COPYSEQ(sq_inplace_concat);
        COPYSEQ(sq_inplace_repeat);
    }

    if (type->tp_as_mapping != NULL && base->tp_as_mapping != NULL) {
        basebase = base->tp_base;
        if (basebase->tp_as_mapping == NULL)
            basebase = NULL;
        COPYMAP(mp_length);
        COPYMAP(mp_subscript);
        COPYMAP(mp_ass_subscript);
    }

    if (type->tp_as_buffer != NULL && base->tp_as_buffer != NULL) {
        basebase = base->tp_base;
        if (basebase->tp_as_buffer == NULL)
            basebase = NULL;
        COPYBUF(bf_getbuffer);
        COPYBUF(bf_releasebuffer);
    }

    basebase = base->tp_base;

    COPYSLOT(tp_dealloc);
    if (type->tp_getattr == NULL && type->tp_getattro == NULL) {
        type->tp_getattr = base->tp_getattr;
        type->tp_getattro = base->tp_getattro;
    }
    if (type->tp_setattr == NULL && type->tp_setattro == NULL) {
        type->tp_setattr = base->tp_setattr;
        type->tp_setattro = base->tp_setattro;
    }
    /* tp_reserved is ignored */
    COPYSLOT(tp_repr);
    /* tp_hash see tp_richcompare */
    COPYSLOT(tp_call);
    COPYSLOT(tp_str);
    {
        /* Copy comparison-related slots only when
           not overriding them anywhere */
        if (type->tp_richcompare == NULL &&
            type->tp_hash == NULL &&
            !overrides_hash(type))
        {
            type->tp_richcompare = base->tp_richcompare;
            type->tp_hash = base->tp_hash;
        }
    }
    {
        COPYSLOT(tp_iter);
        COPYSLOT(tp_iternext);
    }
    {
        COPYSLOT(tp_descr_get);
        COPYSLOT(tp_descr_set);
        COPYSLOT(tp_dictoffset);
        COPYSLOT(tp_init);
        COPYSLOT(tp_alloc);
        COPYSLOT(tp_is_gc);
        if ((type->tp_flags & Py_TPFLAGS_HAVE_GC) ==
            (base->tp_flags & Py_TPFLAGS_HAVE_GC)) {
            /* They agree about gc. */
            COPYSLOT(tp_free);
        }
        else if ((type->tp_flags & Py_TPFLAGS_HAVE_GC) &&
                 type->tp_free == NULL &&
                 base->tp_free == PyObject_Free) {
            /* A bit of magic to plug in the correct default
             * tp_free function when a derived class adds gc,
             * didn't define tp_free, and the base uses the
             * default non-gc tp_free.
             */
            type->tp_free = PyObject_GC_Del;
        }
        /* else they didn't agree about gc, and there isn't something
         * obvious to be done -- the type is on its own.
         */
    }
}

static int add_operators(PyTypeObject *);

int
PyType_Ready(PyTypeObject *type)
{
    PyObject *dict, *bases;
    PyTypeObject *base;
    Py_ssize_t i, n;

    if (type->tp_flags & Py_TPFLAGS_READY) {
        assert(type->tp_dict != NULL);
        return 0;
    }
    assert((type->tp_flags & Py_TPFLAGS_READYING) == 0);

    type->tp_flags |= Py_TPFLAGS_READYING;

#ifdef Py_TRACE_REFS
    /* PyType_Ready is the closest thing we have to a choke point
     * for type objects, so is the best place I can think of to try
     * to get type objects into the doubly-linked list of all objects.
     * Still, not all type objects go thru PyType_Ready.
     */
    _Py_AddToAllObjects((PyObject *)type, 0);
#endif

    /* Initialize tp_base (defaults to BaseObject unless that's us) */
    base = type->tp_base;
    if (base == NULL && type != &PyBaseObject_Type) {
        base = type->tp_base = &PyBaseObject_Type;
        Py_INCREF(base);
    }

    /* Now the only way base can still be NULL is if type is
     * &PyBaseObject_Type.
     */

    /* Initialize the base class */
    if (base != NULL && base->tp_dict == NULL) {
        if (PyType_Ready(base) < 0)
            goto error;
    }

    /* Initialize ob_type if NULL.      This means extensions that want to be
       compilable separately on Windows can call PyType_Ready() instead of
       initializing the ob_type field of their type objects. */
    /* The test for base != NULL is really unnecessary, since base is only
       NULL when type is &PyBaseObject_Type, and we know its ob_type is
       not NULL (it's initialized to &PyType_Type).      But coverity doesn't
       know that. */
    if (Py_TYPE(type) == NULL && base != NULL)
        Py_TYPE(type) = Py_TYPE(base);

    /* Initialize tp_bases */
    bases = type->tp_bases;
    if (bases == NULL) {
        if (base == NULL)
            bases = PyTuple_New(0);
        else
            bases = PyTuple_Pack(1, base);
        if (bases == NULL)
            goto error;
        type->tp_bases = bases;
    }

    /* Initialize tp_dict */
    dict = type->tp_dict;
    if (dict == NULL) {
        dict = PyDict_New();
        if (dict == NULL)
            goto error;
        type->tp_dict = dict;
    }

    /* Add type-specific descriptors to tp_dict */
    if (add_operators(type) < 0)
        goto error;
    if (type->tp_methods != NULL) {
        if (add_methods(type, type->tp_methods) < 0)
            goto error;
    }
    if (type->tp_members != NULL) {
        if (add_members(type, type->tp_members) < 0)
            goto error;
    }
    if (type->tp_getset != NULL) {
        if (add_getset(type, type->tp_getset) < 0)
            goto error;
    }

    /* Calculate method resolution order */
    if (mro_internal(type) < 0) {
        goto error;
    }

    /* Inherit special flags from dominant base */
    if (type->tp_base != NULL)
        inherit_special(type, type->tp_base);

    /* Initialize tp_dict properly */
    bases = type->tp_mro;
    assert(bases != NULL);
    assert(PyTuple_Check(bases));
    n = PyTuple_GET_SIZE(bases);
    for (i = 1; i < n; i++) {
        PyObject *b = PyTuple_GET_ITEM(bases, i);
        if (PyType_Check(b))
            inherit_slots(type, (PyTypeObject *)b);
    }

    /* Sanity check for tp_free. */
    if (PyType_IS_GC(type) && (type->tp_flags & Py_TPFLAGS_BASETYPE) &&
        (type->tp_free == NULL || type->tp_free == PyObject_Del)) {
        /* This base class needs to call tp_free, but doesn't have
         * one, or its tp_free is for non-gc'ed objects.
         */
        PyErr_Format(PyExc_TypeError, "type '%.100s' participates in "
                     "gc and is a base type but has inappropriate "
                     "tp_free slot",
                     type->tp_name);
        goto error;
    }

    /* if the type dictionary doesn't contain a __doc__, set it from
       the tp_doc slot.
     */
    if (_PyDict_GetItemId(type->tp_dict, &PyId___doc__) == NULL) {
        if (type->tp_doc != NULL) {
            PyObject *doc = PyUnicode_FromString(type->tp_doc);
            if (doc == NULL)
                goto error;
            _PyDict_SetItemId(type->tp_dict, &PyId___doc__, doc);
            Py_DECREF(doc);
        } else {
            _PyDict_SetItemId(type->tp_dict,
                              &PyId___doc__, Py_None);
        }
    }

    /* Hack for tp_hash and __hash__.
       If after all that, tp_hash is still NULL, and __hash__ is not in
       tp_dict, set tp_hash to PyObject_HashNotImplemented and
       tp_dict['__hash__'] equal to None.
       This signals that __hash__ is not inherited.
     */
    if (type->tp_hash == NULL) {
        if (_PyDict_GetItemId(type->tp_dict, &PyId___hash__) == NULL) {
            if (_PyDict_SetItemId(type->tp_dict, &PyId___hash__, Py_None) < 0)
                goto error;
            type->tp_hash = PyObject_HashNotImplemented;
        }
    }

    /* Some more special stuff */
    base = type->tp_base;
    if (base != NULL) {
        if (type->tp_as_number == NULL)
            type->tp_as_number = base->tp_as_number;
        if (type->tp_as_sequence == NULL)
            type->tp_as_sequence = base->tp_as_sequence;
        if (type->tp_as_mapping == NULL)
            type->tp_as_mapping = base->tp_as_mapping;
        if (type->tp_as_buffer == NULL)
            type->tp_as_buffer = base->tp_as_buffer;
    }

    /* Link into each base class's list of subclasses */
    bases = type->tp_bases;
    n = PyTuple_GET_SIZE(bases);
    for (i = 0; i < n; i++) {
        PyObject *b = PyTuple_GET_ITEM(bases, i);
        if (PyType_Check(b) &&
            add_subclass((PyTypeObject *)b, type) < 0)
            goto error;
    }

    /* Warn for a type that implements tp_compare (now known as
       tp_reserved) but not tp_richcompare. */
    if (type->tp_reserved && !type->tp_richcompare) {
        int error;
        error = PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
            "Type %.100s defines tp_reserved (formerly tp_compare) "
            "but not tp_richcompare. Comparisons may not behave as intended.",
            type->tp_name);
        if (error == -1)
            goto error;
    }

    /* All done -- set the ready flag */
    assert(type->tp_dict != NULL);
    type->tp_flags =
        (type->tp_flags & ~Py_TPFLAGS_READYING) | Py_TPFLAGS_READY;
    return 0;

  error:
    type->tp_flags &= ~Py_TPFLAGS_READYING;
    return -1;
}

static int
add_subclass(PyTypeObject *base, PyTypeObject *type)
{
    Py_ssize_t i;
    int result;
    PyObject *list, *ref, *newobj;

    list = base->tp_subclasses;
    if (list == NULL) {
        base->tp_subclasses = list = PyList_New(0);
        if (list == NULL)
            return -1;
    }
    assert(PyList_Check(list));
    newobj = PyWeakref_NewRef((PyObject *)type, NULL);
    i = PyList_GET_SIZE(list);
    while (--i >= 0) {
        ref = PyList_GET_ITEM(list, i);
        assert(PyWeakref_CheckRef(ref));
        if (PyWeakref_GET_OBJECT(ref) == Py_None)
            return PyList_SetItem(list, i, newobj);
    }
    result = PyList_Append(list, newobj);
    Py_DECREF(newobj);
    return result;
}

static void
remove_subclass(PyTypeObject *base, PyTypeObject *type)
{
    Py_ssize_t i;
    PyObject *list, *ref;

    list = base->tp_subclasses;
    if (list == NULL) {
        return;
    }
    assert(PyList_Check(list));
    i = PyList_GET_SIZE(list);
    while (--i >= 0) {
        ref = PyList_GET_ITEM(list, i);
        assert(PyWeakref_CheckRef(ref));
        if (PyWeakref_GET_OBJECT(ref) == (PyObject*)type) {
            /* this can't fail, right? */
            PySequence_DelItem(list, i);
            return;
        }
    }
}

static int
check_num_args(PyObject *ob, int n)
{
    if (!PyTuple_CheckExact(ob)) {
        PyErr_SetString(PyExc_SystemError,
            "PyArg_UnpackTuple() argument list is not a tuple");
        return 0;
    }
    if (n == PyTuple_GET_SIZE(ob))
        return 1;
    PyErr_Format(
        PyExc_TypeError,
        "expected %d arguments, got %zd", n, PyTuple_GET_SIZE(ob));
    return 0;
}

