gh-151722: Do not track the frozendict in the GC in _PyDict_FromKeys() (#152067)

vstinner · corona10 · methane · web-flow · commit 55bc3126e0a0 · 2026-06-25T18:06:07.000Z
_PyDict_FromKeys() now creates a frozendict copy which is
not tracked by the GC.

dict_merge() no longer requires the dictionary to be tracked by the
GC.

Co-authored-by: Donghee Na &lt;donghee.na@python.org&gt;
Co-authored-by: Inada Naoki &lt;songofacandy@gmail.com&gt;
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
@@ -5757,6 +5757,13 @@ Frozen dictionaries
    Like dictionaries, frozendicts are :ref:`generic <generics>` over two types,
    signifying (respectively) the types of the frozendict's keys and values.
 
+   .. classmethod:: fromkeys(iterable, value=None, /)
+
+      Similar to :meth:`dict.fromkeys`, but call again the type constructor
+      with an initialized :class:`frozendict` if the type is a
+      :class:`frozendict` subclass or if the constructor returned a
+      :class:`frozendict`.
+
    .. versionadded:: 3.15
 
 
diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
@@ -1939,8 +1939,11 @@ def test_fromkeys(self):
         # Subclass which overrides the constructor
         created = frozendict(x=1)
         class FrozenDictSubclass(frozendict):
-            def __new__(self):
-                return created
+            def __new__(cls, *args, **kwargs):
+                if args or kwargs:
+                    return super().__new__(cls, *args, **kwargs)
+                else:
+                    return created
 
         fd = FrozenDictSubclass.fromkeys("abc")
         self.assertEqual(fd, frozendict(x=1, a=None, b=None, c=None))
@@ -1952,6 +1955,20 @@ def __new__(self):
         self.assertEqual(type(fd), FrozenDictSubclass)
         self.assertEqual(created, frozendict(x=1))
 
+        # Dict subclass with a constructor which returns a frozendict
+        # by default
+        class DictSubclass(dict):
+            def __new__(cls, *args, **kwargs):
+                if args or kwargs:
+                    return super().__new__(cls, *args, **kwargs)
+                else:
+                    return created
+
+        fd = DictSubclass.fromkeys("abc")
+        self.assertEqual(fd, frozendict(x=1, a=None, b=None, c=None))
+        self.assertEqual(type(fd), DictSubclass)
+        self.assertEqual(created, frozendict(x=1))
+
         # Subclass which doesn't override the constructor
         class FrozenDictSubclass2(frozendict):
             pass
@@ -1960,16 +1977,6 @@ class FrozenDictSubclass2(frozendict):
         self.assertEqual(fd, frozendict(a=None, b=None, c=None))
         self.assertEqual(type(fd), FrozenDictSubclass2)
 
-        # Dict subclass which overrides the constructor
-        class DictSubclass(dict):
-            def __new__(self):
-                return created
-
-        fd = DictSubclass.fromkeys("abc")
-        self.assertEqual(fd, frozendict(x=1, a=None, b=None, c=None))
-        self.assertEqual(type(fd), DictSubclass)
-        self.assertEqual(created, frozendict(x=1))
-
     def test_pickle(self):
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
             for fd in (
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-06-24-13-36-31.gh-issue-151722.lWKfE1.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-06-24-13-36-31.gh-issue-151722.lWKfE1.rst
@@ -0,0 +1,3 @@
+:meth:`!frozendict.fromkeys` now only tracks the :class:`frozendict` in the
+garbage collector once the dictionary is fully initialized. Patch by Donghee Na
+and Victor Stinner.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
@@ -140,6 +140,7 @@ static PyObject* frozendict_new(PyTypeObject *type, PyObject *args,
                                 PyObject *kwds);
 static PyObject* frozendict_new_untracked(PyTypeObject *type);
 static PyObject* dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
+static PyObject* dict_new_untracked(PyTypeObject *type);
 static int dict_merge(PyObject *a, PyObject *b, int override, PyObject **dupkey);
 static int dict_contains(PyObject *op, PyObject *key);
 static int dict_merge_from_seq2(PyObject *d, PyObject *seq2, int override);
@@ -3431,40 +3432,47 @@ dict_set_fromkeys(PyDictObject *mp, PyObject *iterable, PyObject *value)
 PyObject *
 _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
 {
-    PyObject *it;       /* iter(iterable) */
-    PyObject *key;
+    PyObject *it = NULL;       /* iter(iterable) */
     PyObject *d;
-    int status;
+    int need_copy = 0;
 
