gh-117566: fix IPv6Address.is_loopback for IPv4-mapped loopbacks (GH-117567)

While properties like IPv6Address.is_private account for IPv4-mapped
IPv6 addresses, such as for example:

    >>> ipaddress.ip_address("192.168.0.1").is_private
    True
    >>> ipaddress.ip_address("::ffff:192.168.0.1").is_private
    True
...the same doesn't currently apply to the is_loopback property:
    >>> ipaddress.ip_address("127.0.0.1").is_loopback
    True
    >>> ipaddress.ip_address("::ffff:127.0.0.1").is_loopback
    False

At minimum, this inconsistency between different properties is
counter-intuitive. Moreover, ::ffff:127.0.0.0/104 is for all intents and
purposes a loopback address, and should be treated as such.

Author: paravoid

Lib/ipaddress.py

@@ -2142,6 +2142,9 @@ def is_loopback(self):
             RFC 2373 2.5.3.
 
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_loopback
         return self._ip == 1
 
     @property
@@ -2258,7 +2261,7 @@ def is_unspecified(self):
 
     @property
     def is_loopback(self):
-        return self._ip == 1 and self.network.is_loopback
+        return super().is_loopback and self.network.is_loopback
 
 
 class IPv6Network(_BaseV6, _BaseNetwork):

Lib/test/test_ipaddress.py

@@ -2446,6 +2446,22 @@ def testIpv4MappedPrivateCheck(self):
         self.assertEqual(
                 False, ipaddress.ip_address('::ffff:172.32.0.0').is_private)
 
+    def testIpv4MappedLoopbackCheck(self):
+        # test networks
+        self.assertEqual(True, ipaddress.ip_network(
+                '::ffff:127.100.200.254/128').is_loopback)
+        self.assertEqual(True, ipaddress.ip_network(
+                '::ffff:127.42.0.0/112').is_loopback)
+        self.assertEqual(False, ipaddress.ip_network(
+                '::ffff:128.0.0.0').is_loopback)
+        # test addresses
+        self.assertEqual(True, ipaddress.ip_address(
+                '::ffff:127.100.200.254').is_loopback)
+        self.assertEqual(True, ipaddress.ip_address(
+                '::ffff:127.42.0.0').is_loopback)
+        self.assertEqual(False, ipaddress.ip_address(
+                '::ffff:128.0.0.0').is_loopback)
+
     def testAddrExclude(self):
         addr1 = ipaddress.ip_network('10.1.1.0/24')
         addr2 = ipaddress.ip_network('10.1.1.0/26')

Misc/NEWS.d/next/Library/2024-04-05-15-51-01.gh-issue-117566.54nABf.rst

@@ -0,0 +1,3 @@
+:meth:`ipaddress.IPv6Address.is_loopback` will now return ``True`` for
+IPv4-mapped loopback addresses, i.e. addresses in the
+``::ffff:127.0.0.0/104`` address space.