/* Generic wrappers for overloadable 'operators' such as __getitem__ */

/* There's a wrapper *function* for each distinct function typedef used
   for type object slots (e.g. binaryfunc, ternaryfunc, etc.).  There's a
   wrapper *table* for each distinct operation (e.g. __len__, __add__).
   Most tables have only one entry; the tables for binary operators have two
   entries, one regular and one with reversed arguments. */

static PyObject *
wrap_lenfunc(PyObject *self, PyObject *args, void *wrapped)
{
    lenfunc func = (lenfunc)wrapped;
    Py_ssize_t res;

    if (!check_num_args(args, 0))
        return NULL;
    res = (*func)(self);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    return PyLong_FromLong((long)res);
}

static PyObject *
wrap_inquirypred(PyObject *self, PyObject *args, void *wrapped)
{
    inquiry func = (inquiry)wrapped;
    int res;

    if (!check_num_args(args, 0))
        return NULL;
    res = (*func)(self);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    return PyBool_FromLong((long)res);
}

static PyObject *
wrap_binaryfunc(PyObject *self, PyObject *args, void *wrapped)
{
    binaryfunc func = (binaryfunc)wrapped;
    PyObject *other;

    if (!check_num_args(args, 1))
        return NULL;
    other = PyTuple_GET_ITEM(args, 0);
    return (*func)(self, other);
}

static PyObject *
wrap_binaryfunc_l(PyObject *self, PyObject *args, void *wrapped)
{
    binaryfunc func = (binaryfunc)wrapped;
    PyObject *other;

    if (!check_num_args(args, 1))
        return NULL;
    other = PyTuple_GET_ITEM(args, 0);
    return (*func)(self, other);
}

static PyObject *
wrap_binaryfunc_r(PyObject *self, PyObject *args, void *wrapped)
{
    binaryfunc func = (binaryfunc)wrapped;
    PyObject *other;

    if (!check_num_args(args, 1))
        return NULL;
    other = PyTuple_GET_ITEM(args, 0);
    return (*func)(other, self);
}

static PyObject *
wrap_ternaryfunc(PyObject *self, PyObject *args, void *wrapped)
{
    ternaryfunc func = (ternaryfunc)wrapped;
    PyObject *other;
    PyObject *third = Py_None;

    /* Note: This wrapper only works for __pow__() */

    if (!PyArg_UnpackTuple(args, "", 1, 2, &other, &third))
        return NULL;
    return (*func)(self, other, third);
}

static PyObject *
wrap_ternaryfunc_r(PyObject *self, PyObject *args, void *wrapped)
{
    ternaryfunc func = (ternaryfunc)wrapped;
    PyObject *other;
    PyObject *third = Py_None;

    /* Note: This wrapper only works for __pow__() */

    if (!PyArg_UnpackTuple(args, "", 1, 2, &other, &third))
        return NULL;
    return (*func)(other, self, third);
}

static PyObject *
wrap_unaryfunc(PyObject *self, PyObject *args, void *wrapped)
{
    unaryfunc func = (unaryfunc)wrapped;

    if (!check_num_args(args, 0))
        return NULL;
    return (*func)(self);
}

static PyObject *
wrap_indexargfunc(PyObject *self, PyObject *args, void *wrapped)
{
    ssizeargfunc func = (ssizeargfunc)wrapped;
    PyObject* o;
    Py_ssize_t i;

    if (!PyArg_UnpackTuple(args, "", 1, 1, &o))
        return NULL;
    i = PyNumber_AsSsize_t(o, PyExc_OverflowError);
    if (i == -1 && PyErr_Occurred())
        return NULL;
    return (*func)(self, i);
}

static Py_ssize_t
getindex(PyObject *self, PyObject *arg)
{
    Py_ssize_t i;

    i = PyNumber_AsSsize_t(arg, PyExc_OverflowError);
    if (i == -1 && PyErr_Occurred())
        return -1;
    if (i < 0) {
        PySequenceMethods *sq = Py_TYPE(self)->tp_as_sequence;
        if (sq && sq->sq_length) {
            Py_ssize_t n = (*sq->sq_length)(self);
            if (n < 0)
                return -1;
            i += n;
        }
    }
    return i;
}

static PyObject *
wrap_sq_item(PyObject *self, PyObject *args, void *wrapped)
{
    ssizeargfunc func = (ssizeargfunc)wrapped;
    PyObject *arg;
    Py_ssize_t i;

    if (PyTuple_GET_SIZE(args) == 1) {
        arg = PyTuple_GET_ITEM(args, 0);
        i = getindex(self, arg);
        if (i == -1 && PyErr_Occurred())
            return NULL;
        return (*func)(self, i);
    }
    check_num_args(args, 1);
    assert(PyErr_Occurred());
    return NULL;
}

static PyObject *
wrap_sq_setitem(PyObject *self, PyObject *args, void *wrapped)
{
    ssizeobjargproc func = (ssizeobjargproc)wrapped;
    Py_ssize_t i;
    int res;
    PyObject *arg, *value;

    if (!PyArg_UnpackTuple(args, "", 2, 2, &arg, &value))
        return NULL;
    i = getindex(self, arg);
    if (i == -1 && PyErr_Occurred())
        return NULL;
    res = (*func)(self, i, value);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
wrap_sq_delitem(PyObject *self, PyObject *args, void *wrapped)
{
    ssizeobjargproc func = (ssizeobjargproc)wrapped;
    Py_ssize_t i;
    int res;
    PyObject *arg;

    if (!check_num_args(args, 1))
        return NULL;
    arg = PyTuple_GET_ITEM(args, 0);
    i = getindex(self, arg);
    if (i == -1 && PyErr_Occurred())
        return NULL;
    res = (*func)(self, i, NULL);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

/* XXX objobjproc is a misnomer; should be objargpred */
static PyObject *
wrap_objobjproc(PyObject *self, PyObject *args, void *wrapped)
{
    objobjproc func = (objobjproc)wrapped;
    int res;
    PyObject *value;

    if (!check_num_args(args, 1))
        return NULL;
    value = PyTuple_GET_ITEM(args, 0);
    res = (*func)(self, value);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    else
        return PyBool_FromLong(res);
}

static PyObject *
wrap_objobjargproc(PyObject *self, PyObject *args, void *wrapped)
{
    objobjargproc func = (objobjargproc)wrapped;
    int res;
    PyObject *key, *value;

    if (!PyArg_UnpackTuple(args, "", 2, 2, &key, &value))
        return NULL;
    res = (*func)(self, key, value);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
wrap_delitem(PyObject *self, PyObject *args, void *wrapped)
{
    objobjargproc func = (objobjargproc)wrapped;
    int res;
    PyObject *key;

    if (!check_num_args(args, 1))
        return NULL;
    key = PyTuple_GET_ITEM(args, 0);
    res = (*func)(self, key, NULL);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

/* Helper to check for object.__setattr__ or __delattr__ applied to a type.
   This is called the Carlo Verre hack after its discoverer. */
static int
hackcheck(PyObject *self, setattrofunc func, char *what)
{
    PyTypeObject *type = Py_TYPE(self);
    while (type && type->tp_flags & Py_TPFLAGS_HEAPTYPE)
        type = type->tp_base;
    /* If type is NULL now, this is a really weird type.
       In the spirit of backwards compatibility (?), just shut up. */
    if (type && type->tp_setattro != func) {
        PyErr_Format(PyExc_TypeError,
                     "can't apply this %s to %s object",
                     what,
                     type->tp_name);
        return 0;
    }
    return 1;
}

static PyObject *
wrap_setattr(PyObject *self, PyObject *args, void *wrapped)
{
    setattrofunc func = (setattrofunc)wrapped;
    int res;
    PyObject *name, *value;

    if (!PyArg_UnpackTuple(args, "", 2, 2, &name, &value))
        return NULL;
    if (!hackcheck(self, func, "__setattr__"))
        return NULL;
    res = (*func)(self, name, value);
    if (res < 0)
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
wrap_delattr(PyObject *self, PyObject *args, void *wrapped)
{
    setattrofunc func = (setattrofunc)wrapped;
    int res;
    PyObject *name;

    if (!check_num_args(args, 1))
        return NULL;
    name = PyTuple_GET_ITEM(args, 0);
    if (!hackcheck(self, func, "__delattr__"))
        return NULL;
    res = (*func)(self, name, NULL);
    if (res < 0)
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
wrap_hashfunc(PyObject *self, PyObject *args, void *wrapped)
{
    hashfunc func = (hashfunc)wrapped;
    Py_hash_t res;

    if (!check_num_args(args, 0))
        return NULL;
    res = (*func)(self);
    if (res == -1 && PyErr_Occurred())
        return NULL;
    return PyLong_FromSsize_t(res);
}

static PyObject *
wrap_call(PyObject *self, PyObject *args, void *wrapped, PyObject *kwds)
{
    ternaryfunc func = (ternaryfunc)wrapped;

    return (*func)(self, args, kwds);
}

static PyObject *
wrap_richcmpfunc(PyObject *self, PyObject *args, void *wrapped, int op)
{
    richcmpfunc func = (richcmpfunc)wrapped;
    PyObject *other;

    if (!check_num_args(args, 1))
        return NULL;
    other = PyTuple_GET_ITEM(args, 0);
    return (*func)(self, other, op);
}

#undef RICHCMP_WRAPPER
#define RICHCMP_WRAPPER(NAME, OP) \
static PyObject * \
richcmp_##NAME(PyObject *self, PyObject *args, void *wrapped) \
{ \
    return wrap_richcmpfunc(self, args, wrapped, OP); \
}

