From fde0337d32843cc47573dd57d4554d2aa40ba428 Mon Sep 17 00:00:00 2001
From: Jarry Shaw <jarryshaw@icloud.com>
Date: Fri, 18 Sep 2026 12:19:38 -0400
Subject: [PATCH] corekit: narrow the interface fields' post_process return
 type via isinstance, not cast (#473)

- IPv4InterfaceField.post_process and IPv6InterfaceField.post_process each
  built their return value with the polymorphic ipaddress.ip_interface(),
  typed IPv4Interface | IPv6Interface, against a declared return type that
  admits only one of the two -- two standing mypy [return-value] errors.
- The two ip_interface() calls in the matching pre_process methods already
  wore a # type: ignore[assignment] for the identical reason, confirmed by
  removing the suppressions and re-running mypy (same errors, different
  code).
- Fixed both pairs the same way: replace the `val.version != self.version`
  comparison with `not isinstance(val, IPv4Interface/IPv6Interface)`, which
  mypy accepts as a narrowing type guard. Rejected constructing the concrete
  class directly (ipaddress.IPv4Interface(...)/IPv6Interface(...)) instead,
  because it changes behaviour: passing an already-constructed interface of
  the *other* concrete type to pre_process would then raise before reaching
  the version check, changing the message from "IP version mismatch: ..."
  to "invalid IP interface: ...", breaking
  test_wrong_version_message_is_not_relabelled_as_a_malformed_value.
- No behaviour change: probed both versions side by side on 16 representative
  inputs (round trips at every extreme prefix length, wrong-version strings,
  wrong-version pre-built Interface objects, malformed values, a non-
  contiguous IPv4 netmask, an out-of-range IPv6 prefix length) -- 0
  mismatches in output or exception message.

mypy pcapkit: 125 errors/40 files -> 123 errors/39 files, diffed line by
line; the only errors removed are the two named in #473, none added.
pytest tests/ (excluding files owned by other in-flight PRs): 963 passed,
17 skipped, 873 subtests passed, 0 failures.
---
 pcapkit/corekit/fields/ipaddress.py | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/pcapkit/corekit/fields/ipaddress.py b/pcapkit/corekit/fields/ipaddress.py
index b2d144695e..f4907de25d 100644
--- a/pcapkit/corekit/fields/ipaddress.py
+++ b/pcapkit/corekit/fields/ipaddress.py
@@ -242,9 +242,10 @@ def pre_process(self, value: 'IPv4Interface | bytes | int | str', packet: 'dict[
             val = value
         else:
             with _reraise_as_field_value_error('invalid IP interface'):
-                val = ipaddress.ip_interface(value)  # type: ignore[assignment]
-            if val.version != self.version:
-                raise FieldValueError(f'IP version mismatch: {val.version} != {self.version}')
+                parsed = ipaddress.ip_interface(value)
+            if not isinstance(parsed, ipaddress.IPv4Interface):
+                raise FieldValueError(f'IP version mismatch: {parsed.version} != {self.version}')
+            val = parsed
 
         ip = val.ip
         mask = val.netmask
@@ -281,7 +282,7 @@ def post_process(self, value: 'bytes', packet: 'dict[str, Any]') -> 'IPv4Interfa
 
         with _reraise_as_field_value_error('invalid IPv4 interface'):
             val = ipaddress.ip_interface(f'{ip}/{mask}')
-        if val.version != self.version:
+        if not isinstance(val, ipaddress.IPv4Interface):
             raise FieldValueError(f'IP version mismatch: {val.version} != {self.version}')
         return val
 
@@ -326,9 +327,10 @@ def pre_process(self, value: 'IPv6Interface | bytes | int | str', packet: 'dict[
             val = value
         else:
             with _reraise_as_field_value_error('invalid IP interface'):
-                val = ipaddress.ip_interface(value)  # type: ignore[assignment]
-            if val.version != self.version:
-                raise FieldValueError(f'IP version mismatch: {val.version} != {self.version}')
+                parsed = ipaddress.ip_interface(value)
+            if not isinstance(parsed, ipaddress.IPv6Interface):
+                raise FieldValueError(f'IP version mismatch: {parsed.version} != {self.version}')
+            val = parsed
 
         ip = val.ip
         prefixlen = cast('int', val._prefixlen)  # type: ignore[attr-defined] # pylint: disable=protected-access
@@ -372,6 +374,6 @@ def post_process(self, value: 'bytes', packet: 'dict[str, Any]') -> 'IPv6Interfa
 
         with _reraise_as_field_value_error('invalid IPv6 interface'):
             val = ipaddress.ip_interface(f'{ip}/{prefixlen}')
-        if val.version != self.version:
+        if not isinstance(val, ipaddress.IPv6Interface):
             raise FieldValueError(f'IP version mismatch: {val.version} != {self.version}')
         return val