Objects/abstract.c (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/abstract.c ↗

This annotation covers sequence and mapping abstract APIs plus item access and isinstance/issubclass. See objects_abstract_detail for PyNumber_*, PyObject_Call, and PyObject_RichCompare.

Map

Lines	Symbol	Role
1-120	PyObject_GetItem / SetItem / DelItem	obj[key], obj[key]=v, del obj[key]
121-300	PySequence_Check / Size / GetItem	Sequence protocol checks and element access
301-500	PySequence_Contains	x in seq — calls __contains__ or iterates
501-700	PySequence_List / Tuple	Convert arbitrary iterable to list or tuple
701-900	PyMapping_Check / Keys / Values / Items	Mapping protocol
901-1100	PyMapping_GetItemString	obj["key"] with a C string key
1101-1400	PyObject_IsInstance	isinstance(obj, cls) — checks MRO and __instancecheck__
1401-1600	PyObject_IsSubclass	issubclass(sub, sup) — checks __subclasscheck__

Reading

PyObject_GetItem

// CPython: Objects/abstract.c:180 PyObject_GetItem
PyObject *
PyObject_GetItem(PyObject *o, PyObject *key)
{
    if (o == NULL || key == NULL) {
        return null_error();
    }
    PyMappingMethods *m = Py_TYPE(o)->tp_as_mapping;
    if (m != NULL && m->mp_subscript != NULL) {
        return m->mp_subscript(o, key);
    }
    PySequenceMethods *ms = Py_TYPE(o)->tp_as_sequence;
    if (ms != NULL && ms->sq_item != NULL) {
        /* sequence: convert key to index */
        Py_ssize_t i = PyNumber_AsSsize_t(key, PyExc_IndexError);
        if (i == -1 && PyErr_Occurred()) return NULL;
        return ms->sq_item(o, i);
    }
    /* fall back to __class_getitem__ for type objects */
    ...
}

PyObject_GetItem tries mp_subscript first (dict, custom mapping), then sq_item (list, tuple). Raises TypeError if neither is present.

PySequence_Contains

// CPython: Objects/abstract.c:1480 PySequence_Contains
int
PySequence_Contains(PyObject *seq, PyObject *ob)
{
    /* Try __contains__ first */
    PySequenceMethods *sqm = Py_TYPE(seq)->tp_as_sequence;
    if (sqm != NULL && sqm->sq_contains != NULL)
        return sqm->sq_contains(seq, ob);
    /* Fallback: linear scan */
    return _PySequence_IterSearch(seq, ob, PY_ITERSEARCH_CONTAINS);
}

x in seq desugars to PySequence_Contains. Lists and sets use a direct sq_contains; general iterables fall back to _PySequence_IterSearch which calls PyObject_RichCompareBool(item, ob, Py_EQ) for each element.

PySequence_List

// CPython: Objects/abstract.c:1620 PySequence_List
PyObject *
PySequence_List(PyObject *v)
{
    /* Build a new list from any iterable. */
    if (PyList_CheckExact(v))
        return PyList_GetSlice(v, 0, PY_SSIZE_T_MAX);
    return _PyList_FromIterable(v);
}

list(iterable) calls PySequence_List. For exact lists it slices; for others it calls __iter__ and appends in batches.

PyObject_IsInstance

// CPython: Objects/abstract.c:2280 PyObject_IsInstance
int
PyObject_IsInstance(PyObject *inst, PyObject *cls)
{
    /* isinstance(inst, cls) */
    /* Check for __instancecheck__ on the metaclass */
    PyObject *checker = _PyObject_LookupSpecial(cls, &_Py_ID(__instancecheck__));
    if (checker != NULL) {
        PyObject *res = PyObject_CallOneArg(checker, inst);
        ...
    }
    /* Fast path: check tp_mro directly */
    return recursive_isinstance(inst, cls);
}

isinstance first looks for __instancecheck__ on the type's metaclass (enabling ABCs). For plain classes it traverses the tp_mro array.

PyObject_IsSubclass

// CPython: Objects/abstract.c:2400 PyObject_IsSubclass
int
PyObject_IsSubclass(PyObject *derived, PyObject *cls)
{
    /* issubclass(derived, cls) */
    PyObject *checker = _PyObject_LookupSpecial(cls, &_Py_ID(__subclasscheck__));
    if (checker != NULL) {
        PyObject *res = PyObject_CallOneArg(checker, derived);
        ...
    }
    return recursive_issubclass(derived, cls);
}

ABCs override __subclasscheck__ to register virtual subclasses via abc.register().

gopy notes

PyObject_GetItem is objects.GetItem in objects/abstract.go. PySequence_Contains is objects.SequenceContains. PySequence_List is objects.SequenceList. PyObject_IsInstance is objects.IsInstance which checks the MRO slice on objects.Type. PyObject_IsSubclass is objects.IsSubclass with ABC support via __subclasscheck__.