RICHCMP_WRAPPER(lt, Py_LT)
RICHCMP_WRAPPER(le, Py_LE)
RICHCMP_WRAPPER(eq, Py_EQ)
RICHCMP_WRAPPER(ne, Py_NE)
RICHCMP_WRAPPER(gt, Py_GT)
RICHCMP_WRAPPER(ge, Py_GE)

static PyObject *
wrap_next(PyObject *self, PyObject *args, void *wrapped)
{
    unaryfunc func = (unaryfunc)wrapped;
    PyObject *res;

    if (!check_num_args(args, 0))
        return NULL;
    res = (*func)(self);
    if (res == NULL && !PyErr_Occurred())
        PyErr_SetNone(PyExc_StopIteration);
    return res;
}

static PyObject *
wrap_descr_get(PyObject *self, PyObject *args, void *wrapped)
{
    descrgetfunc func = (descrgetfunc)wrapped;
    PyObject *obj;
    PyObject *type = NULL;

    if (!PyArg_UnpackTuple(args, "", 1, 2, &obj, &type))
        return NULL;
    if (obj == Py_None)
        obj = NULL;
    if (type == Py_None)
        type = NULL;
    if (type == NULL &&obj == NULL) {
        PyErr_SetString(PyExc_TypeError,
                        "__get__(None, None) is invalid");
        return NULL;
    }
    return (*func)(self, obj, type);
}

static PyObject *
wrap_descr_set(PyObject *self, PyObject *args, void *wrapped)
{
    descrsetfunc func = (descrsetfunc)wrapped;
    PyObject *obj, *value;
    int ret;

    if (!PyArg_UnpackTuple(args, "", 2, 2, &obj, &value))
        return NULL;
    ret = (*func)(self, obj, value);
    if (ret < 0)
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
wrap_descr_delete(PyObject *self, PyObject *args, void *wrapped)
{
    descrsetfunc func = (descrsetfunc)wrapped;
    PyObject *obj;
    int ret;

    if (!check_num_args(args, 1))
        return NULL;
    obj = PyTuple_GET_ITEM(args, 0);
    ret = (*func)(self, obj, NULL);
    if (ret < 0)
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
wrap_init(PyObject *self, PyObject *args, void *wrapped, PyObject *kwds)
{
    initproc func = (initproc)wrapped;

    if (func(self, args, kwds) < 0)
        return NULL;
    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
tp_new_wrapper(PyObject *self, PyObject *args, PyObject *kwds)
{
    PyTypeObject *type, *subtype, *staticbase;
    PyObject *arg0, *res;

    if (self == NULL || !PyType_Check(self))
        Py_FatalError("__new__() called with non-type 'self'");
    type = (PyTypeObject *)self;
    if (!PyTuple_Check(args) || PyTuple_GET_SIZE(args) < 1) {
        PyErr_Format(PyExc_TypeError,
                     "%s.__new__(): not enough arguments",
                     type->tp_name);
        return NULL;
    }
    arg0 = PyTuple_GET_ITEM(args, 0);
    if (!PyType_Check(arg0)) {
        PyErr_Format(PyExc_TypeError,
                     "%s.__new__(X): X is not a type object (%s)",
                     type->tp_name,
                     Py_TYPE(arg0)->tp_name);
        return NULL;
    }
    subtype = (PyTypeObject *)arg0;
    if (!PyType_IsSubtype(subtype, type)) {
        PyErr_Format(PyExc_TypeError,
                     "%s.__new__(%s): %s is not a subtype of %s",
                     type->tp_name,
                     subtype->tp_name,
                     subtype->tp_name,
                     type->tp_name);
        return NULL;
    }

    /* Check that the use doesn't do something silly and unsafe like
       object.__new__(dict).  To do this, we check that the
       most derived base that's not a heap type is this type. */
    staticbase = subtype;
    while (staticbase && (staticbase->tp_flags & Py_TPFLAGS_HEAPTYPE))
        staticbase = staticbase->tp_base;
    /* If staticbase is NULL now, it is a really weird type.
       In the spirit of backwards compatibility (?), just shut up. */
    if (staticbase && staticbase->tp_new != type->tp_new) {
        PyErr_Format(PyExc_TypeError,
                     "%s.__new__(%s) is not safe, use %s.__new__()",
                     type->tp_name,
                     subtype->tp_name,
                     staticbase == NULL ? "?" : staticbase->tp_name);
        return NULL;
    }

    args = PyTuple_GetSlice(args, 1, PyTuple_GET_SIZE(args));
    if (args == NULL)
        return NULL;
    res = type->tp_new(subtype, args, kwds);
    Py_DECREF(args);
    return res;
}

static struct PyMethodDef tp_new_methoddef[] = {
    {"__new__", (PyCFunction)tp_new_wrapper, METH_VARARGS|METH_KEYWORDS,
     PyDoc_STR("T.__new__(S, ...) -> "
               "a new object with type S, a subtype of T")},
    {0}
};

static int
add_tp_new_wrapper(PyTypeObject *type)
{
    PyObject *func;

    if (_PyDict_GetItemId(type->tp_dict, &PyId___new__) != NULL)
        return 0;
    func = PyCFunction_New(tp_new_methoddef, (PyObject *)type);
    if (func == NULL)
        return -1;
    if (_PyDict_SetItemId(type->tp_dict, &PyId___new__, func)) {
        Py_DECREF(func);
        return -1;
    }
    Py_DECREF(func);
    return 0;
}

/* Slot wrappers that call the corresponding __foo__ slot.  See comments
   below at override_slots() for more explanation. */

#define SLOT0(FUNCNAME, OPSTR) \
static PyObject * \
FUNCNAME(PyObject *self) \
{ \
    _Py_static_string(id, OPSTR); \
    return call_method(self, &id, "()"); \
}

#define SLOT1(FUNCNAME, OPSTR, ARG1TYPE, ARGCODES) \
static PyObject * \
FUNCNAME(PyObject *self, ARG1TYPE arg1) \
{ \
    _Py_static_string(id, OPSTR); \
    return call_method(self, &id, "(" ARGCODES ")", arg1); \
}

/* Boolean helper for SLOT1BINFULL().
   right.__class__ is a nontrivial subclass of left.__class__. */
static int
method_is_overloaded(PyObject *left, PyObject *right, struct _Py_Identifier *name)
{
    PyObject *a, *b;
    int ok;

    b = _PyObject_GetAttrId((PyObject *)(Py_TYPE(right)), name);
    if (b == NULL) {
        PyErr_Clear();
        /* If right doesn't have it, it's not overloaded */
        return 0;
    }

    a = _PyObject_GetAttrId((PyObject *)(Py_TYPE(left)), name);
    if (a == NULL) {
        PyErr_Clear();
        Py_DECREF(b);
        /* If right has it but left doesn't, it's overloaded */
        return 1;
    }

    ok = PyObject_RichCompareBool(a, b, Py_NE);
    Py_DECREF(a);
    Py_DECREF(b);
    if (ok < 0) {
        PyErr_Clear();
        return 0;
    }

    return ok;
}


#define SLOT1BINFULL(FUNCNAME, TESTFUNC, SLOTNAME, OPSTR, ROPSTR) \
static PyObject * \
FUNCNAME(PyObject *self, PyObject *other) \
{ \
    _Py_static_string(op_id, OPSTR); \
    _Py_static_string(rop_id, ROPSTR); \
    int do_other = Py_TYPE(self) != Py_TYPE(other) && \
        Py_TYPE(other)->tp_as_number != NULL && \
        Py_TYPE(other)->tp_as_number->SLOTNAME == TESTFUNC; \
    if (Py_TYPE(self)->tp_as_number != NULL && \
        Py_TYPE(self)->tp_as_number->SLOTNAME == TESTFUNC) { \
        PyObject *r; \
        if (do_other && \
            PyType_IsSubtype(Py_TYPE(other), Py_TYPE(self)) && \
            method_is_overloaded(self, other, &rop_id)) { \
            r = call_maybe(other, &rop_id, "(O)", self); \
            if (r != Py_NotImplemented) \
                return r; \
            Py_DECREF(r); \
            do_other = 0; \
        } \
        r = call_maybe(self, &op_id, "(O)", other); \
        if (r != Py_NotImplemented || \
            Py_TYPE(other) == Py_TYPE(self)) \
            return r; \
        Py_DECREF(r); \
    } \
    if (do_other) { \
        return call_maybe(other, &rop_id, "(O)", self); \
    } \
    Py_RETURN_NOTIMPLEMENTED; \
}

#define SLOT1BIN(FUNCNAME, SLOTNAME, OPSTR, ROPSTR) \
    SLOT1BINFULL(FUNCNAME, FUNCNAME, SLOTNAME, OPSTR, ROPSTR)

#define SLOT2(FUNCNAME, OPSTR, ARG1TYPE, ARG2TYPE, ARGCODES) \
static PyObject * \
FUNCNAME(PyObject *self, ARG1TYPE arg1, ARG2TYPE arg2) \
{ \
    _Py_static_string(id, #OPSTR); \
    return call_method(self, &id, "(" ARGCODES ")", arg1, arg2); \
}

static Py_ssize_t
slot_sq_length(PyObject *self)
{
    _Py_IDENTIFIER(__len__);
    PyObject *res = call_method(self, &PyId___len__, "()");
    Py_ssize_t len;

    if (res == NULL)
        return -1;
    len = PyNumber_AsSsize_t(res, PyExc_OverflowError);
    Py_DECREF(res);
    if (len < 0) {
        if (!PyErr_Occurred())
            PyErr_SetString(PyExc_ValueError,
                            "__len__() should return >= 0");
        return -1;
    }
    return len;
}

/* Super-optimized version of slot_sq_item.
   Other slots could do the same... */
static PyObject *
slot_sq_item(PyObject *self, Py_ssize_t i)
{
    PyObject *func, *args = NULL, *ival = NULL, *retval = NULL;
    descrgetfunc f;

    func = _PyType_LookupId(Py_TYPE(self), &PyId___getitem__);
    if (func != NULL) {
        if ((f = Py_TYPE(func)->tp_descr_get) == NULL)
            Py_INCREF(func);
        else {
            func = f(func, self, (PyObject *)(Py_TYPE(self)));
            if (func == NULL) {
                return NULL;
            }
        }
        ival = PyLong_FromSsize_t(i);
        if (ival != NULL) {
            args = PyTuple_New(1);
            if (args != NULL) {
                PyTuple_SET_ITEM(args, 0, ival);
                retval = PyObject_Call(func, args, NULL);
                Py_XDECREF(args);
                Py_XDECREF(func);
                return retval;
            }
        }
    }
    else {
        PyObject *getitem_str = _PyUnicode_FromId(&PyId___getitem__);
        PyErr_SetObject(PyExc_AttributeError, getitem_str);
    }
    Py_XDECREF(args);
    Py_XDECREF(ival);
    Py_XDECREF(func);
    return NULL;
}

static int
slot_sq_ass_item(PyObject *self, Py_ssize_t index, PyObject *value)
{
    PyObject *res;
    _Py_IDENTIFIER(__delitem__);
    _Py_IDENTIFIER(__setitem__);

    if (value == NULL)
        res = call_method(self, &PyId___delitem__, "(n)", index);
    else
        res = call_method(self, &PyId___setitem__, "(nO)", index, value);
    if (res == NULL)
        return -1;
    Py_DECREF(res);
    return 0;
}

static int
slot_sq_contains(PyObject *self, PyObject *value)
{
    PyObject *func, *res, *args;
    int result = -1;
    _Py_IDENTIFIER(__contains__);

    func = lookup_maybe(self, &PyId___contains__);
    if (func != NULL) {
        args = PyTuple_Pack(1, value);
        if (args == NULL)
            res = NULL;
        else {
            res = PyObject_Call(func, args, NULL);
            Py_DECREF(args);
        }
        Py_DECREF(func);
        if (res != NULL) {
            result = PyObject_IsTrue(res);
            Py_DECREF(res);
        }
    }
    else if (! PyErr_Occurred()) {
        /* Possible results: -1 and 1 */
        result = (int)_PySequence_IterSearch(self, value,
                                         PY_ITERSEARCH_CONTAINS);
    }
    return result;
}

