Only improve the message, keep the AttributeError handler

An isinstance() check on _BaseNetwork rejects a network-like object that is
not a subclass, which works today. Keeping the handler and suppressing the
context keeps that working and still gives the caller one clear error.

Author: fedonman

Lib/ipaddress.py

@@ -1026,14 +1026,16 @@ def is_multicast(self):
 
     @staticmethod
     def _is_subnet_of(a, b):
-        if not (isinstance(a, _BaseNetwork) and isinstance(b, _BaseNetwork)):
-            raise TypeError(f"Unable to test subnet containment between "
-                            f"{a} and {b}: both must be network objects")
-        # Always false if one is v4 and the other is v6.
-        if a.version != b.version:
-            raise TypeError(f"{a} and {b} are not of the same version")
-        return (b.network_address <= a.network_address and
-                b.broadcast_address >= a.broadcast_address)
+        try:
+            # Always false if one is v4 and the other is v6.
+            if a.version != b.version:
+                raise TypeError(f"{a} and {b} are not of the same version")
+            return (b.network_address <= a.network_address and
+                    b.broadcast_address >= a.broadcast_address)
+        except AttributeError:
+            raise TypeError(
+                f"Unable to test subnet containment between {a} and {b}: "
+                f"both must be network objects") from None
 
     def subnet_of(self, other):
         """Return True if this network is a subnet of other."""

Lib/test/test_ipaddress.py

@@ -733,9 +733,8 @@ def test_subnet_of_non_network(self):
                 with self.assertRaises(TypeError) as cm:
                     method(other)
                 self.assertIn('network', str(cm.exception))
-                # The error should not be a swallowed AttributeError.
-                self.assertNotIsInstance(cm.exception.__context__,
-                                         AttributeError)
+                # The internal AttributeError is not shown to the caller.
+                self.assertTrue(cm.exception.__suppress_context__)
 
 
 class NetmaskTestMixin_v6(CommonTestMixin_v6):

Misc/NEWS.d/next/Library/2026-07-19-16-30-00.gh-issue-153769.STygbq.rst

@@ -1,5 +1,4 @@
 :meth:`~ipaddress.IPv4Network.subnet_of` and
-:meth:`~ipaddress.IPv4Network.supernet_of` now raise a clear
-:exc:`TypeError` when passed an argument that is not a network object,
-such as an address, instead of a confusing error that masked an internal
-:exc:`AttributeError`.
+:meth:`~ipaddress.IPv4Network.supernet_of` now say that both arguments must be
+network objects when passed something else, such as an address, and no longer
+show the internal :exc:`AttributeError` behind it.