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gh-141510: Optimize frozendict(frozendict) #145592

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99d7ca6
gh-141510: Optimize frozendict(frozendict)
vstinner Mar 6, 2026
5b07811
Optimize also PyFrozenDict_New(frozendict)
vstinner Mar 6, 2026
114ffb5
Document PyFrozenSet_New() optimization
vstinner Mar 6, 2026
9de1f08
Simplify the doc; omit subclass special case
vstinner Mar 6, 2026
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4 changes: 4 additions & 0 deletions Doc/c-api/dict.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -542,6 +542,10 @@ Frozen dictionary objects

Create an empty dictionary if *iterable* is ``NULL``.

.. impl-detail::
If *iterable* is a :class:`frozendict`, return the same object
unmodified.


Ordered dictionaries
^^^^^^^^^^^^^^^^^^^^
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10 changes: 10 additions & 0 deletions Lib/test/test_capi/test_dict.py
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Expand Up @@ -619,6 +619,16 @@ def test_frozendict_new(self):
self.assertEqual(dct, frozendict(x=1, y=2))
self.assertIs(type(dct), frozendict)

# PyFrozenDict_New(frozendict) returns the same object unmodified
fd = frozendict(a=1, b=2, c=3)
fd2 = frozendict_new(fd)
self.assertIs(fd2, fd)

fd = FrozenDictSubclass(a=1, b=2, c=3)
fd2 = frozendict_new(fd)
self.assertIsNot(fd2, fd)
self.assertEqual(fd2, fd)

# PyFrozenDict_New(NULL) creates an empty dictionary
dct = frozendict_new(NULL)
self.assertEqual(dct, frozendict())
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7 changes: 7 additions & 0 deletions Lib/test/test_dict.py
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Expand Up @@ -1829,6 +1829,13 @@ def test_constructor(self):
with self.assertRaises(TypeError):
dict.__init__(d, x=1)

# Avoid copy if it's frozendict type
d2 = frozendict(d)
self.assertIs(d2, d)
d2 = FrozenDict(d)
self.assertIsNot(d2, d)
self.assertEqual(d2, d)

def test_copy(self):
d = frozendict(x=1, y=2)
d2 = d.copy()
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53 changes: 45 additions & 8 deletions Objects/dictobject.c
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Expand Up @@ -5169,15 +5169,47 @@ dict_vectorcall(PyObject *type, PyObject * const*args,
return NULL;
}

PyObject *self;
if (Py_Is((PyTypeObject*)type, &PyFrozenDict_Type)
|| PyType_IsSubtype((PyTypeObject*)type, &PyFrozenDict_Type))
{
self = frozendict_new(_PyType_CAST(type), NULL, NULL);
PyObject *self = dict_new(_PyType_CAST(type), NULL, NULL);
if (self == NULL) {
return NULL;
}
else {
self = dict_new(_PyType_CAST(type), NULL, NULL);
if (nargs == 1) {
if (dict_update_arg(self, args[0]) < 0) {
Py_DECREF(self);
return NULL;
}
args++;
}
if (kwnames != NULL) {
for (Py_ssize_t i = 0; i < PyTuple_GET_SIZE(kwnames); i++) {
PyObject *key = PyTuple_GET_ITEM(kwnames, i); // borrowed
if (PyDict_SetItem(self, key, args[i]) < 0) {
Py_DECREF(self);
return NULL;
}
}
}
return self;
}

static PyObject *
frozendict_vectorcall(PyObject *type, PyObject * const*args,
size_t nargsf, PyObject *kwnames)
{
Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
if (!_PyArg_CheckPositional("frozendict", nargs, 0, 1)) {
return NULL;
}

if (nargs == 1 && kwnames == NULL
&& PyFrozenDict_CheckExact(args[0])
&& Py_Is((PyTypeObject*)type, &PyFrozenDict_Type))
{
// frozendict(frozendict) returns the same object unmodified
return Py_NewRef(args[0]);
}

PyObject *self = frozendict_new(_PyType_CAST(type), NULL, NULL);
if (self == NULL) {
return NULL;
}
Expand Down Expand Up @@ -8171,6 +8203,11 @@ PyObject*
PyFrozenDict_New(PyObject *iterable)
{
if (iterable != NULL) {
if (PyFrozenDict_CheckExact(iterable)) {
// PyFrozenDict_New(frozendict) returns the same object unmodified
return Py_NewRef(iterable);
}
Comment on lines +8206 to +8209
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@encukou encukou Mar 6, 2026

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This is how I'd like it, but do note that PyTuple_New or PyFrozenSet_New will always return a new object.

It's probably worth noting this in documentation.

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@vstinner vstinner Mar 6, 2026

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Hmm, usually we don't document such optimization, but ok, I added it to PyFrozenDict_New() documentation as a "CPython implementation detail".

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@kumaraditya303 kumaraditya303 Mar 7, 2026

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Why document it if it is an internal implementation detail anyways? It gives the wrong sense that users can rely on this in CPython


PyObject *args = PyTuple_Pack(1, iterable);
if (args == NULL) {
return NULL;
Expand Down Expand Up @@ -8228,6 +8265,6 @@ PyTypeObject PyFrozenDict_Type = {
.tp_alloc = _PyType_AllocNoTrack,
.tp_new = frozendict_new,
.tp_free = PyObject_GC_Del,
.tp_vectorcall = dict_vectorcall,
.tp_vectorcall = frozendict_vectorcall,
.tp_version_tag = _Py_TYPE_VERSION_FROZENDICT,
};
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