#define slot_mp_length slot_sq_length

SLOT1(slot_mp_subscript, "__getitem__", PyObject *, "O")

static int
slot_mp_ass_subscript(PyObject *self, PyObject *key, PyObject *value)
{
    PyObject *res;
    _Py_IDENTIFIER(__delitem__);
    _Py_IDENTIFIER(__setitem__);

    if (value == NULL)
        res = call_method(self, &PyId___delitem__, "(O)", key);
    else
        res = call_method(self, &PyId___setitem__, "(OO)", key, value);

    if (res == NULL)
        return -1;
    Py_DECREF(res);
    return 0;
}

SLOT1BIN(slot_nb_add, nb_add, "__add__", "__radd__")
SLOT1BIN(slot_nb_subtract, nb_subtract, "__sub__", "__rsub__")
SLOT1BIN(slot_nb_multiply, nb_multiply, "__mul__", "__rmul__")
SLOT1BIN(slot_nb_remainder, nb_remainder, "__mod__", "__rmod__")
SLOT1BIN(slot_nb_divmod, nb_divmod, "__divmod__", "__rdivmod__")

static PyObject *slot_nb_power(PyObject *, PyObject *, PyObject *);

SLOT1BINFULL(slot_nb_power_binary, slot_nb_power,
             nb_power, "__pow__", "__rpow__")

static PyObject *
slot_nb_power(PyObject *self, PyObject *other, PyObject *modulus)
{
    _Py_IDENTIFIER(__pow__);

    if (modulus == Py_None)
        return slot_nb_power_binary(self, other);
    /* Three-arg power doesn't use __rpow__.  But ternary_op
       can call this when the second argument's type uses
       slot_nb_power, so check before calling self.__pow__. */
    if (Py_TYPE(self)->tp_as_number != NULL &&
        Py_TYPE(self)->tp_as_number->nb_power == slot_nb_power) {
        return call_method(self, &PyId___pow__, "(OO)", other, modulus);
    }
    Py_RETURN_NOTIMPLEMENTED;
}

SLOT0(slot_nb_negative, "__neg__")
SLOT0(slot_nb_positive, "__pos__")
SLOT0(slot_nb_absolute, "__abs__")

static int
slot_nb_bool(PyObject *self)
{
    PyObject *func, *args;
    int result = -1;
    int using_len = 0;
    _Py_IDENTIFIER(__len__);
    _Py_IDENTIFIER(__bool__);

    func = lookup_maybe(self, &PyId___bool__);
    if (func == NULL) {
        if (PyErr_Occurred())
            return -1;
        func = lookup_maybe(self, &PyId___len__);
        if (func == NULL)
            return PyErr_Occurred() ? -1 : 1;
        using_len = 1;
    }
    args = PyTuple_New(0);
    if (args != NULL) {
        PyObject *temp = PyObject_Call(func, args, NULL);
        Py_DECREF(args);
        if (temp != NULL) {
            if (using_len) {
                /* enforced by slot_nb_len */
                result = PyObject_IsTrue(temp);
            }
            else if (PyBool_Check(temp)) {
                result = PyObject_IsTrue(temp);
            }
            else {
                PyErr_Format(PyExc_TypeError,
                             "__bool__ should return "
                             "bool, returned %s",
                             Py_TYPE(temp)->tp_name);
                result = -1;
            }
            Py_DECREF(temp);
        }
    }
    Py_DECREF(func);
    return result;
}


static PyObject *
slot_nb_index(PyObject *self)
{
    _Py_IDENTIFIER(__index__);
    return call_method(self, &PyId___index__, "()");
}


SLOT0(slot_nb_invert, "__invert__")
SLOT1BIN(slot_nb_lshift, nb_lshift, "__lshift__", "__rlshift__")
SLOT1BIN(slot_nb_rshift, nb_rshift, "__rshift__", "__rrshift__")
SLOT1BIN(slot_nb_and, nb_and, "__and__", "__rand__")
SLOT1BIN(slot_nb_xor, nb_xor, "__xor__", "__rxor__")
SLOT1BIN(slot_nb_or, nb_or, "__or__", "__ror__")

SLOT0(slot_nb_int, "__int__")
SLOT0(slot_nb_float, "__float__")
SLOT1(slot_nb_inplace_add, "__iadd__", PyObject *, "O")
SLOT1(slot_nb_inplace_subtract, "__isub__", PyObject *, "O")
SLOT1(slot_nb_inplace_multiply, "__imul__", PyObject *, "O")
SLOT1(slot_nb_inplace_remainder, "__imod__", PyObject *, "O")
/* Can't use SLOT1 here, because nb_inplace_power is ternary */
static PyObject *
slot_nb_inplace_power(PyObject *self, PyObject * arg1, PyObject *arg2)
{
    _Py_IDENTIFIER(__ipow__);
    return call_method(self, &PyId___ipow__, "(" "O" ")", arg1);
}
SLOT1(slot_nb_inplace_lshift, "__ilshift__", PyObject *, "O")
SLOT1(slot_nb_inplace_rshift, "__irshift__", PyObject *, "O")
SLOT1(slot_nb_inplace_and, "__iand__", PyObject *, "O")
SLOT1(slot_nb_inplace_xor, "__ixor__", PyObject *, "O")
SLOT1(slot_nb_inplace_or, "__ior__", PyObject *, "O")
SLOT1BIN(slot_nb_floor_divide, nb_floor_divide,
         "__floordiv__", "__rfloordiv__")
SLOT1BIN(slot_nb_true_divide, nb_true_divide, "__truediv__", "__rtruediv__")
SLOT1(slot_nb_inplace_floor_divide, "__ifloordiv__", PyObject *, "O")
SLOT1(slot_nb_inplace_true_divide, "__itruediv__", PyObject *, "O")

static PyObject *
slot_tp_repr(PyObject *self)
{
    PyObject *func, *res;
    _Py_IDENTIFIER(__repr__);

    func = lookup_method(self, &PyId___repr__);
    if (func != NULL) {
        res = PyEval_CallObject(func, NULL);
        Py_DECREF(func);
        return res;
    }
    PyErr_Clear();
    return PyUnicode_FromFormat("<%s object at %p>",
                               Py_TYPE(self)->tp_name, self);
}

static PyObject *
slot_tp_str(PyObject *self)
{
    PyObject *func, *res;
    _Py_IDENTIFIER(__str__);

    func = lookup_method(self, &PyId___str__);
    if (func != NULL) {
        res = PyEval_CallObject(func, NULL);
        Py_DECREF(func);
        return res;
    }
    else {
        /* PyObject *ress; */
        PyErr_Clear();
        res = slot_tp_repr(self);
        if (!res)
            return NULL;
        /* XXX this is non-sensical. Why should we return
           a bytes object from __str__. Is this code even
           used? - mvl */
        assert(0);
        return res;
        /*
        ress = _PyUnicode_AsDefaultEncodedString(res);
        Py_DECREF(res);
        return ress;
        */
    }
}

static Py_hash_t
slot_tp_hash(PyObject *self)
{
    PyObject *func, *res;
    Py_ssize_t h;

    func = lookup_method(self, &PyId___hash__);

    if (func == Py_None) {
        Py_DECREF(func);
        func = NULL;
    }

    if (func == NULL) {
        return PyObject_HashNotImplemented(self);
    }

    res = PyEval_CallObject(func, NULL);
    Py_DECREF(func);
    if (res == NULL)
        return -1;

    if (!PyLong_Check(res)) {
        PyErr_SetString(PyExc_TypeError,
                        "__hash__ method should return an integer");
        return -1;
    }
    /* Transform the PyLong `res` to a Py_hash_t `h`.  For an existing
       hashable Python object x, hash(x) will always lie within the range of
       Py_hash_t.  Therefore our transformation must preserve values that
       already lie within this range, to ensure that if x.__hash__() returns
       hash(y) then hash(x) == hash(y). */
    h = PyLong_AsSsize_t(res);
    if (h == -1 && PyErr_Occurred()) {
        /* res was not within the range of a Py_hash_t, so we're free to
           use any sufficiently bit-mixing transformation;
           long.__hash__ will do nicely. */
        PyErr_Clear();
        h = PyLong_Type.tp_hash(res);
    }
    /* -1 is reserved for errors. */
    if (h == -1)
        h = -2;
    Py_DECREF(res);
    return h;
}

static PyObject *
slot_tp_call(PyObject *self, PyObject *args, PyObject *kwds)
{
    _Py_IDENTIFIER(__call__);
    PyObject *meth = lookup_method(self, &PyId___call__);
    PyObject *res;

    if (meth == NULL)
        return NULL;

    res = PyObject_Call(meth, args, kwds);

    Py_DECREF(meth);
    return res;
}

/* There are two slot dispatch functions for tp_getattro.

   - slot_tp_getattro() is used when __getattribute__ is overridden
     but no __getattr__ hook is present;

   - slot_tp_getattr_hook() is used when a __getattr__ hook is present.

   The code in update_one_slot() always installs slot_tp_getattr_hook(); this
   detects the absence of __getattr__ and then installs the simpler slot if
   necessary. */

static PyObject *
slot_tp_getattro(PyObject *self, PyObject *name)
{
    return call_method(self, &PyId___getattribute__, "(O)", name);
}

static PyObject *
call_attribute(PyObject *self, PyObject *attr, PyObject *name)
{
    PyObject *res, *descr = NULL;
    descrgetfunc f = Py_TYPE(attr)->tp_descr_get;

    if (f != NULL) {
        descr = f(attr, self, (PyObject *)(Py_TYPE(self)));
        if (descr == NULL)
            return NULL;
        else
            attr = descr;
    }
    res = PyObject_CallFunctionObjArgs(attr, name, NULL);
    Py_XDECREF(descr);
    return res;
}

static PyObject *
slot_tp_getattr_hook(PyObject *self, PyObject *name)
{
    PyTypeObject *tp = Py_TYPE(self);
    PyObject *getattr, *getattribute, *res;
    _Py_IDENTIFIER(__getattr__);

    /* speed hack: we could use lookup_maybe, but that would resolve the
       method fully for each attribute lookup for classes with
       __getattr__, even when the attribute is present. So we use
       _PyType_Lookup and create the method only when needed, with
       call_attribute. */
    getattr = _PyType_LookupId(tp, &PyId___getattr__);
    if (getattr == NULL) {
        /* No __getattr__ hook: use a simpler dispatcher */
        tp->tp_getattro = slot_tp_getattro;
        return slot_tp_getattro(self, name);
    }
    Py_INCREF(getattr);
    /* speed hack: we could use lookup_maybe, but that would resolve the
       method fully for each attribute lookup for classes with
       __getattr__, even when self has the default __getattribute__
       method. So we use _PyType_Lookup and create the method only when
       needed, with call_attribute. */
    getattribute = _PyType_LookupId(tp, &PyId___getattribute__);
    if (getattribute == NULL ||
        (Py_TYPE(getattribute) == &PyWrapperDescr_Type &&
         ((PyWrapperDescrObject *)getattribute)->d_wrapped ==
         (void *)PyObject_GenericGetAttr))
        res = PyObject_GenericGetAttr(self, name);
    else {
        Py_INCREF(getattribute);
        res = call_attribute(self, getattribute, name);
        Py_DECREF(getattribute);
    }
    if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
        PyErr_Clear();
        res = call_attribute(self, getattr, name);
    }
    Py_DECREF(getattr);
    return res;
}

static int
slot_tp_setattro(PyObject *self, PyObject *name, PyObject *value)
{
    PyObject *res;
    _Py_IDENTIFIER(__delattr__);
    _Py_IDENTIFIER(__setattr__);

