Issue #21481: Teach argparse equality tests to return NotImplemented when comparing to unknown types.

Lib/argparse.py

@@ -1198,9 +1198,13 @@ class Namespace(_AttributeHolder):
             setattr(self, name, kwargs[name])
 
     def __eq__(self, other):
+        if not isinstance(other, Namespace):
+            return NotImplemented
         return vars(self) == vars(other)
 
     def __ne__(self, other):
+        if not isinstance(other, Namespace):
+            return NotImplemented
         return not (self == other)
 
     def __contains__(self, key):

Lib/test/test_argparse.py

@@ -4551,6 +4551,12 @@ class TestNamespace(TestCase):
         self.assertTrue(ns2 != ns3)
         self.assertTrue(ns2 != ns4)
 
+    def test_equality_returns_notimplemeted(self):
+        # See issue 21481
+        ns = argparse.Namespace(a=1, b=2)
+        self.assertIs(ns.__eq__(None), NotImplemented)
+        self.assertIs(ns.__ne__(None), NotImplemented)
+
 
 # ===================
 # File encoding tests

Misc/NEWS

@@ -24,6 +24,9 @@ Library
 - Issue #14710: pkgutil.find_loader() no longer raises an exception when a
   module doesn't exist.
 
+- Issue #21481:  Argparse equality and inequality tests now return 
+  NotImplemented when comparing to an unknown type.
+
 - Issue #8743: Fix interoperability between set objects and the
   collections.Set() abstract base class.