-    d = _PyObject_CallNoArgs(cls);
+    if (cls == (PyObject*)&PyFrozenDict_Type) {
+        // gh-151722: Create a frozendict which is not tracked by the GC.
+        d = frozendict_new_untracked(&PyFrozenDict_Type);
+    }
+    else {
+        // Dict subclass, or frozendict subclass which overrides
+        // the constructor.
+        d = _PyObject_CallNoArgs(cls);
+    }
     if (d == NULL) {
         return NULL;
     }
 
-    // If cls is a dict or frozendict subclass with overridden constructor,
-    // copy the frozendict.
-    PyTypeObject *cls_type = _PyType_CAST(cls);
-    if (PyFrozenDict_Check(d) && cls_type->tp_new != frozendict_new) {
-        // Subclass-friendly copy
-        PyObject *copy;
-        if (PyObject_IsSubclass(cls, (PyObject*)&PyFrozenDict_Type)) {
-            copy = frozendict_new(cls_type, NULL, NULL);
-        }
-        else {
-            copy = dict_new(cls_type, NULL, NULL);
-        }
+    // gh-151722: If cls constructor returns a frozendict which is tracked by
+    // the GC, create a frozendict copy which is not tracked by the GC.
+    //
+    // At the function exit, return cls(fd) where fd is a frozendict.
+    //
+    // Untracking the frozendict requires tracking again the frozendict on
+    // error which is more complicated. It's easier to work on a copy.
+    if (PyFrozenDict_Check(d) && _PyObject_GC_IS_TRACKED(d)) {
+        need_copy = 1;
+
+        PyObject *copy = frozendict_new_untracked(&PyFrozenDict_Type);
         if (copy == NULL) {
-            Py_DECREF(d);
-            return NULL;
+            goto Fail;
         }
         if (dict_merge(copy, d, 1, NULL) < 0) {
-            Py_DECREF(d);
             Py_DECREF(copy);
-            return NULL;
+            goto Fail;
         }
         Py_SETREF(d, copy);
     }
-    assert(!PyFrozenDict_Check(d) || can_modify_dict((PyDictObject*)d));
+    if (PyFrozenDict_Check(d)) {
+        assert(can_modify_dict((PyDictObject*)d));
+        assert(!_PyObject_GC_IS_TRACKED(d));
+    }
 
     if (PyDict_CheckExact(d)) {
         if (PyDict_CheckExact(iterable)) {
@@ -3473,23 +3481,23 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
             Py_BEGIN_CRITICAL_SECTION2(d, iterable);
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION2();
-            return d;
+            goto Done;
         }
         else if (PyFrozenDict_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
 
             Py_BEGIN_CRITICAL_SECTION(d);
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION();
-            return d;
+            goto Done;
         }
         else if (PyAnySet_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
 
             Py_BEGIN_CRITICAL_SECTION2(d, iterable);
             d = (PyObject *)dict_set_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION2();
-            return d;
+            goto Done;
         }
     }
     else if (PyFrozenDict_CheckExact(d)) {
@@ -3499,71 +3507,113 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
             Py_BEGIN_CRITICAL_SECTION(iterable);
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION();
-            return d;
+            goto Done;
         }
         else if (PyFrozenDict_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
-            return d;
+            goto Done;
         }
         else if (PyAnySet_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
 
             Py_BEGIN_CRITICAL_SECTION(iterable);
             d = (PyObject *)dict_set_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION();
-            return d;
+            goto Done;
         }
     }
 
     it = PyObject_GetIter(iterable);
     if (it == NULL){
-        Py_DECREF(d);
-        return NULL;
+        goto Fail;
     }
 
     if (PyDict_CheckExact(d)) {
+        int status = 0;
+
         Py_BEGIN_CRITICAL_SECTION(d);
-        while ((key = PyIter_Next(it)) != NULL) {
+        while (1) {
+            PyObject *key;
+            status = PyIter_NextItem(it, &key);
+            if (status <= 0) {
+                break;
+            }
+
             status = setitem_lock_held((PyDictObject *)d, key, value);
             Py_DECREF(key);
             if (status < 0) {
-                assert(PyErr_Occurred());
-                goto dict_iter_exit;
+                break;
             }
         }
-dict_iter_exit:;
         Py_END_CRITICAL_SECTION();
+
+        if (status < 0) {
+            goto Fail;
+        }
     }
     else if (PyFrozenDict_Check(d)) {
-        while ((key = PyIter_Next(it)) != NULL) {
+        while (1) {
+            PyObject *key;
+            int status = PyIter_NextItem(it, &key);
+            if (status < 0) {
+                goto Fail;
+            }
+            if (status == 0) {
+                break;
+            }
+
             // setitem_take2_lock_held consumes a reference to key
             status = setitem_take2_lock_held((PyDictObject *)d,
                                              key, Py_NewRef(value));
             if (status < 0) {
-                assert(PyErr_Occurred());
                 goto Fail;
             }
         }
     }
     else {
-        while ((key = PyIter_Next(it)) != NULL) {
+        while (1) {
+            PyObject *key;
+            int status = PyIter_NextItem(it, &key);
+            if (status < 0) {
+                goto Fail;
+            }
+            if (status == 0) {
+                break;
+            }
+
             status = PyObject_SetItem(d, key, value);
             Py_DECREF(key);
-            if (status < 0)
+            if (status < 0) {
                 goto Fail;
+            }
         }
+
     }
 
