From bb2c546a21022b6bf09410211094ffd006f760cc Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Wed, 18 Feb 2026 11:27:46 +0100
Subject: [PATCH 1/2] gh-141510: Add frozendict_check_mutable()

frozendict.fromkeys() now checks if it can mutate the newly created
frozendict.
---
 Lib/test/test_dict.py | 18 ++++++++++++++++++
 Objects/dictobject.c  | 24 ++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 21f8bb11071c90..5c0916d5802be0 100644
--- a/Lib/test/test_dict.py
+++ b/Lib/test/test_dict.py
@@ -1787,6 +1787,24 @@ def test_hash(self):
         with self.assertRaisesRegex(TypeError, "unhashable type: 'list'"):
             hash(fd)
 
+    def test_fromkeys(self):
+        self.assertEqual(frozendict.fromkeys('abc'),
+                         frozendict(a=None, b=None, c=None))
+
+        # special class which keeps a reference to the created dictionary
+        fd = None
+        class SpecialDict(frozendict):
+            def __new__(self):
+                nonlocal fd
+                fd = frozendict()
+                return fd
+
+        errmsg = "cannot mutate frozendict already exposed in Python"
+        with self.assertRaisesRegex(RuntimeError, errmsg):
+            SpecialDict.fromkeys(frozendict(x=1))
+        with self.assertRaisesRegex(RuntimeError, errmsg):
+            SpecialDict.fromkeys("def")
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 0959e2c78a3289..80a81a1226bed6 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -138,6 +138,7 @@ As a consequence of this, split keys have a maximum size of 16.
 // Forward declarations
 static PyObject* frozendict_new(PyTypeObject *type, PyObject *args,
                                 PyObject *kwds);
+static int frozendict_check_mutable(PyObject *self);
 
 
 /*[clinic input]
@@ -3317,6 +3318,11 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
         }
     }
     else if (PyFrozenDict_CheckExact(d)) {
+        // Check if the class constructor kept a reference to the frozendict
+        if (frozendict_check_mutable(d) < 0) {
+            return NULL;
+        }
+
         if (PyDict_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
 
@@ -3360,6 +3366,11 @@ dict_iter_exit:;
         Py_END_CRITICAL_SECTION();
     }
     else if (PyFrozenDict_CheckExact(d)) {
+        // Check if the class constructor kept a reference to the frozendict
+        if (frozendict_check_mutable(d) < 0) {
+            goto Fail;
+        }
+
         while ((key = PyIter_Next(it)) != NULL) {
             // anydict_setitem_take2 consumes a reference to key
             status = anydict_setitem_take2((PyDictObject *)d,
@@ -7889,6 +7900,17 @@ _PyObject_InlineValuesConsistencyCheck(PyObject *obj)
 
 // --- frozendict implementation ---------------------------------------------
 
+static int
+frozendict_check_mutable(PyObject *self)
+{
+    if (Py_REFCNT(self) > 1) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "cannot mutate frozendict already exposed in Python");
+        return -1;
+    }
+    return 0;
+}
+
 static PyNumberMethods frozendict_as_number = {
     .nb_or = frozendict_or,
 };
@@ -7994,6 +8016,8 @@ frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     if (d == NULL) {
         return NULL;
     }
+    assert(Py_REFCNT(self) == 1);
+
     PyFrozenDictObject *self = _PyFrozenDictObject_CAST(d);
     self->ma_hash = -1;
 

From 5ecf0f951667024fad98b75b0a94a651a3f55818 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Wed, 18 Feb 2026 11:37:58 +0100
Subject: [PATCH 2/2] Fix typo

---
 Objects/dictobject.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 80a81a1226bed6..053d5a465329ee 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -8016,7 +8016,7 @@ frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     if (d == NULL) {
         return NULL;
     }
-    assert(Py_REFCNT(self) == 1);
+    assert(Py_REFCNT(d) == 1);
 
     PyFrozenDictObject *self = _PyFrozenDictObject_CAST(d);
     self->ma_hash = -1;