gh-153769: Clarify the TypeError from ipaddress subnet_of()/supernet_of()

Passing something that is not a network object, such as an address, to
IPv4Network.subnet_of() or supernet_of() made _is_subnet_of() reach for
the network_address attribute, fail with an AttributeError, and turn that
into a TypeError with a message that read like a real containment result
and hid the actual cause.

Check that both arguments are network objects up front and raise a clear
TypeError explaining what is required, so nothing masks an internal
AttributeError anymore.

Author: fedonman

Lib/ipaddress.py

@@ -1026,15 +1026,14 @@ def is_multicast(self):
 
     @staticmethod
     def _is_subnet_of(a, b):
-        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 "
-                            f"between {a} and {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)
 
     def subnet_of(self, other):
         """Return True if this network is a subnet of other."""

Lib/test/test_ipaddress.py

@@ -723,6 +723,20 @@ def test_subnet_of_mixed_types(self):
             ipaddress.IPv6Network('::1/128').subnet_of(
                 ipaddress.IPv4Network('10.0.0.0/30'))
 
+    def test_subnet_of_non_network(self):
+        # Passing something that is not a network object (e.g. an address)
+        # must raise a clear TypeError rather than masking an internal
+        # AttributeError (gh-153769).
+        net = ipaddress.IPv4Network('10.0.0.0/30')
+        for other in (ipaddress.IPv4Address('10.0.0.1'), '10.0.0.0/30', 42):
+            for method in (net.subnet_of, net.supernet_of):
+                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)
+
 
 class NetmaskTestMixin_v6(CommonTestMixin_v6):
     """Input validation on interfaces and networks is very similar"""

Misc/ACKS

@@ -2008,6 +2008,7 @@ Wm. Keith van der Meulen
 Eric N. Vander Weele
 Andrew Vant
 Atul Varma
+Vyron Vasileiadis
 Dmitry Vasiliev
 Sebastian Ortiz Vasquez
 Alexandre Vassalotti

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

@@ -0,0 +1,5 @@
+: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`.