    if (value == NULL)
        res = call_method(self, &PyId___delattr__, "(O)", name);
    else
        res = call_method(self, &PyId___setattr__, "(OO)", name, value);
    if (res == NULL)
        return -1;
    Py_DECREF(res);
    return 0;
}

static _Py_Identifier name_op[] = {
    {0, "__lt__", 0},
    {0, "__le__", 0},
    {0, "__eq__", 0},
    {0, "__ne__", 0},
    {0, "__gt__", 0},
    {0, "__ge__", 0}
};

static PyObject *
slot_tp_richcompare(PyObject *self, PyObject *other, int op)
{
    PyObject *func, *args, *res;

    func = lookup_method(self, &name_op[op]);
    if (func == NULL) {
        PyErr_Clear();
        Py_RETURN_NOTIMPLEMENTED;
    }
    args = PyTuple_Pack(1, other);
    if (args == NULL)
        res = NULL;
    else {
        res = PyObject_Call(func, args, NULL);
        Py_DECREF(args);
    }
    Py_DECREF(func);
    return res;
}

static PyObject *
slot_tp_iter(PyObject *self)
{
    PyObject *func, *res;
    _Py_IDENTIFIER(__iter__);

    func = lookup_method(self, &PyId___iter__);
    if (func != NULL) {
        PyObject *args;
        args = res = PyTuple_New(0);
        if (args != NULL) {
            res = PyObject_Call(func, args, NULL);
            Py_DECREF(args);
        }
        Py_DECREF(func);
        return res;
    }
    PyErr_Clear();
    func = lookup_method(self, &PyId___getitem__);
    if (func == NULL) {
        PyErr_Format(PyExc_TypeError,
                     "'%.200s' object is not iterable",
                     Py_TYPE(self)->tp_name);
        return NULL;
    }
    Py_DECREF(func);
    return PySeqIter_New(self);
}

static PyObject *
slot_tp_iternext(PyObject *self)
{
    _Py_IDENTIFIER(__next__);
    return call_method(self, &PyId___next__, "()");
}

static PyObject *
slot_tp_descr_get(PyObject *self, PyObject *obj, PyObject *type)
{
    PyTypeObject *tp = Py_TYPE(self);
    PyObject *get;
    _Py_IDENTIFIER(__get__);

    get = _PyType_LookupId(tp, &PyId___get__);
    if (get == NULL) {
        /* Avoid further slowdowns */
        if (tp->tp_descr_get == slot_tp_descr_get)
            tp->tp_descr_get = NULL;
        Py_INCREF(self);
        return self;
    }
    if (obj == NULL)
        obj = Py_None;
    if (type == NULL)
        type = Py_None;
    return PyObject_CallFunctionObjArgs(get, self, obj, type, NULL);
}

static int
slot_tp_descr_set(PyObject *self, PyObject *target, PyObject *value)
{
    PyObject *res;
    _Py_IDENTIFIER(__delete__);
    _Py_IDENTIFIER(__set__);

    if (value == NULL)
        res = call_method(self, &PyId___delete__, "(O)", target);
    else
        res = call_method(self, &PyId___set__, "(OO)", target, value);
    if (res == NULL)
        return -1;
    Py_DECREF(res);
    return 0;
}

static int
slot_tp_init(PyObject *self, PyObject *args, PyObject *kwds)
{
    _Py_IDENTIFIER(__init__);
    PyObject *meth = lookup_method(self, &PyId___init__);
    PyObject *res;

    if (meth == NULL)
        return -1;
    res = PyObject_Call(meth, args, kwds);
    Py_DECREF(meth);
    if (res == NULL)
        return -1;
    if (res != Py_None) {
        PyErr_Format(PyExc_TypeError,
                     "__init__() should return None, not '%.200s'",
                     Py_TYPE(res)->tp_name);
        Py_DECREF(res);
        return -1;
    }
    Py_DECREF(res);
    return 0;
}

static PyObject *
slot_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    PyObject *func;
    PyObject *newargs, *x;
    Py_ssize_t i, n;
    _Py_IDENTIFIER(__new__);

    func = _PyObject_GetAttrId((PyObject *)type, &PyId___new__);
    if (func == NULL)
        return NULL;
    assert(PyTuple_Check(args));
    n = PyTuple_GET_SIZE(args);
    newargs = PyTuple_New(n+1);
    if (newargs == NULL)
        return NULL;
    Py_INCREF(type);
    PyTuple_SET_ITEM(newargs, 0, (PyObject *)type);
    for (i = 0; i < n; i++) {
        x = PyTuple_GET_ITEM(args, i);
        Py_INCREF(x);
        PyTuple_SET_ITEM(newargs, i+1, x);
    }
    x = PyObject_Call(func, newargs, kwds);
    Py_DECREF(newargs);
    Py_DECREF(func);
    return x;
}

static void
slot_tp_del(PyObject *self)
{
    _Py_IDENTIFIER(__del__);
    PyObject *del, *res;
    PyObject *error_type, *error_value, *error_traceback;

    /* Temporarily resurrect the object. */
    assert(self->ob_refcnt == 0);
    self->ob_refcnt = 1;

    /* Save the current exception, if any. */
    PyErr_Fetch(&error_type, &error_value, &error_traceback);

    /* Execute __del__ method, if any. */
    del = lookup_maybe(self, &PyId___del__);
    if (del != NULL) {
        res = PyEval_CallObject(del, NULL);
        if (res == NULL)
            PyErr_WriteUnraisable(del);
        else
            Py_DECREF(res);
        Py_DECREF(del);
    }

    /* Restore the saved exception. */
    PyErr_Restore(error_type, error_value, error_traceback);

    /* Undo the temporary resurrection; can't use DECREF here, it would
     * cause a recursive call.
     */
    assert(self->ob_refcnt > 0);
    if (--self->ob_refcnt == 0)
        return;         /* this is the normal path out */

    /* __del__ resurrected it!  Make it look like the original Py_DECREF
     * never happened.
     */
    {
        Py_ssize_t refcnt = self->ob_refcnt;
        _Py_NewReference(self);
        self->ob_refcnt = refcnt;
    }
    assert(!PyType_IS_GC(Py_TYPE(self)) ||
           _Py_AS_GC(self)->gc.gc_refs != _PyGC_REFS_UNTRACKED);
    /* If Py_REF_DEBUG, _Py_NewReference bumped _Py_RefTotal, so
     * we need to undo that. */
    _Py_DEC_REFTOTAL;
    /* If Py_TRACE_REFS, _Py_NewReference re-added self to the object
     * chain, so no more to do there.
     * If COUNT_ALLOCS, the original decref bumped tp_frees, and
     * _Py_NewReference bumped tp_allocs:  both of those need to be
     * undone.
     */
#ifdef COUNT_ALLOCS
    --Py_TYPE(self)->tp_frees;
    --Py_TYPE(self)->tp_allocs;
#endif
}


/* Table mapping __foo__ names to tp_foo offsets and slot_tp_foo wrapper
   functions.  The offsets here are relative to the 'PyHeapTypeObject'
   structure, which incorporates the additional structures used for numbers,
   sequences and mappings.
   Note that multiple names may map to the same slot (e.g. __eq__,
   __ne__ etc. all map to tp_richcompare) and one name may map to multiple
   slots (e.g. __str__ affects tp_str as well as tp_repr). The table is
   terminated with an all-zero entry.  (This table is further initialized and
   sorted in init_slotdefs() below.) */

typedef struct wrapperbase slotdef;

#undef TPSLOT
#undef FLSLOT
#undef ETSLOT
#undef SQSLOT
#undef MPSLOT
#undef NBSLOT
#undef UNSLOT
#undef IBSLOT
#undef BINSLOT
#undef RBINSLOT

#define TPSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
    {NAME, offsetof(PyTypeObject, SLOT), (void *)(FUNCTION), WRAPPER, \
     PyDoc_STR(DOC)}
#define FLSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC, FLAGS) \
    {NAME, offsetof(PyTypeObject, SLOT), (void *)(FUNCTION), WRAPPER, \
     PyDoc_STR(DOC), FLAGS}
#define ETSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
    {NAME, offsetof(PyHeapTypeObject, SLOT), (void *)(FUNCTION), WRAPPER, \
     PyDoc_STR(DOC)}
#define SQSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
    ETSLOT(NAME, as_sequence.SLOT, FUNCTION, WRAPPER, DOC)
#define MPSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
    ETSLOT(NAME, as_mapping.SLOT, FUNCTION, WRAPPER, DOC)
#define NBSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
    ETSLOT(NAME, as_number.SLOT, FUNCTION, WRAPPER, DOC)
#define UNSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
    ETSLOT(NAME, as_number.SLOT, FUNCTION, WRAPPER, \
           "x." NAME "() <==> " DOC)
#define IBSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
    ETSLOT(NAME, as_number.SLOT, FUNCTION, WRAPPER, \
           "x." NAME "(y) <==> x" DOC "y")
#define BINSLOT(NAME, SLOT, FUNCTION, DOC) \
    ETSLOT(NAME, as_number.SLOT, FUNCTION, wrap_binaryfunc_l, \
           "x." NAME "(y) <==> x" DOC "y")
#define RBINSLOT(NAME, SLOT, FUNCTION, DOC) \
    ETSLOT(NAME, as_number.SLOT, FUNCTION, wrap_binaryfunc_r, \
           "x." NAME "(y) <==> y" DOC "x")
#define BINSLOTNOTINFIX(NAME, SLOT, FUNCTION, DOC) \
    ETSLOT(NAME, as_number.SLOT, FUNCTION, wrap_binaryfunc_l, \
           "x." NAME "(y) <==> " DOC)
#define RBINSLOTNOTINFIX(NAME, SLOT, FUNCTION, DOC) \
    ETSLOT(NAME, as_number.SLOT, FUNCTION, wrap_binaryfunc_r, \
           "x." NAME "(y) <==> " DOC)

static slotdef slotdefs[] = {
    SQSLOT("__len__", sq_length, slot_sq_length, wrap_lenfunc,
           "x.__len__() <==> len(x)"),
    /* Heap types defining __add__/__mul__ have sq_concat/sq_repeat == NULL.
       The logic in abstract.c always falls back to nb_add/nb_multiply in
       this case.  Defining both the nb_* and the sq_* slots to call the
       user-defined methods has unexpected side-effects, as shown by
       test_descr.notimplemented() */
    SQSLOT("__add__", sq_concat, NULL, wrap_binaryfunc,
      "x.__add__(y) <==> x+y"),
    SQSLOT("__mul__", sq_repeat, NULL, wrap_indexargfunc,
      "x.__mul__(n) <==> x*n"),
    SQSLOT("__rmul__", sq_repeat, NULL, wrap_indexargfunc,
      "x.__rmul__(n) <==> n*x"),
    SQSLOT("__getitem__", sq_item, slot_sq_item, wrap_sq_item,
           "x.__getitem__(y) <==> x[y]"),
    SQSLOT("__setitem__", sq_ass_item, slot_sq_ass_item, wrap_sq_setitem,
           "x.__setitem__(i, y) <==> x[i]=y"),
    SQSLOT("__delitem__", sq_ass_item, slot_sq_ass_item, wrap_sq_delitem,
           "x.__delitem__(y) <==> del x[y]"),
    SQSLOT("__contains__", sq_contains, slot_sq_contains, wrap_objobjproc,
           "x.__contains__(y) <==> y in x"),
    SQSLOT("__iadd__", sq_inplace_concat, NULL,
      wrap_binaryfunc, "x.__iadd__(y) <==> x+=y"),
    SQSLOT("__imul__", sq_inplace_repeat, NULL,
      wrap_indexargfunc, "x.__imul__(y) <==> x*=y"),

