gh-142085: Avoid raising in ChainMap.__ior__ before trying other.__ror__

Lib/collections/__init__.py

@@ -1151,7 +1151,10 @@ def clear(self):
         self.maps[0].clear()
 
     def __ior__(self, other):
-        self.maps[0].update(other)
+        try:
+            self.maps[0].update(other)
+        except TypeError:
+            return NotImplemented
         return self
 
     def __or__(self, other):

Lib/test/test_collections.py

@@ -309,6 +309,19 @@ def __ror__(self, other):
         self.assertIs(type(tmp), SubclassRor)
         self.assertIs(type(tmp.maps[0]), dict)
 
+    def test_ior_fallback(self):
+        # Verify that __ior__ returns NotImplemented for unrecognized types,
+        # allowing the other operand to handle the operation via __ror__.
+        class Rescuer:
+            def __ror__(self, other):
+                return "fallback"
+
+        cm = ChainMap()
+        # This should not raise TypeError.
+        # Since ChainMap.__ior__ returns NotImplemented, Python calls Rescuer.__ror__,
+        # and the result ("fallback") is assigned back to 'cm'.
+        cm |= Rescuer()
+        self.assertEqual(cm, "fallback")
 
 ################################################################################
 ### Named Tuples

Misc/NEWS.d/next/Library/2025-11-29-22-23-00.gh-issue-142085.7u8i9o.rst

@@ -0,0 +1,2 @@
+Fix :class:`ChainMap.__ior__ <collections.ChainMap>` to return :data:`NotImplemented`
+instead of raising :exc:`TypeError` for unsupported types.