https://github.com/python/cpython/commit/d8994b0a77cc9821772d05db00a6ab23382fa17d
commit: d8994b0a77cc9821772d05db00a6ab23382fa17d
branch: main
author: Sam Gross <colesb...@gmail.com>
committer: colesbury <colesb...@gmail.com>
date: 2025-06-16T12:55:20-04:00
summary:
gh-132617: Fix `dict.update()` mutation check (gh-134815)
Use `ma_used` instead of `ma_keys->dk_nentries` for modification check
so that we only check if the dictionary is modified, not if new keys are
added to a different dictionary that shared the same keys object.
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2025-05-27-20-29-00.gh-issue-132617.EmUfQQ.rst
M Lib/test/test_dict.py
M Objects/dictobject.c
diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 52c38e42eca02d..60c62430370e96 100644
--- a/Lib/test/test_dict.py
+++ b/Lib/test/test_dict.py
@@ -290,6 +290,38 @@ def badgen():
['Cannot convert dictionary update sequence element #0 to a
sequence'],
)
+ def test_update_shared_keys(self):
+ class MyClass: pass
+
+ # Subclass str to enable us to create an object during the
+ # dict.update() call.
+ class MyStr(str):
+ def __hash__(self):
+ return super().__hash__()
+
+ def __eq__(self, other):
+ # Create an object that shares the same PyDictKeysObject as
+ # obj.__dict__.
+ obj2 = MyClass()
+ obj2.a = "a"
+ obj2.b = "b"
+ obj2.c = "c"
+ return super().__eq__(other)
+
+ obj = MyClass()
+ obj.a = "a"
+ obj.b = "b"
+
+ x = {}
+ x[MyStr("a")] = MyStr("a")
+
+ # gh-132617: this previously raised "dict mutated during update" error
+ x.update(obj.__dict__)
+
+ self.assertEqual(x, {
+ MyStr("a"): "a",
+ "b": "b",
+ })
def test_fromkeys(self):
self.assertEqual(dict.fromkeys('abc'), {'a':None, 'b':None, 'c':None})
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-27-20-29-00.gh-issue-132617.EmUfQQ.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-27-20-29-00.gh-issue-132617.EmUfQQ.rst
new file mode 100644
index 00000000000000..53aef541e64c23
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-27-20-29-00.gh-issue-132617.EmUfQQ.rst
@@ -0,0 +1,3 @@
+Fix :meth:`dict.update` modification check that could incorrectly raise a
+"dict mutated during update" error when a different dictionary was modified
+that happens to share the same underlying keys object.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index fd8ccf56324207..72901377cbb3da 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -3858,7 +3858,7 @@ dict_dict_merge(PyInterpreterState *interp, PyDictObject
*mp, PyDictObject *othe
}
}
- Py_ssize_t orig_size = other->ma_keys->dk_nentries;
+ Py_ssize_t orig_size = other->ma_used;
Py_ssize_t pos = 0;
Py_hash_t hash;
PyObject *key, *value;
@@ -3892,7 +3892,7 @@ dict_dict_merge(PyInterpreterState *interp, PyDictObject
*mp, PyDictObject *othe
if (err != 0)
return -1;
- if (orig_size != other->ma_keys->dk_nentries) {
+ if (orig_size != other->ma_used) {
PyErr_SetString(PyExc_RuntimeError,
"dict mutated during update");
return -1;
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