Python/typeobject.c (part 4)

Source:

cpython 3.14 @ ab2d84fe1023/Python/typeobject.c ↗

This annotation covers type creation hooks. See objects_typeobject3_detail for type.__call__, tp_new, tp_init, and type.__new__.

Map

Lines	Symbol	Role
1-100	type.__init_subclass__	Hook called when a class is subclassed
101-220	type.__set_name__	Notify descriptors of their name in the owning class
221-360	__slots__ processing	Allocate member descriptors; set tp_basicsize
361-500	__init_subclass__ dispatch	Call in MRO order; pass keyword arguments
501-600	type.__subclasses__	Return direct subclasses

Reading

type.__init_subclass__

// CPython: Python/typeobject.c:7480 type_init_subclass
static int
type_init_subclass(PyTypeObject *type, PyObject *kwds)
{
    /* Call type.__init_subclass__ on each base in MRO.
       PEP 487: called after the class body has been executed. */
    PyObject *super = PyObject_CallOneArg((PyObject *)&PySuper_Type,
                                           (PyObject *)type);
    PyObject *func = PyObject_GetAttr(super, &_Py_ID(__init_subclass__));
    return PyObject_Call(func, _PyTuple_EMPTY, kwds) != NULL ? 0 : -1;
}

class Foo(Base, metaclass=Meta, keyword=value): passes keyword=value to Base.__init_subclass__. This is how dataclasses receives eq=True and order=False and abc.ABCMeta registers abstract methods.

type.__set_name__

// CPython: Python/typeobject.c:7380 set_names
static int
set_names(PyTypeObject *type)
{
    /* For each attribute in the class dict that implements __set_name__,
       call descriptor.__set_name__(owner_type, attr_name). */
    PyObject *key, *value;
    Py_ssize_t i = 0;
    while (PyDict_Next(type->tp_dict, &i, &key, &value)) {
        PyObject *set_name = _PyObject_LookupSpecial(value, &_Py_ID(__set_name__));
        if (set_name != NULL) {
            PyObject *res = PyObject_CallFunctionObjArgs(set_name, type, key, NULL);
            if (res == NULL) return -1;
            Py_DECREF(res);
        }
    }
    return 0;
}

__set_name__ allows descriptors to know which attribute name they were assigned to. dataclasses.field() uses this to record the field name without requiring explicit name= argument. Called during class creation, before __init_subclass__.

__slots__ processing

// CPython: Python/typeobject.c:2880 type_new_slots
static int
type_new_slots(PyTypeObject *type, PyObject *slots)
{
    /* For each name in __slots__:
       1. Check it's a valid identifier and not a duplicate
       2. Create a PyMemberDef at offset (basicsize + i * sizeof(PyObject*))
       3. Add a member_descriptor to tp_dict */
    Py_ssize_t nslot = PyTuple_GET_SIZE(slots);
    type->tp_basicsize += nslot * sizeof(PyObject *);
    for (int i = 0; i < nslot; i++) {
        PyObject *name = PyTuple_GET_ITEM(slots, i);
        ...
        type->tp_members[i] = ...;
    }
}

__slots__ prevents __dict__ creation per instance, saving ~200 bytes per instance. Each slot becomes a C-level member at a fixed offset in the object struct. Inheritance with __slots__ in parent but not child still creates __dict__ in the child.

gopy notes

type.__init_subclass__ is objects.TypeInitSubclass in objects/type.go. It calls super().__init_subclass__(**kwargs). __set_name__ dispatch is objects.TypeSetNames. __slots__ processing creates objects.MemberDescriptor objects and adjusts objects.Type.BasicSize.