From c0d42e964c86e59da46e82ccaabcdea005beb77a Mon Sep 17 00:00:00 2001
From: Pranav Mishra <pranav@harperinsure.com>
Date: Tue, 7 Jul 2026 13:56:06 -0700
Subject: [PATCH] Match filter attributes by identity instead of equality

attrs.filters.include/exclude stored the passed Attribute instances in a
frozenset and tested membership with `attribute in attrs`. Attribute
equality ignores the owning class, so two identically-configured
attributes on different classes compare equal - filtering by one class's
attribute also matched the other class's. Keep the attributes in a tuple
and match them by identity instead. Field-name strings still match by
name across classes, which covers the cross-class use case.

Fixes #864

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 changelog.d/864.change.md |  1 +
 src/attr/filters.py       | 14 ++++++++++----
 tests/test_filters.py     | 40 ++++++++++++++++++++++++++++++++++++++-
 3 files changed, 50 insertions(+), 5 deletions(-)
 create mode 100644 changelog.d/864.change.md

diff --git a/changelog.d/864.change.md b/changelog.d/864.change.md
new file mode 100644
index 000000000..73b2e3620
--- /dev/null
+++ b/changelog.d/864.change.md
@@ -0,0 +1 @@
+`attrs.filters.include` and `attrs.filters.exclude` now match `attrs.Attribute` instances by identity instead of equality, so filtering by an attribute of one class no longer also matches an identically-configured attribute of a different class.
diff --git a/src/attr/filters.py b/src/attr/filters.py
index 689b1705a..3ac4f6e6c 100644
--- a/src/attr/filters.py
+++ b/src/attr/filters.py
@@ -9,12 +9,18 @@
 
 def _split_what(what):
     """
-    Returns a tuple of `frozenset`s of classes and attributes.
+    Returns a `frozenset` of classes, a `frozenset` of names, and a `tuple` of
+    attributes.
+
+    Attributes are kept as a tuple and matched by identity rather than put in a
+    `frozenset`: `Attribute` equality ignores the owning class, so two
+    identically-configured attributes on different classes compare equal and
+    set membership would match the wrong one (see #864).
     """
     return (
         frozenset(cls for cls in what if isinstance(cls, type)),
         frozenset(cls for cls in what if isinstance(cls, str)),
-        frozenset(cls for cls in what if isinstance(cls, Attribute)),
+        tuple(cls for cls in what if isinstance(cls, Attribute)),
     )
 
 
@@ -39,7 +45,7 @@ def include_(attribute, value):
         return (
             value.__class__ in cls
             or attribute.name in names
-            or attribute in attrs
+            or any(attribute is a for a in attrs)
         )
 
     return include_
@@ -66,7 +72,7 @@ def exclude_(attribute, value):
         return not (
             value.__class__ in cls
             or attribute.name in names
-            or attribute in attrs
+            or any(attribute is a for a in attrs)
         )
 
     return exclude_
diff --git a/tests/test_filters.py b/tests/test_filters.py
index 08314fa88..f1ca7e237 100644
--- a/tests/test_filters.py
+++ b/tests/test_filters.py
@@ -30,7 +30,7 @@ def test_splits(self):
         assert (
             frozenset((int, str)),
             frozenset(("abcd", "123")),
-            frozenset((fields(C).a,)),
+            (fields(C).a,),
         ) == _split_what((str, "123", fields(C).a, int, "abcd"))
 
 
@@ -79,6 +79,24 @@ def test_drop_class(self, incl, value):
         i = include(*incl)
         assert i(fields(C).a, value) is False
 
+    def test_matches_attribute_by_identity(self):
+        """
+        An identically-configured attribute on a different class is not
+        included: attributes are matched by identity, not equality (#864).
+        """
+
+        @attr.s
+        class D:
+            a = attr.ib()
+            b = attr.ib()
+
+        assert fields(C).a == fields(D).a
+        assert fields(C).a is not fields(D).a
+
+        i = include(fields(C).a)
+        assert i(fields(C).a, 42) is True
+        assert i(fields(D).a, 42) is False
+
 
 class TestExclude:
     """
@@ -124,3 +142,23 @@ def test_drop_class(self, excl, value):
         """
         e = exclude(*excl)
         assert e(fields(C).a, value) is False
+
+    def test_matches_attribute_by_identity(self):
+        """
+        An identically-configured attribute on a different class is not
+        excluded: attributes are matched by identity, not equality (#864).
+        """
+
+        @attr.s
+        class D:
+            a = attr.ib()
+            b = attr.ib()
+
+        assert fields(C).a == fields(D).a
+        assert fields(C).a is not fields(D).a
+
+        e = exclude(fields(C).a)
+        # C.a is excluded ...
+        assert e(fields(C).a, 42) is False
+        # ... but the equal-but-distinct D.a is not.
+        assert e(fields(D).a, 42) is True