    MPSLOT("__len__", mp_length, slot_mp_length, wrap_lenfunc,
           "x.__len__() <==> len(x)"),
    MPSLOT("__getitem__", mp_subscript, slot_mp_subscript,
           wrap_binaryfunc,
           "x.__getitem__(y) <==> x[y]"),
    MPSLOT("__setitem__", mp_ass_subscript, slot_mp_ass_subscript,
           wrap_objobjargproc,
           "x.__setitem__(i, y) <==> x[i]=y"),
    MPSLOT("__delitem__", mp_ass_subscript, slot_mp_ass_subscript,
           wrap_delitem,
           "x.__delitem__(y) <==> del x[y]"),

    BINSLOT("__add__", nb_add, slot_nb_add,
        "+"),
    RBINSLOT("__radd__", nb_add, slot_nb_add,
             "+"),
    BINSLOT("__sub__", nb_subtract, slot_nb_subtract,
        "-"),
    RBINSLOT("__rsub__", nb_subtract, slot_nb_subtract,
             "-"),
    BINSLOT("__mul__", nb_multiply, slot_nb_multiply,
        "*"),
    RBINSLOT("__rmul__", nb_multiply, slot_nb_multiply,
             "*"),
    BINSLOT("__mod__", nb_remainder, slot_nb_remainder,
        "%"),
    RBINSLOT("__rmod__", nb_remainder, slot_nb_remainder,
             "%"),
    BINSLOTNOTINFIX("__divmod__", nb_divmod, slot_nb_divmod,
        "divmod(x, y)"),
    RBINSLOTNOTINFIX("__rdivmod__", nb_divmod, slot_nb_divmod,
             "divmod(y, x)"),
    NBSLOT("__pow__", nb_power, slot_nb_power, wrap_ternaryfunc,
           "x.__pow__(y[, z]) <==> pow(x, y[, z])"),
    NBSLOT("__rpow__", nb_power, slot_nb_power, wrap_ternaryfunc_r,
           "y.__rpow__(x[, z]) <==> pow(x, y[, z])"),
    UNSLOT("__neg__", nb_negative, slot_nb_negative, wrap_unaryfunc, "-x"),
    UNSLOT("__pos__", nb_positive, slot_nb_positive, wrap_unaryfunc, "+x"),
    UNSLOT("__abs__", nb_absolute, slot_nb_absolute, wrap_unaryfunc,
           "abs(x)"),
    UNSLOT("__bool__", nb_bool, slot_nb_bool, wrap_inquirypred,
           "x != 0"),
    UNSLOT("__invert__", nb_invert, slot_nb_invert, wrap_unaryfunc, "~x"),
    BINSLOT("__lshift__", nb_lshift, slot_nb_lshift, "<<"),
    RBINSLOT("__rlshift__", nb_lshift, slot_nb_lshift, "<<"),
    BINSLOT("__rshift__", nb_rshift, slot_nb_rshift, ">>"),
    RBINSLOT("__rrshift__", nb_rshift, slot_nb_rshift, ">>"),
    BINSLOT("__and__", nb_and, slot_nb_and, "&"),
    RBINSLOT("__rand__", nb_and, slot_nb_and, "&"),
    BINSLOT("__xor__", nb_xor, slot_nb_xor, "^"),
    RBINSLOT("__rxor__", nb_xor, slot_nb_xor, "^"),
    BINSLOT("__or__", nb_or, slot_nb_or, "|"),
    RBINSLOT("__ror__", nb_or, slot_nb_or, "|"),
    UNSLOT("__int__", nb_int, slot_nb_int, wrap_unaryfunc,
           "int(x)"),
    UNSLOT("__float__", nb_float, slot_nb_float, wrap_unaryfunc,
           "float(x)"),
    NBSLOT("__index__", nb_index, slot_nb_index, wrap_unaryfunc,
           "x[y:z] <==> x[y.__index__():z.__index__()]"),
    IBSLOT("__iadd__", nb_inplace_add, slot_nb_inplace_add,
           wrap_binaryfunc, "+="),
    IBSLOT("__isub__", nb_inplace_subtract, slot_nb_inplace_subtract,
           wrap_binaryfunc, "-="),
    IBSLOT("__imul__", nb_inplace_multiply, slot_nb_inplace_multiply,
           wrap_binaryfunc, "*="),
    IBSLOT("__imod__", nb_inplace_remainder, slot_nb_inplace_remainder,
           wrap_binaryfunc, "%="),
    IBSLOT("__ipow__", nb_inplace_power, slot_nb_inplace_power,
           wrap_binaryfunc, "**="),
    IBSLOT("__ilshift__", nb_inplace_lshift, slot_nb_inplace_lshift,
           wrap_binaryfunc, "<<="),
    IBSLOT("__irshift__", nb_inplace_rshift, slot_nb_inplace_rshift,
           wrap_binaryfunc, ">>="),
    IBSLOT("__iand__", nb_inplace_and, slot_nb_inplace_and,
           wrap_binaryfunc, "&="),
    IBSLOT("__ixor__", nb_inplace_xor, slot_nb_inplace_xor,
           wrap_binaryfunc, "^="),
    IBSLOT("__ior__", nb_inplace_or, slot_nb_inplace_or,
           wrap_binaryfunc, "|="),
    BINSLOT("__floordiv__", nb_floor_divide, slot_nb_floor_divide, "//"),
    RBINSLOT("__rfloordiv__", nb_floor_divide, slot_nb_floor_divide, "//"),
    BINSLOT("__truediv__", nb_true_divide, slot_nb_true_divide, "/"),
    RBINSLOT("__rtruediv__", nb_true_divide, slot_nb_true_divide, "/"),
    IBSLOT("__ifloordiv__", nb_inplace_floor_divide,
           slot_nb_inplace_floor_divide, wrap_binaryfunc, "//"),
    IBSLOT("__itruediv__", nb_inplace_true_divide,
           slot_nb_inplace_true_divide, wrap_binaryfunc, "/"),

    TPSLOT("__str__", tp_str, slot_tp_str, wrap_unaryfunc,
           "x.__str__() <==> str(x)"),
    TPSLOT("__repr__", tp_repr, slot_tp_repr, wrap_unaryfunc,
           "x.__repr__() <==> repr(x)"),
    TPSLOT("__hash__", tp_hash, slot_tp_hash, wrap_hashfunc,
           "x.__hash__() <==> hash(x)"),
    FLSLOT("__call__", tp_call, slot_tp_call, (wrapperfunc)wrap_call,
           "x.__call__(...) <==> x(...)", PyWrapperFlag_KEYWORDS),
    TPSLOT("__getattribute__", tp_getattro, slot_tp_getattr_hook,
           wrap_binaryfunc, "x.__getattribute__('name') <==> x.name"),
    TPSLOT("__getattribute__", tp_getattr, NULL, NULL, ""),
    TPSLOT("__getattr__", tp_getattro, slot_tp_getattr_hook, NULL, ""),
    TPSLOT("__getattr__", tp_getattr, NULL, NULL, ""),
    TPSLOT("__setattr__", tp_setattro, slot_tp_setattro, wrap_setattr,
           "x.__setattr__('name', value) <==> x.name = value"),
    TPSLOT("__setattr__", tp_setattr, NULL, NULL, ""),
    TPSLOT("__delattr__", tp_setattro, slot_tp_setattro, wrap_delattr,
           "x.__delattr__('name') <==> del x.name"),
    TPSLOT("__delattr__", tp_setattr, NULL, NULL, ""),
    TPSLOT("__lt__", tp_richcompare, slot_tp_richcompare, richcmp_lt,
           "x.__lt__(y) <==> x<y"),
    TPSLOT("__le__", tp_richcompare, slot_tp_richcompare, richcmp_le,
           "x.__le__(y) <==> x<=y"),
    TPSLOT("__eq__", tp_richcompare, slot_tp_richcompare, richcmp_eq,
           "x.__eq__(y) <==> x==y"),
    TPSLOT("__ne__", tp_richcompare, slot_tp_richcompare, richcmp_ne,
           "x.__ne__(y) <==> x!=y"),
    TPSLOT("__gt__", tp_richcompare, slot_tp_richcompare, richcmp_gt,
           "x.__gt__(y) <==> x>y"),
    TPSLOT("__ge__", tp_richcompare, slot_tp_richcompare, richcmp_ge,
           "x.__ge__(y) <==> x>=y"),
    TPSLOT("__iter__", tp_iter, slot_tp_iter, wrap_unaryfunc,
           "x.__iter__() <==> iter(x)"),
    TPSLOT("__next__", tp_iternext, slot_tp_iternext, wrap_next,
           "x.__next__() <==> next(x)"),
    TPSLOT("__get__", tp_descr_get, slot_tp_descr_get, wrap_descr_get,
           "descr.__get__(obj[, type]) -> value"),
    TPSLOT("__set__", tp_descr_set, slot_tp_descr_set, wrap_descr_set,
           "descr.__set__(obj, value)"),
    TPSLOT("__delete__", tp_descr_set, slot_tp_descr_set,
           wrap_descr_delete, "descr.__delete__(obj)"),
    FLSLOT("__init__", tp_init, slot_tp_init, (wrapperfunc)wrap_init,
           "x.__init__(...) initializes x; "
           "see help(type(x)) for signature",
           PyWrapperFlag_KEYWORDS),
    TPSLOT("__new__", tp_new, slot_tp_new, NULL, ""),
    TPSLOT("__del__", tp_del, slot_tp_del, NULL, ""),
    {NULL}
};

/* Given a type pointer and an offset gotten from a slotdef entry, return a
   pointer to the actual slot.  This is not quite the same as simply adding
   the offset to the type pointer, since it takes care to indirect through the
   proper indirection pointer (as_buffer, etc.); it returns NULL if the
   indirection pointer is NULL. */
static void **
slotptr(PyTypeObject *type, int ioffset)
{
    char *ptr;
    long offset = ioffset;

    /* Note: this depends on the order of the members of PyHeapTypeObject! */
    assert(offset >= 0);
    assert((size_t)offset < offsetof(PyHeapTypeObject, as_buffer));
    if ((size_t)offset >= offsetof(PyHeapTypeObject, as_sequence)) {
        ptr = (char *)type->tp_as_sequence;
        offset -= offsetof(PyHeapTypeObject, as_sequence);
    }
    else if ((size_t)offset >= offsetof(PyHeapTypeObject, as_mapping)) {
        ptr = (char *)type->tp_as_mapping;
        offset -= offsetof(PyHeapTypeObject, as_mapping);
    }
    else if ((size_t)offset >= offsetof(PyHeapTypeObject, as_number)) {
        ptr = (char *)type->tp_as_number;
        offset -= offsetof(PyHeapTypeObject, as_number);
    }
    else {
        ptr = (char *)type;
    }
    if (ptr != NULL)
        ptr += offset;
    return (void **)ptr;
}

