Objects/typeobject.c

Source:

cpython 3.14 @ ab2d84fe1023/Objects/typeobject.c ↗

Objects/typeobject.c is one of the largest files in CPython. It implements PyTypeObject, the metaclass machinery, MRO computation, slot inheritance, the descriptor protocol for attribute lookup, type.__new__, object.__init__, and super.

Map

Lines	Symbol	Role
1-500	slot wrappers, slotdefs	Wrapper functions bridging Python methods to C slots
501-1200	type_new, type_new_impl	type(name, bases, dict) metaclass constructor
1201-1800	mro_internal, calculate_mro	C3 linearization
1801-2500	type_ready, inherit_slots	Finalize type; copy slots from base types
2501-3500	type_getattro, _PyType_Lookup	MRO attribute lookup; descriptor priority
3501-5000	object_init, object_new, object_repr	object base-class methods
5001-7000	slot_tp_*, update_one_slot	Dynamic slot updaters for Python-defined types
7001-9000	super_getattro, supercheck	super() proxy implementation

Reading

C3 MRO computation

calculate_mro implements the C3 linearization algorithm. It starts with the direct bases' MROs and the bases list, iteratively picks the head of the first list that does not appear in the tail of any other list, and appends it to the result.

// Objects/typeobject.c:1201 mro_internal
static PyObject *
mro_internal(PyTypeObject *type, PyObject **p_old_mro)
{
    PyObject *mro = mro_implementation(type);
    PyObject *old_mro = type->tp_mro;
    type->tp_mro = mro;  /* atomic swap for free-threaded safety */
    ...
}

_PyType_Lookup: descriptor search

_PyType_Lookup(type, name) walks the MRO tuple and returns the first entry in any class's __dict__ that matches name. The result is cached in the type's attribute lookup cache (tp_cache), keyed on the type's version tag. The cache is invalidated when the type's dict is modified.

// Objects/typeobject.c:2501 _PyType_Lookup
PyObject *
_PyType_Lookup(PyTypeObject *type, PyObject *name)
{
    PyObject *mro = type->tp_mro;
    for (Py_ssize_t i = 0; i < PyTuple_GET_SIZE(mro); i++) {
        PyTypeObject *base = (PyTypeObject *)PyTuple_GET_ITEM(mro, i);
        PyObject *dict = base->tp_dict;
        PyObject *res = PyDict_GetItemWithError(dict, name);
        if (res != NULL) return res;
    }
    return NULL;
}

slot inheritance and update_one_slot

When a Python subclass defines __add__, update_one_slot is called to update the nb_add slot on the subclass's type object. This is the bridge between Python-level dunder methods and C-level slot function pointers. inherit_slots copies slots from the base type for slots not explicitly defined by the subclass.

// Objects/typeobject.c:5001 update_one_slot (excerpt)
static slotdef *
update_one_slot(PyTypeObject *type, slotdef *p)
{
    /* look up p->name_strobj in the MRO */
    /* if found and is a wrapper, set p->slot to the wrapper's C function */
    ...
}

gopy notes

The gopy type system is in objects/type.go and objects/usertype.go. MRO computation is in objects/type.go:computeMRO and follows the same C3 algorithm. Slot inheritance is handled by objects/type.go:inherit. The _PyType_Lookup fast path maps to objects/type.go:Lookup with a version-tag cache.