Objects/dictobject.c (part 11)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/dictobject.c ↗

This annotation covers dict construction helpers and view objects. See objects_dictobject10_detail for dict.__or__, dict.update, dict.__ior__, and compact dict internals.

Map

Lines	Symbol	Role
1-80	dict.fromkeys	Build a dict from an iterable of keys
81-160	dict.__reversed__	Iterate keys in insertion-reverse order
161-240	dict_keys view	Live view of keys
241-360	dict_values view	Live view of values
361-500	dict_items view	Live view of (key, value) pairs

Reading

dict.fromkeys

// CPython: Objects/dictobject.c:3280 dict_fromkeys
static PyObject *
dict_fromkeys(PyTypeObject *type, PyObject *const *args, Py_ssize_t nargs)
{
    PyObject *seq = args[0];
    PyObject *value = nargs >= 2 ? args[1] : Py_None;
    PyObject *d = type->tp_new(type, NULL, NULL);
    PyObject *it = PyObject_GetIter(seq);
    PyObject *key;
    while ((key = PyIter_Next(it)) != NULL) {
        PyDict_SetItem(d, key, value);
        Py_DECREF(key);
    }
    Py_DECREF(it);
    return d;
}

dict.fromkeys(['a', 'b', 'c'], 0) creates {'a': 0, 'b': 0, 'c': 0}. All keys map to the same value object (not copies). Mutating value after calling fromkeys with a mutable default affects all entries.

dict.__reversed__

// CPython: Objects/dictobject.c:3360 dictreviter_new
static PyObject *
dictreviter_new(PyDictObject *dict, PyObject *Py_UNUSED(ignored))
{
    dictiterobject *di = PyObject_GC_New(dictiterobject, &PyDictRevIterKey_Type);
    di->dv_dict = dict;
    Py_INCREF(dict);
    di->di_pos = dict->ma_used - 1;  /* start from last */
    di->len = dict->ma_used;
    return (PyObject *)di;
}

reversed(d) iterates keys from the last-inserted to the first. Since Python 3.8 dict is insertion-ordered. The reverse iterator starts at ma_used - 1 and steps backward through the compact entries.

dict_keys view

// CPython: Objects/dictobject.c:3480 dictkeys_iter
static PyObject *
dictkeys_iter(dictviewobject *dv)
{
    /* Return a new dict_keyiterator */
    return dictiter_new(dv->dv_dict, &PyDictIterKey_Type);
}

static PyObject *
dictkeys_contains(dictviewobject *dv, PyObject *key)
{
    return PyBool_FromLong(PyDict_Contains(dv->dv_dict, key));
}

d.keys() returns a dict_keys view object that does not copy the keys. It supports len, in, and iteration. Because it is live, iterating after modification raises RuntimeError: dictionary changed size during iteration.

dict_values view

// CPython: Objects/dictobject.c:3540 dictvalues_iter
static PyObject *
dictvalues_iter(dictviewobject *dv)
{
    return dictiter_new(dv->dv_dict, &PyDictIterValue_Type);
}

d.values() is a live view of values. Unlike d.keys(), dict_values does not support __contains__ or set operations because values are not unique or hashable.

dict_items view

// CPython: Objects/dictobject.c:3600 dictitems_contains
static int
dictitems_contains(dictviewobject *dv, PyObject *obj)
{
    PyObject *key, *value, *found;
    if (!PyTuple_Check(obj) || PyTuple_GET_SIZE(obj) != 2)
        return 0;
    key = PyTuple_GET_ITEM(obj, 0);
    value = PyTuple_GET_ITEM(obj, 1);
    found = PyDict_GetItemWithError(dv->dv_dict, key);
    if (found == NULL) return 0;
    return PyObject_RichCompareBool(found, value, Py_EQ);
}

(key, value) in d.items() checks both the key and value. dict_items supports set operations (&, |, -) because each (k, v) pair is unique if values are hashable.

gopy notes

dict.fromkeys is objects.DictFromKeys in objects/dict.go. __reversed__ returns objects.DictRevKeyIter. dict.keys() returns objects.DictKeysView. dict.values() returns objects.DictValuesView. dict.items() returns objects.DictItemsView. All views hold a reference to the dict.