/* Length of array of slotdef pointers used to store slots with the
   same __name__.  There should be at most MAX_EQUIV-1 slotdef entries with
   the same __name__, for any __name__. Since that's a static property, it is
   appropriate to declare fixed-size arrays for this. */
#define MAX_EQUIV 10

/* Return a slot pointer for a given name, but ONLY if the attribute has
   exactly one slot function.  The name must be an interned string. */
static void **
resolve_slotdups(PyTypeObject *type, PyObject *name)
{
    /* XXX Maybe this could be optimized more -- but is it worth it? */

    /* pname and ptrs act as a little cache */
    static PyObject *pname;
    static slotdef *ptrs[MAX_EQUIV];
    slotdef *p, **pp;
    void **res, **ptr;

    if (pname != name) {
        /* Collect all slotdefs that match name into ptrs. */
        pname = name;
        pp = ptrs;
        for (p = slotdefs; p->name_strobj; p++) {
            if (p->name_strobj == name)
                *pp++ = p;
        }
        *pp = NULL;
    }

    /* Look in all matching slots of the type; if exactly one of these has
       a filled-in slot, return its value.      Otherwise return NULL. */
    res = NULL;
    for (pp = ptrs; *pp; pp++) {
        ptr = slotptr(type, (*pp)->offset);
        if (ptr == NULL || *ptr == NULL)
            continue;
        if (res != NULL)
            return NULL;
        res = ptr;
    }
    return res;
}

/* Common code for update_slots_callback() and fixup_slot_dispatchers().  This
   does some incredibly complex thinking and then sticks something into the
   slot.  (It sees if the adjacent slotdefs for the same slot have conflicting
   interests, and then stores a generic wrapper or a specific function into
   the slot.)  Return a pointer to the next slotdef with a different offset,
   because that's convenient  for fixup_slot_dispatchers(). */
static slotdef *
update_one_slot(PyTypeObject *type, slotdef *p)
{
    PyObject *descr;
    PyWrapperDescrObject *d;
    void *generic = NULL, *specific = NULL;
    int use_generic = 0;
    int offset = p->offset;
    void **ptr = slotptr(type, offset);

    if (ptr == NULL) {
        do {
            ++p;
        } while (p->offset == offset);
        return p;
    }
    do {
        descr = _PyType_Lookup(type, p->name_strobj);
        if (descr == NULL) {
            if (ptr == (void**)&type->tp_iternext) {
                specific = _PyObject_NextNotImplemented;
            }
            continue;
        }
        if (Py_TYPE(descr) == &PyWrapperDescr_Type) {
            void **tptr = resolve_slotdups(type, p->name_strobj);
            if (tptr == NULL || tptr == ptr)
                generic = p->function;
            d = (PyWrapperDescrObject *)descr;
            if (d->d_base->wrapper == p->wrapper &&
                PyType_IsSubtype(type, PyDescr_TYPE(d)))
            {
                if (specific == NULL ||
                    specific == d->d_wrapped)
                    specific = d->d_wrapped;
                else
                    use_generic = 1;
            }
        }
        else if (Py_TYPE(descr) == &PyCFunction_Type &&
                 PyCFunction_GET_FUNCTION(descr) ==
                 (PyCFunction)tp_new_wrapper &&
                 ptr == (void**)&type->tp_new)
        {
            /* The __new__ wrapper is not a wrapper descriptor,
               so must be special-cased differently.
               If we don't do this, creating an instance will
               always use slot_tp_new which will look up
               __new__ in the MRO which will call tp_new_wrapper
               which will look through the base classes looking
               for a static base and call its tp_new (usually
               PyType_GenericNew), after performing various
               sanity checks and constructing a new argument
               list.  Cut all that nonsense short -- this speeds
               up instance creation tremendously. */
            specific = (void *)type->tp_new;
            /* XXX I'm not 100% sure that there isn't a hole
               in this reasoning that requires additional
               sanity checks.  I'll buy the first person to
               point out a bug in this reasoning a beer. */
        }
        else if (descr == Py_None &&
                 ptr == (void**)&type->tp_hash) {
            /* We specifically allow __hash__ to be set to None
               to prevent inheritance of the default
               implementation from object.__hash__ */
            specific = PyObject_HashNotImplemented;
        }
        else {
            use_generic = 1;
            generic = p->function;
        }
    } while ((++p)->offset == offset);
    if (specific && !use_generic)
        *ptr = specific;
    else
        *ptr = generic;
    return p;
}

/* In the type, update the slots whose slotdefs are gathered in the pp array.
   This is a callback for update_subclasses(). */
static int
update_slots_callback(PyTypeObject *type, void *data)
{
    slotdef **pp = (slotdef **)data;

    for (; *pp; pp++)
        update_one_slot(type, *pp);
    return 0;
}

/* Comparison function for qsort() to compare slotdefs by their offset, and
   for equal offset by their address (to force a stable sort). */
static int
slotdef_cmp(const void *aa, const void *bb)
{
    const slotdef *a = (const slotdef *)aa, *b = (const slotdef *)bb;
    int c = a->offset - b->offset;
    if (c != 0)
        return c;
    else
        /* Cannot use a-b, as this gives off_t,
           which may lose precision when converted to int. */
        return (a > b) ? 1 : (a < b) ? -1 : 0;
}

/* Initialize the slotdefs table by adding interned string objects for the
   names and sorting the entries. */
static void
init_slotdefs(void)
{
    slotdef *p;
    static int initialized = 0;

    if (initialized)
        return;
    for (p = slotdefs; p->name; p++) {
        p->name_strobj = PyUnicode_InternFromString(p->name);
        if (!p->name_strobj)
            Py_FatalError("Out of memory interning slotdef names");
    }
    qsort((void *)slotdefs, (size_t)(p-slotdefs), sizeof(slotdef),
          slotdef_cmp);
    initialized = 1;
}

/* Update the slots after assignment to a class (type) attribute. */
static int
update_slot(PyTypeObject *type, PyObject *name)
{
    slotdef *ptrs[MAX_EQUIV];
    slotdef *p;
    slotdef **pp;
    int offset;

    /* Clear the VALID_VERSION flag of 'type' and all its
       subclasses.  This could possibly be unified with the
       update_subclasses() recursion below, but carefully:
       they each have their own conditions on which to stop
       recursing into subclasses. */
    PyType_Modified(type);

    init_slotdefs();
    pp = ptrs;
    for (p = slotdefs; p->name; p++) {
        /* XXX assume name is interned! */
        if (p->name_strobj == name)
            *pp++ = p;
    }
    *pp = NULL;
    for (pp = ptrs; *pp; pp++) {
        p = *pp;
        offset = p->offset;
        while (p > slotdefs && (p-1)->offset == offset)
            --p;
        *pp = p;
    }
    if (ptrs[0] == NULL)
        return 0; /* Not an attribute that affects any slots */
    return update_subclasses(type, name,
                             update_slots_callback, (void *)ptrs);
}

/* Store the proper functions in the slot dispatches at class (type)
   definition time, based upon which operations the class overrides in its
   dict. */
static void
fixup_slot_dispatchers(PyTypeObject *type)
{
    slotdef *p;

    init_slotdefs();
    for (p = slotdefs; p->name; )
        p = update_one_slot(type, p);
}

static void
update_all_slots(PyTypeObject* type)
{
    slotdef *p;

    init_slotdefs();
    for (p = slotdefs; p->name; p++) {
        /* update_slot returns int but can't actually fail */
        update_slot(type, p->name_strobj);
    }
}

/* recurse_down_subclasses() and update_subclasses() are mutually
   recursive functions to call a callback for all subclasses,
   but refraining from recursing into subclasses that define 'name'. */

static int
update_subclasses(PyTypeObject *type, PyObject *name,
                  update_callback callback, void *data)
{
    if (callback(type, data) < 0)
        return -1;
    return recurse_down_subclasses(type, name, callback, data);
}

static int
recurse_down_subclasses(PyTypeObject *type, PyObject *name,
                        update_callback callback, void *data)
{
    PyTypeObject *subclass;
    PyObject *ref, *subclasses, *dict;
    Py_ssize_t i, n;

    subclasses = type->tp_subclasses;
    if (subclasses == NULL)
        return 0;
    assert(PyList_Check(subclasses));
    n = PyList_GET_SIZE(subclasses);
    for (i = 0; i < n; i++) {
        ref = PyList_GET_ITEM(subclasses, i);
        assert(PyWeakref_CheckRef(ref));
        subclass = (PyTypeObject *)PyWeakref_GET_OBJECT(ref);
        assert(subclass != NULL);
        if ((PyObject *)subclass == Py_None)
            continue;
        assert(PyType_Check(subclass));
        /* Avoid recursing down into unaffected classes */
        dict = subclass->tp_dict;
        if (dict != NULL && PyDict_Check(dict) &&
            PyDict_GetItem(dict, name) != NULL)
            continue;
        if (update_subclasses(subclass, name, callback, data) < 0)
            return -1;
    }
    return 0;
}

/* This function is called by PyType_Ready() to populate the type's
   dictionary with method descriptors for function slots.  For each
   function slot (like tp_repr) that's defined in the type, one or more
   corresponding descriptors are added in the type's tp_dict dictionary
   under the appropriate name (like __repr__).  Some function slots
   cause more than one descriptor to be added (for example, the nb_add
   slot adds both __add__ and __radd__ descriptors) and some function
   slots compete for the same descriptor (for example both sq_item and
   mp_subscript generate a __getitem__ descriptor).

   In the latter case, the first slotdef entry encountered wins.  Since
   slotdef entries are sorted by the offset of the slot in the
   PyHeapTypeObject, this gives us some control over disambiguating
   between competing slots: the members of PyHeapTypeObject are listed
   from most general to least general, so the most general slot is
   preferred.  In particular, because as_mapping comes before as_sequence,
   for a type that defines both mp_subscript and sq_item, mp_subscript
   wins.

