Objects/typeobject.c (part 5)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/typeobject.c ↗

This annotation covers isinstance and issubclass internals. See objects_typeobject4_detail for __init_subclass__, __set_name__, and __slots__.

Map

Lines	Symbol	Role
1-80	type.__instancecheck__	isinstance(obj, cls) default implementation
81-180	type.__subclasscheck__	issubclass(sub, cls) default implementation
181-300	Abstract class protocol	Check __abstractmethods__ before instantiation
301-420	PyObject_IsInstance	C API that respects __instancecheck__
421-500	MRO-based subclass check	Scan type(obj).__mro__ for cls

Reading

type.__instancecheck__

// CPython: Objects/typeobject.c:3880 type___instancecheck___impl
static PyObject *
type___instancecheck___impl(PyTypeObject *self, PyObject *instance)
{
    return PyBool_FromLong(_PyObject_RealIsInstance(instance, (PyObject *)self));
}

static int
_PyObject_RealIsInstance(PyObject *inst, PyObject *cls)
{
    /* Fast path: exact type match */
    if (Py_IS_TYPE(inst, (PyTypeObject *)cls)) return 1;
    /* Walk the MRO */
    PyObject *mro = ((PyTypeObject *)Py_TYPE(inst))->tp_mro;
    for (int i = 0; i < PyTuple_GET_SIZE(mro); i++) {
        if (PyTuple_GET_ITEM(mro, i) == cls) return 1;
    }
    return 0;
}

The default __instancecheck__ does an MRO walk. isinstance(True, int) returns True because bool.__mro__ = [bool, int, object]. ABCMeta overrides __instancecheck__ to also check the virtual subclass registry.

Abstract class protocol

// CPython: Objects/typeobject.c:4020 object_new
static PyObject *
object_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    /* Refuse to instantiate abstract classes. */
    if (type->tp_flags & Py_TPFLAGS_IS_ABSTRACT) {
        PyObject *abstract_methods = type->tp_dict; /* __abstractmethods__ */
        ...
        if (PySet_GET_SIZE(abstract_methods) > 0) {
            PyErr_Format(PyExc_TypeError,
                "Can't instantiate abstract class %s "
                "with abstract method%s %U",
                type->tp_name,
                PySet_GET_SIZE(abstract_methods) > 1 ? "s" : "",
                joined);
            return NULL;
        }
    }
    ...
}

class Foo(ABC): @abstractmethod def bar(self): pass sets Foo.__abstractmethods__ = frozenset({'bar'}). Attempting Foo() raises TypeError. Once all abstract methods are overridden in a subclass, __abstractmethods__ becomes empty.

PyObject_IsInstance

// CPython: Objects/typeobject.c:4200 PyObject_IsInstance
int
PyObject_IsInstance(PyObject *inst, PyObject *cls)
{
    /* Check for __instancecheck__ override first. */
    PyObject *checker = _PyObject_LookupSpecial(cls, &_Py_ID(__instancecheck__));
    if (checker != NULL) {
        PyObject *res = PyObject_CallOneArg(checker, inst);
        int ok = PyObject_IsTrue(res);
        Py_DECREF(res);
        return ok;
    }
    /* Fall back to MRO walk */
    return _PyObject_RealIsInstance(inst, cls);
}

isinstance first checks for __instancecheck__ on the class (or metaclass). This allows ABCMeta, Union, and custom metaclasses to override the behavior.

gopy notes

type.__instancecheck__ is objects.TypeInstanceCheck in objects/type.go. PyObject_IsInstance is objects.IsInstance, which checks for __instancecheck__ via objects.LookupSpecial then falls back to MRO scan. Abstract class enforcement is in objects.ObjectNew.