gh-129538: Fix IPv6Interface.ip losing scope ID information

Lib/ipaddress.py

@@ -2199,6 +2199,7 @@ def __init__(self, address):
         self.network = IPv6Network((addr, mask), strict=False)
         self.netmask = self.network.netmask
         self._prefixlen = self.network._prefixlen
+        self._scope_id = addr.scope_id if isinstance(addr, IPv6Address) else None
 
     @functools.cached_property
     def hostmask(self):
@@ -2239,7 +2240,8 @@ def __hash__(self):
 
     @property
     def ip(self):
-        return IPv6Address(self._ip)
+        addr_str = self._string_from_ip_int(self._ip) + (f'%{self._scope_id}' if self._scope_id else '')
+        return IPv6Address(addr_str)
 
     @property
     def with_prefixlen(self):

Lib/test/test_ipaddress.py

@@ -562,6 +562,12 @@ def test_copy(self):
         self.assertEqual(addr, copy.copy(addr))
         self.assertEqual(addr, copy.deepcopy(addr))
 
+    # issue: 129538
+    def test_ipv6_interface_scope_id(self):
+        addr = ipaddress.IPv6Address("fe80::%10")
+        addr_with_prefix = ipaddress.IPv6Interface((addr, 64))
+        self.assertEqual(addr_with_prefix.ip, ipaddress.IPv6Address("fe80::%10"))
+
 
 class NetmaskTestMixin_v4(CommonTestMixin_v4):
     """Input validation on interfaces and networks is very similar"""