gh-149534: Fix unification of defaultdict and frozendict with | (#149539)

Lib/test/test_defaultdict.py

@@ -186,6 +186,18 @@ def test_union(self):
         with self.assertRaises(TypeError):
             i |= None
 
+        # frozendict
+        i_fd = i | frozendict(s)
+        self.assertIs(type(i_fd), defaultdict)
+        self.assertIs(i_fd.default_factory, int)
+        self.assertDictEqual(i_fd, {1: "one", 2: 2, 0: "zero"})
+        self.assertEqual(list(i_fd), [1, 2, 0])
+
+        fd_i = frozendict(s) | i
+        self.assertIs(type(fd_i), frozendict)
+        self.assertEqual(fd_i, {1: "one", 2: 2, 0: "zero"})
+        self.assertEqual(list(fd_i), [0, 1, 2])
+
     def test_factory_conflict_with_set_value(self):
         key = "conflict_test"
         count = 0

Misc/NEWS.d/next/Library/2026-05-08-09-11-48.gh-issue-149534.Tw7eeY.rst

@@ -0,0 +1 @@
+Fix merging of :class:`collections.defaultdict` and :class:`frozendict`.

Modules/_collectionsmodule.c

@@ -2421,7 +2421,7 @@ defdict_or(PyObject* left, PyObject* right)
         self = right;
         other = left;
     }
-    if (!PyDict_Check(other)) {
+    if (!PyAnyDict_Check(other)) {
         Py_RETURN_NOTIMPLEMENTED;
     }
     // Like copy(), this calls the object's class.