From 2751a66ea88aff146136db11e91b6fdf6476c909 Mon Sep 17 00:00:00 2001
From: Samuel-Hills <samhills004@gmail.com>
Date: Fri, 2 Oct 2026 21:12:24 -0400
Subject: [PATCH 1/4] fix: preserve explicitly declared slots

---
 changelog.d/1637.change.md |  1 +
 src/attr/_make.py          | 18 ++++++++++++++++--
 tests/test_slots.py        | 17 +++++++++++++++++
 3 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/1637.change.md

diff --git a/changelog.d/1637.change.md b/changelog.d/1637.change.md
new file mode 100644
index 000000000..9c530dfe0
--- /dev/null
+++ b/changelog.d/1637.change.md
@@ -0,0 +1 @@
+Preserve slots declared directly on a class decorated with `@attrs.define` or `@attr.s`.
diff --git a/src/attr/_make.py b/src/attr/_make.py
index afbca4635..8404166d4 100644
--- a/src/attr/_make.py
+++ b/src/attr/_make.py
@@ -860,10 +860,22 @@ def _create_slots_class(self):
         """
         Build and return a new class with a `__slots__` attribute.
         """
+        own_slots = self._cls.__dict__.get("__slots__", ())
+        if isinstance(own_slots, str):
+            own_slots = (own_slots,)
+        else:
+            own_slots = tuple(own_slots)
+
         cd = {
             k: v
             for k, v in self._cls_dict.items()
-            if k not in (*tuple(self._attr_names), "__dict__", "__weakref__")
+            if k
+            not in (
+                *tuple(self._attr_names),
+                *own_slots,
+                "__dict__",
+                "__weakref__",
+            )
         }
 
         # 3.14.0rc2+
@@ -903,7 +915,9 @@ def _create_slots_class(self):
 
         base_names = set(self._base_names)
 
-        names = self._attr_names
+        names = self._attr_names + tuple(
+            name for name in own_slots if name not in self._attr_names
+        )
         if (
             self._weakref_slot
             and "__weakref__" not in getattr(self._cls, "__slots__", ())
diff --git a/tests/test_slots.py b/tests/test_slots.py
index a74c32b03..07a577d49 100644
--- a/tests/test_slots.py
+++ b/tests/test_slots.py
@@ -104,6 +104,23 @@ def test_slots_being_used():
     assert attr.asdict(slot_instance) == attr.asdict(non_slot_instance)
 
 
+def test_explicit_slots_are_preserved():
+    """Slots declared on the class are retained when attrs creates the class."""
+
+    @attrs.define
+    class ExplicitSlots:
+        __slots__ = ("private",)
+
+        public: str = "foo"
+
+    instance = ExplicitSlots()
+    instance.private = 1
+
+    assert instance.private == 1
+    assert instance.public == "foo"
+    assert {"__weakref__", "private", "public"} == set(ExplicitSlots.__slots__)
+
+
 def test_slots_base_of_slotted():
     """
     The (hopefully gc'ed) temporary base class of a slotted class contains a

From a514fb9d7b4c58d524396eccd3066b5d68ad8ed3 Mon Sep 17 00:00:00 2001
From: Samuel-Hills <samhills004@gmail.com>
Date: Fri, 23 Oct 2026 07:24:07 +0200
Subject: [PATCH 2/4] test: cover declared slot variants

---
 tests/test_slots.py | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/tests/test_slots.py b/tests/test_slots.py
index 07a577d49..0a3a13011 100644
--- a/tests/test_slots.py
+++ b/tests/test_slots.py
@@ -121,6 +121,36 @@ class ExplicitSlots:
     assert {"__weakref__", "private", "public"} == set(ExplicitSlots.__slots__)
 
 
+def test_explicit_string_slot_is_preserved():
+    @attrs.define
+    class ExplicitStringSlot:
+        __slots__ = "private"
+
+        public: str = "foo"
+
+    instance = ExplicitStringSlot()
+    instance.private = 1
+
+    assert instance.private == 1
+    assert instance.public == "foo"
+    assert {"__weakref__", "private", "public"} == set(
+        ExplicitStringSlot.__slots__
+    )
+
+
+def test_explicit_slot_for_attribute_is_preserved():
+    @attrs.define
+    class ExplicitAttributeSlot:
+        __slots__ = ("public",)
+
+        public: str
+
+    instance = ExplicitAttributeSlot("foo")
+
+    assert instance.public == "foo"
+    assert {"__weakref__", "public"} == set(ExplicitAttributeSlot.__slots__)
+
+
 def test_slots_base_of_slotted():
     """
     The (hopefully gc'ed) temporary base class of a slotted class contains a

From e80642262f067b61486008792e56381299615041 Mon Sep 17 00:00:00 2001
From: Samuel-Hills <samhills004@gmail.com>
Date: Fri, 23 Oct 2026 07:27:19 +0200
Subject: [PATCH 3/4] test: satisfy lint for string slots case

---
 tests/test_slots.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_slots.py b/tests/test_slots.py
index 0a3a13011..70037a2e2 100644
--- a/tests/test_slots.py
+++ b/tests/test_slots.py
@@ -124,7 +124,7 @@ class ExplicitSlots:
 def test_explicit_string_slot_is_preserved():
     @attrs.define
     class ExplicitStringSlot:
-        __slots__ = "private"
+        __slots__ = "private"  # noqa: PLC0205 - exercise the supported string form
 
         public: str = "foo"
 

From d96c51fef05df5e6715d7c1b4f6afec4491e449e Mon Sep 17 00:00:00 2001
From: Samuel-Hills <samhills004@gmail.com>
Date: Fri, 23 Oct 2026 07:32:24 +0200
Subject: [PATCH 4/4] test: document declared slot coverage

---
 tests/test_slots.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tests/test_slots.py b/tests/test_slots.py
index 70037a2e2..2ab11fd28 100644
--- a/tests/test_slots.py
+++ b/tests/test_slots.py
@@ -122,6 +122,8 @@ class ExplicitSlots:
 
 
 def test_explicit_string_slot_is_preserved():
+    """A single string slot declaration survives attrs class rebuilding."""
+
     @attrs.define
     class ExplicitStringSlot:
         __slots__ = "private"  # noqa: PLC0205 - exercise the supported string form
@@ -139,6 +141,8 @@ class ExplicitStringSlot:
 
 
 def test_explicit_slot_for_attribute_is_preserved():
+    """A declared slot matching an attrs field is not duplicated."""
+
     @attrs.define
     class ExplicitAttributeSlot:
         __slots__ = ("public",)