Objects/dictobject.c (part 13)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/dictobject.c ↗

This annotation covers dict creation helpers and views. See objects_dictobject12_detail for dict merge, update, and __or__.

Map

Lines	Symbol	Role
1-80	dict.fromkeys	Class method: create dict from keys iterable
81-160	dict.__reversed__	Reverse insertion-order iterator
161-280	dict.keys / dict.values / dict.items	View objects
281-400	dictview.__and__ / __or__	Set-like operations on dict views
401-500	dictiterobject	Shared iterator implementation

Reading

dict.fromkeys

// CPython: Objects/dictobject.c:2280 dict_fromkeys
static PyObject *
dict_fromkeys(PyObject *type, PyObject *const *args, Py_ssize_t nargs)
{
    PyObject *iterable = args[0];
    PyObject *value = nargs >= 2 ? args[1] : Py_None;
    PyObject *d = PyObject_CallOneArg(type, NULL);
    PyObject *it = PyObject_GetIter(iterable);
    PyObject *key;
    while ((key = PyIter_Next(it)) != NULL) {
        if (PyDict_SetItem(d, key, value) < 0) { ... }
        Py_DECREF(key);
    }
    return d;
}

dict.fromkeys(['a', 'b', 'c'], 0) creates {'a': 0, 'b': 0, 'c': 0}. All values are the same object (not copies), which is a common gotcha with mutable defaults: dict.fromkeys(keys, []) creates a dict where all values share the same list.

dict.__reversed__

// CPython: Objects/dictobject.c:2640 dict_reversed
static PyObject *
dict_reversed(PyObject *self, PyObject *Py_UNUSED(ignored))
{
    return dictiter_new((PyDictObject *)self, &PyDictRevIterKey_Type);
}

static PyObject *
dictreviter_iternext(dictiterobject *di)
{
    Py_ssize_t i = di->di_pos;
    /* Walk backwards through the compact array */
    while (i >= 0) {
        PyDictKeysObject *k = di->di_dict->ma_keys;
        PyObject *key = DK_ENTRIES(k)[i].me_key;
        i--;
        if (key == NULL || key == dummy) continue;
        di->di_pos = i;
        return Py_NewRef(key);
    }
    ...
}

reversed(d) yields keys in reverse insertion order. The reverse iterator walks the compact entry array backward, skipping deleted slots. dict.__reversed__ was added in Python 3.8 (dict is ordered since 3.7).

Dict views

// CPython: Objects/dictobject.c:2720 dictkeys_new
static PyObject *
dictkeys_new(PyObject *self, PyObject *Py_UNUSED(ignored))
{
    return _PyDictView_New(self, &PyDictKeys_Type);
}

d.keys(), d.values(), d.items() return view objects that reflect the live state of the dict. They support len, iter, in, and (for keys and items) set operations. Views hold a reference to the dict but do not copy it.

dictview.__and__

// CPython: Objects/dictobject.c:2860 dictviews_and
static PyObject *
dictviews_and(PyObject *self, PyObject *other)
{
    /* keys & other_set: intersection */
    PyObject *result = PySet_New(self);
    PyObject *tmp = PyObject_CallMethodOneArg(result, &_Py_ID(intersection_update), other);
    Py_DECREF(tmp);
    return result;
}

d1.keys() & d2.keys() returns a set of keys common to both dicts. d1.items() - d2.items() returns items in d1 but not d2. These operations work because keys and items views implement the Set abstract base class protocol.

gopy notes

dict.fromkeys is objects.DictFromKeys in objects/dictobject.go. dict.__reversed__ is objects.DictReversed returning an objects.DictRevIter. Dict views are objects.DictKeysView, objects.DictValuesView, objects.DictItemsView. Set operations on views call objects.SetNew and objects.SetIntersectionUpdate.