   This only adds new descriptors and doesn't overwrite entries in
   tp_dict that were previously defined.  The descriptors contain a
   reference to the C function they must call, so that it's safe if they
   are copied into a subtype's __dict__ and the subtype has a different
   C function in its slot -- calling the method defined by the
   descriptor will call the C function that was used to create it,
   rather than the C function present in the slot when it is called.
   (This is important because a subtype may have a C function in the
   slot that calls the method from the dictionary, and we want to avoid
   infinite recursion here.) */

static int
add_operators(PyTypeObject *type)
{
    PyObject *dict = type->tp_dict;
    slotdef *p;
    PyObject *descr;
    void **ptr;

    init_slotdefs();
    for (p = slotdefs; p->name; p++) {
        if (p->wrapper == NULL)
            continue;
        ptr = slotptr(type, p->offset);
        if (!ptr || !*ptr)
            continue;
        if (PyDict_GetItem(dict, p->name_strobj))
            continue;
        if (*ptr == PyObject_HashNotImplemented) {
            /* Classes may prevent the inheritance of the tp_hash
               slot by storing PyObject_HashNotImplemented in it. Make it
               visible as a None value for the __hash__ attribute. */
            if (PyDict_SetItem(dict, p->name_strobj, Py_None) < 0)
                return -1;
        }
        else {
            descr = PyDescr_NewWrapper(type, p, *ptr);
            if (descr == NULL)
                return -1;
            if (PyDict_SetItem(dict, p->name_strobj, descr) < 0)
                return -1;
            Py_DECREF(descr);
        }
    }
    if (type->tp_new != NULL) {
        if (add_tp_new_wrapper(type) < 0)
            return -1;
    }
    return 0;
}


/* Cooperative 'super' */

typedef struct {
    PyObject_HEAD
    PyTypeObject *type;
    PyObject *obj;
    PyTypeObject *obj_type;
} superobject;

static PyMemberDef super_members[] = {
    {"__thisclass__", T_OBJECT, offsetof(superobject, type), READONLY,
     "the class invoking super()"},
    {"__self__",  T_OBJECT, offsetof(superobject, obj), READONLY,
     "the instance invoking super(); may be None"},
    {"__self_class__", T_OBJECT, offsetof(superobject, obj_type), READONLY,
     "the type of the instance invoking super(); may be None"},
    {0}
};

static void
super_dealloc(PyObject *self)
{
    superobject *su = (superobject *)self;

    _PyObject_GC_UNTRACK(self);
    Py_XDECREF(su->obj);
    Py_XDECREF(su->type);
    Py_XDECREF(su->obj_type);
    Py_TYPE(self)->tp_free(self);
}

static PyObject *
super_repr(PyObject *self)
{
    superobject *su = (superobject *)self;

    if (su->obj_type)
        return PyUnicode_FromFormat(
            "<super: <class '%s'>, <%s object>>",
            su->type ? su->type->tp_name : "NULL",
            su->obj_type->tp_name);
    else
        return PyUnicode_FromFormat(
            "<super: <class '%s'>, NULL>",
            su->type ? su->type->tp_name : "NULL");
}

static PyObject *
super_getattro(PyObject *self, PyObject *name)
{
    superobject *su = (superobject *)self;
    int skip = su->obj_type == NULL;

    if (!skip) {
        /* We want __class__ to return the class of the super object
           (i.e. super, or a subclass), not the class of su->obj. */
        skip = (PyUnicode_Check(name) &&
            PyUnicode_GET_LENGTH(name) == 9 &&
            PyUnicode_CompareWithASCIIString(name, "__class__") == 0);
    }

    if (!skip) {
        PyObject *mro, *res, *tmp, *dict;
        PyTypeObject *starttype;
        descrgetfunc f;
        Py_ssize_t i, n;

        starttype = su->obj_type;
        mro = starttype->tp_mro;

        if (mro == NULL)
            n = 0;
        else {
            assert(PyTuple_Check(mro));
            n = PyTuple_GET_SIZE(mro);
        }
        for (i = 0; i < n; i++) {
            if ((PyObject *)(su->type) == PyTuple_GET_ITEM(mro, i))
                break;
        }
        i++;
        res = NULL;
        /* keep a strong reference to mro because starttype->tp_mro can be
           replaced during PyDict_GetItem(dict, name)  */
        Py_INCREF(mro);
        for (; i < n; i++) {
            tmp = PyTuple_GET_ITEM(mro, i);
            if (PyType_Check(tmp))
                dict = ((PyTypeObject *)tmp)->tp_dict;
            else
                continue;
            res = PyDict_GetItem(dict, name);
            if (res != NULL) {
                Py_INCREF(res);
                f = Py_TYPE(res)->tp_descr_get;
                if (f != NULL) {
                    tmp = f(res,
                        /* Only pass 'obj' param if
                           this is instance-mode super
                           (See SF ID #743627)
                        */
                        (su->obj == (PyObject *)
                                    su->obj_type
                            ? (PyObject *)NULL
                            : su->obj),
                        (PyObject *)starttype);
                    Py_DECREF(res);
                    res = tmp;
                }
                Py_DECREF(mro);
                return res;
            }
        }
        Py_DECREF(mro);
    }
    return PyObject_GenericGetAttr(self, name);
}

static PyTypeObject *
supercheck(PyTypeObject *type, PyObject *obj)
{
    /* Check that a super() call makes sense.  Return a type object.

       obj can be a class, or an instance of one:

       - If it is a class, it must be a subclass of 'type'.      This case is
         used for class methods; the return value is obj.

       - If it is an instance, it must be an instance of 'type'.  This is
         the normal case; the return value is obj.__class__.

       But... when obj is an instance, we want to allow for the case where
       Py_TYPE(obj) is not a subclass of type, but obj.__class__ is!
       This will allow using super() with a proxy for obj.
    */

    /* Check for first bullet above (special case) */
    if (PyType_Check(obj) && PyType_IsSubtype((PyTypeObject *)obj, type)) {
        Py_INCREF(obj);
        return (PyTypeObject *)obj;
    }

    /* Normal case */
    if (PyType_IsSubtype(Py_TYPE(obj), type)) {
        Py_INCREF(Py_TYPE(obj));
        return Py_TYPE(obj);
    }
    else {
        /* Try the slow way */
        PyObject *class_attr;

        class_attr = _PyObject_GetAttrId(obj, &PyId___class__);
        if (class_attr != NULL &&
            PyType_Check(class_attr) &&
            (PyTypeObject *)class_attr != Py_TYPE(obj))
        {
            int ok = PyType_IsSubtype(
                (PyTypeObject *)class_attr, type);
            if (ok)
                return (PyTypeObject *)class_attr;
        }

        if (class_attr == NULL)
            PyErr_Clear();
        else
            Py_DECREF(class_attr);
    }

    PyErr_SetString(PyExc_TypeError,
                    "super(type, obj): "
                    "obj must be an instance or subtype of type");
    return NULL;
}

static PyObject *
super_descr_get(PyObject *self, PyObject *obj, PyObject *type)
{
    superobject *su = (superobject *)self;
    superobject *newobj;

    if (obj == NULL || obj == Py_None || su->obj != NULL) {
        /* Not binding to an object, or already bound */
        Py_INCREF(self);
        return self;
    }
    if (Py_TYPE(su) != &PySuper_Type)
        /* If su is an instance of a (strict) subclass of super,
           call its type */
        return PyObject_CallFunctionObjArgs((PyObject *)Py_TYPE(su),
                                            su->type, obj, NULL);
    else {
        /* Inline the common case */
        PyTypeObject *obj_type = supercheck(su->type, obj);
        if (obj_type == NULL)
            return NULL;
        newobj = (superobject *)PySuper_Type.tp_new(&PySuper_Type,
                                                 NULL, NULL);
        if (newobj == NULL)
            return NULL;
        Py_INCREF(su->type);
        Py_INCREF(obj);
        newobj->type = su->type;
        newobj->obj = obj;
        newobj->obj_type = obj_type;
        return (PyObject *)newobj;
    }
}

static int
super_init(PyObject *self, PyObject *args, PyObject *kwds)
{
    superobject *su = (superobject *)self;
    PyTypeObject *type = NULL;
    PyObject *obj = NULL;
    PyTypeObject *obj_type = NULL;

    if (!_PyArg_NoKeywords("super", kwds))
        return -1;
    if (!PyArg_ParseTuple(args, "|O!O:super", &PyType_Type, &type, &obj))
        return -1;

    if (type == NULL) {
        /* Call super(), without args -- fill in from __class__
           and first local variable on the stack. */
        PyFrameObject *f = PyThreadState_GET()->frame;
        PyCodeObject *co = f->f_code;
        Py_ssize_t i, n;
        if (co == NULL) {
            PyErr_SetString(PyExc_SystemError,
                            "super(): no code object");
            return -1;
        }
        if (co->co_argcount == 0) {
            PyErr_SetString(PyExc_SystemError,
                            "super(): no arguments");
            return -1;
        }
        obj = f->f_localsplus[0];
        if (obj == NULL) {
            PyErr_SetString(PyExc_SystemError,
                            "super(): arg[0] deleted");
            return -1;
        }
        if (co->co_freevars == NULL)
            n = 0;
        else {
            assert(PyTuple_Check(co->co_freevars));
            n = PyTuple_GET_SIZE(co->co_freevars);
        }
        for (i = 0; i < n; i++) {
            PyObject *name = PyTuple_GET_ITEM(co->co_freevars, i);
            assert(PyUnicode_Check(name));
            if (!PyUnicode_CompareWithASCIIString(name,
                                                  "@__class__")) {
                Py_ssize_t index = co->co_nlocals +
                    PyTuple_GET_SIZE(co->co_cellvars) + i;
                PyObject *cell = f->f_localsplus[index];
                if (cell == NULL || !PyCell_Check(cell)) {
                    PyErr_SetString(PyExc_SystemError,
                      "super(): bad __class__ cell");
                    return -1;
                }
                type = (PyTypeObject *) PyCell_GET(cell);
                if (type == NULL) {
                    PyErr_SetString(PyExc_SystemError,
                      "super(): empty __class__ cell");
                    return -1;
                }
                if (!PyType_Check(type)) {
                    PyErr_Format(PyExc_SystemError,
                      "super(): __class__ is not a type (%s)",
                      Py_TYPE(type)->tp_name);
                    return -1;
                }
                break;
            }
        }
        if (type == NULL) {
            PyErr_SetString(PyExc_SystemError,
                            "super(): __class__ cell not found");
            return -1;
        }
    }

    if (obj == Py_None)
        obj = NULL;
    if (obj != NULL) {
        obj_type = supercheck(type, obj);
        if (obj_type == NULL)
            return -1;
        Py_INCREF(obj);
    }
    Py_INCREF(type);
    su->type = type;
    su->obj = obj;
    su->obj_type = obj_type;
    return 0;
}

PyDoc_STRVAR(super_doc,
"super() -> same as super(__class__, <first argument>)\n"
"super(type) -> unbound super object\n"
"super(type, obj) -> bound super object; requires isinstance(obj, type)\n"
"super(type, type2) -> bound super object; requires issubclass(type2, type)\n"
"Typical use to call a cooperative superclass method:\n"
"class C(B):\n"
"    def meth(self, arg):\n"
"        super().meth(arg)\n"
"This works for class methods too:\n"
"class C(B):\n"
"    @classmethod\n"
"    def cmeth(cls, arg):\n"
"        super().cmeth(arg)\n");

static int
super_traverse(PyObject *self, visitproc visit, void *arg)
{
    superobject *su = (superobject *)self;

    Py_VISIT(su->obj);
    Py_VISIT(su->type);
    Py_VISIT(su->obj_type);

    return 0;
}

PyTypeObject PySuper_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "super",                                    /* tp_name */
    sizeof(superobject),                        /* tp_basicsize */
    0,                                          /* tp_itemsize */
    /* methods */
    super_dealloc,                              /* tp_dealloc */
    0,                                          /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_reserved */
    super_repr,                                 /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    super_getattro,                             /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
        Py_TPFLAGS_BASETYPE,                    /* tp_flags */
    super_doc,                                  /* tp_doc */
    super_traverse,                             /* tp_traverse */
    0,                                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    0,                                          /* tp_methods */
    super_members,                              /* tp_members */
    0,                                          /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    super_descr_get,                            /* tp_descr_get */
    0,                                          /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    super_init,                                 /* tp_init */
    PyType_GenericAlloc,                        /* tp_alloc */
    PyType_GenericNew,                          /* tp_new */
    PyObject_GC_Del,                            /* tp_free */
};