-    if (PyErr_Occurred())
-        goto Fail;
+    assert(!PyErr_Occurred());
     Py_DECREF(it);
-    return d;
+    goto Done;
 
 Fail:
-    Py_DECREF(it);
+    assert(PyErr_Occurred());
+    Py_XDECREF(it);
     Py_DECREF(d);
     return NULL;
+
+Done:
+    if (d == NULL) {
+        return NULL;
+    }
+
+    if (need_copy) {
+        PyObject *copy = _PyObject_CallOneArg(cls, d);
+        Py_SETREF(d, copy);
+    }
+    else if (!_PyObject_GC_IS_TRACKED(d)) {
+        _PyObject_GC_TRACK(d);
+    }
+    return d;
 }
 
 /* Methods */
@@ -4164,9 +4214,6 @@ dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override, PyObject **
             set_keys(mp, keys);
             STORE_USED(mp, other->ma_used);
             ASSERT_CONSISTENT(mp);
-            if (PyDict_Check(mp)) {
-                assert(_PyObject_GC_IS_TRACKED(mp));
-            }
             return 0;
         }
     }
@@ -4333,7 +4380,12 @@ dict_merge_api(PyObject *a, PyObject *b, int override, PyObject **dupkey)
         }
         return -1;
     }
-    return dict_merge(a, b, override, dupkey);
+
+    int res = dict_merge(a, b, override, dupkey);
+    if (PyDict_Check(a)) {
+        assert(_PyObject_GC_IS_TRACKED(a));
+    }
+    return res;
 }
 
 int
@@ -4492,10 +4544,15 @@ copy_lock_held(PyObject *o, int as_frozendict)
     }
     if (copy == NULL)
         return NULL;
-    if (dict_merge(copy, o, 1, NULL) == 0)
-        return copy;
-    Py_DECREF(copy);
-    return NULL;
+    if (dict_merge(copy, o, 1, NULL) < 0) {
+        Py_DECREF(copy);
+        return NULL;
+    }
+
+    if (PyDict_Check(copy)) {
+        assert(_PyObject_GC_IS_TRACKED(copy));
+    }
+    return copy;
 }
 
 PyObject *
@@ -5256,11 +5313,11 @@ static PyNumberMethods dict_as_number = {
     .nb_inplace_or = _PyDict_IOr,
 };
 
-static PyObject *
-dict_new_untracked(PyTypeObject *type)
+static PyObject*
+anydict_new_untracked(PyTypeObject *type)
 {
     assert(type != NULL);
-    // dict subclasses must implement the GC protocol
+    // dict and frozendict subclasses must implement the GC protocol
     assert(_PyType_IS_GC(type));
 
     PyObject *self = _PyType_AllocNoTrack(type, 0);
@@ -5279,6 +5336,14 @@ dict_new_untracked(PyTypeObject *type)
     return self;
 }
 
+static PyObject*
+dict_new_untracked(PyTypeObject *type)
+{
+    assert(PyObject_IsSubclass((PyObject*)type, (PyObject*)&PyDict_Type));
+
+    return anydict_new_untracked(type);
+}
+
 static PyObject *
 dict_new(PyTypeObject *type, PyObject *Py_UNUSED(args), PyObject *Py_UNUSED(kwds))
 {
@@ -8394,7 +8459,9 @@ frozendict_hash(PyObject *op)
 static PyObject *
 frozendict_new_untracked(PyTypeObject *type)
 {
-    PyObject *d = dict_new_untracked(type);
+    assert(PyObject_IsSubclass((PyObject*)type, (PyObject*)&PyFrozenDict_Type));
+
+    PyObject *d = anydict_new_untracked(type);
     if (d == NULL) {
         return NULL;
     }

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+:meth:`!frozendict.fromkeys` now only tracks the :class:`frozendict` in the
	`2`	`+garbage collector once the dictionary is fully initialized. Patch by Donghee Na`
	`3`	`+and Victor Stinner.`