Objects/dictobject.c (part 6)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/dictobject.c ↗

This annotation covers dict view objects and LIFO operations. See objects_dictobject5_detail for dict.update, dict.__or__, dict.setdefault, and dict merging.

Map

Lines	Symbol	Role
1-80	dict.popitem	Remove and return the last inserted (key, value) pair
81-180	dict.__reversed__	Iterate keys in LIFO order
181-300	dict.keys / dict.values / dict.items	Return view objects
301-420	View iteration	dictitemsiter, dictkeysiter, dictvaluesiter
421-500	View set operations	keys_view & other_set, keys_view - other

Reading

dict.popitem

// CPython: Objects/dictobject.c:3280 dict_popitem
static PyObject *
dict_popitem(PyDictObject *mp, PyObject *Py_UNUSED(ignored))
{
    /* LIFO: remove the last inserted item. */
    if (mp->ma_used == 0) {
        PyErr_SetString(PyExc_KeyError, "popitem(): dictionary is empty");
        return NULL;
    }
    /* Find the last non-empty slot in dk_entries */
    Py_ssize_t i = mp->ma_keys->dk_nentries - 1;
    PyDictKeyEntry *ep = &DK_ENTRIES(mp->ma_keys)[i];
    while (ep->me_value == NULL) { i--; ep--; }
    PyObject *res = PyTuple_New(2);
    PyTuple_SET_ITEM(res, 0, ep->me_key);
    PyTuple_SET_ITEM(res, 1, ep->me_value);
    /* Delete the entry */
    _PyDict_NotifyEvent(mp, PyDict_EVENT_DELETED, ep->me_key, NULL);
    ep->me_value = NULL;
    ep->me_key = NULL;
    mp->ma_used--;
    return res;
}

dict.popitem() is O(1) because it always removes the most recently inserted item (last in dk_entries). This is guaranteed since Python 3.7. Used for destructively iterating a dict.

dict.__reversed__

// CPython: Objects/dictobject.c:3480 dict_reversed
static PyObject *
dict_reversed(PyDictObject *mp, PyObject *Py_UNUSED(ignored))
{
    /* Return a reverse iterator over keys (LIFO order). */
    dictreversekeysobject *di;
    di = PyObject_New(dictreversekeysobject, &PyDictRevIterKey_Type);
    di->dv_dict = Py_NewRef(mp);
    di->di_pos = mp->ma_keys->dk_nentries;
    return (PyObject *)di;
}

reversed(d) (Python 3.8+) iterates keys from last-inserted to first. Used for stack-like patterns: insert multiple items, then process in reverse order.

View set operations

// CPython: Objects/dictobject.c:3680 dictviews_and
static PyObject *
dictviews_and(PyObject *self, PyObject *other)
{
    /* dict.keys() & set or dict.keys() & dict.keys()
       Returns the intersection as a new set. */
    PyObject *result = PySet_New(self);
    PyObject_CallMethodOneArg(result, &_Py_ID(intersection_update), other);
    return result;
}

d.keys() & s returns a set of keys in both d and s. d.keys() - s returns keys in d but not s. These operations make dict.keys() behave like a frozen set for membership testing.

gopy notes

dict.popitem is objects.DictPopItem in objects/dict.go. It scans backward through the entries array. dict.__reversed__ returns objects.DictReverseKeysIterator. View objects (dict_keys, dict_values, dict_items) are objects.DictKeysView, etc. View set operations use objects.SetNew and objects.SetIntersectionUpdate.