From: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Date: Tue, 16 Dec 2025 15:31:05 +0000 (+0100)
Subject: [3.14] gh-142495: Make `defaultdict` keep existed value when racing with `__missing__... 
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d089cad6564f6ef713b5acab85cb42b28611f5e8;p=thirdparty%2FPython%2Fcpython.git

[3.14] gh-142495: Make `defaultdict` keep existed value when racing with `__missing__` (GH-142668) (GH-142832)

(cherry picked from commit a0434075108efe6acdfba34f42545f4d80ac9a5e)

Co-authored-by: Edward Xu <xuxiangad@gmail.com>
---

diff --git a/Lib/test/test_defaultdict.py b/Lib/test/test_defaultdict.py
index bdbe9b81e8fb..fbd7354a915a 100644
--- a/Lib/test/test_defaultdict.py
+++ b/Lib/test/test_defaultdict.py
@@ -186,5 +186,23 @@ class TestDefaultDict(unittest.TestCase):
         with self.assertRaises(TypeError):
             i |= None
 
+    def test_factory_conflict_with_set_value(self):
+        key = "conflict_test"
+        count = 0
+
+        def default_factory():
+            nonlocal count
+            count += 1
+            local_count = count
+            if count == 1:
+                test_dict[key]
+            return local_count
+
+        test_dict = defaultdict(default_factory)
+
+        self.assertEqual(count, 0)
+        self.assertEqual(test_dict[key], 2)
+        self.assertEqual(count, 2)
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2025-12-13-23-26-42.gh-issue-142495.I88Uv_.rst b/Misc/NEWS.d/next/Library/2025-12-13-23-26-42.gh-issue-142495.I88Uv_.rst
new file mode 100644
index 000000000000..3e1a624fe564
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-13-23-26-42.gh-issue-142495.I88Uv_.rst
@@ -0,0 +1,4 @@
+:class:`collections.defaultdict` now prioritizes :meth:`~object.__setitem__`
+when inserting default values from ``default_factory``. This prevents race
+conditions where a default value would overwrite a value set before
+``default_factory`` returns.
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index 3ba48d5d9d3c..3b14a21fa842 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -2231,11 +2231,11 @@ defdict_missing(PyObject *op, PyObject *key)
     value = _PyObject_CallNoArgs(factory);
     if (value == NULL)
         return value;
-    if (PyObject_SetItem(op, key, value) < 0) {
-        Py_DECREF(value);
-        return NULL;
-    }
-    return value;
+    PyObject *result = NULL;
+    (void)PyDict_SetDefaultRef(op, key, value, &result);
+    // 'result' is NULL, or a strong reference to 'value' or 'op[key]'
+    Py_DECREF(value);
+    return result;
 }
 
 static inline PyObject*