python · sobolevn · Apr 26, 2025
@@ -766,7 +766,7 @@ def _is_type(annotation, cls, a_module, a_type, is_type_predicate):
     return False
 
 
-def _get_field(cls, a_name, a_type, default_kw_only):
+def _get_field(cls, a_name, a_type, default_kw_only, slots):
     # Return a Field object for this field name and type.  ClassVars and
     # InitVars are also returned, but marked as such (see f._field_type).
     # default_kw_only is the value of kw_only to use if there isn't a field()
@@ -861,6 +861,18 @@ def _get_field(cls, a_name, a_type, default_kw_only):
         raise ValueError(f'mutable default {type(f.default)} for field '
                          f'{f.name} is not allowed: use default_factory')
 
+    # Validate that you can't set descriptors with `__set__`
+    # when using `slots=True`. Because `__slots__` will override
+    # this descriptor and it can hide a bug from users.
+    if slots:
+        static_default = inspect.getattr_static(cls, a_name, MISSING)
+        if (
+            not inspect.ismemberdescriptor(static_default)
+            and inspect.isdatadescriptor(static_default)
+        ):
+            raise ValueError(f'data descriptor {type(static_default).__name__!r} '
+                             f'in {f.name!r} will be overriden when slots=True')
+
     return f
 
 def _set_new_attribute(cls, name, value):
@@ -1007,7 +1019,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
             kw_only = True
         else:
             # Otherwise it's a field of some type.
-            cls_fields.append(_get_field(cls, name, type, kw_only))
+            cls_fields.append(_get_field(cls, name, type, kw_only, slots))
 
     for f in cls_fields:
         fields[f.name] = f

@@ -3383,6 +3383,53 @@ class Root2(Root):
             class C(Root2):
                 x: int
 
+    def test_data_descriptor_with_slots(self):
+        class DescriptorFieldType:
+            def __get__(self, instance, owner):
+                return 1
+
+            def __set__(self, instance, value):
+                ...  # this method must just be present
+
+        @dataclass
+        class Regular:
+            one: DescriptorFieldType = DescriptorFieldType()
+
+        self.assertEqual(Regular.one, 1)
+        self.assertEqual(Regular().one, 1)
+
+        with self.assertRaisesRegex(
+            ValueError,
+            (
+                "data descriptor 'DescriptorFieldType' in 'two' "
+                "will be overriden when slots=True"
+            ),
+        ):
+            @dataclass(slots=True)
+            class WithSlots:
+                two: DescriptorFieldType = DescriptorFieldType()
+
+    def test_regular_descriptor_with_slots(self):
+        class DescriptorFieldType:
+            def __get__(self, instance, owner):
+                return 1
+
+            # no __set__
+
+        @dataclass
+        class Regular:
+            one: DescriptorFieldType = DescriptorFieldType()
+
+        self.assertEqual(Regular.one, 1)
+        self.assertEqual(Regular().one, 1)
+
+        @dataclass(slots=True)
+        class WithSlots:
+            two: DescriptorFieldType = DescriptorFieldType()
+
+        self.assertIsInstance(WithSlots.two, types.MemberDescriptorType)
+        self.assertEqual(WithSlots().two, 1)
+
     def test_returns_new_class(self):
         class A:
             x: int

diff --git a/Misc/NEWS.d/next/Library/2025-04-26-15-01-09.gh-issue-132946.2q6MNT.rst b/Misc/NEWS.d/next/Library/2025-04-26-15-01-09.gh-issue-132946.2q6MNT.rst
@@ -0,0 +1,2 @@
+Forbid setting :term:`Data descriptors <descriptor>` in
+:func:`dataclasses.dataclass` with ``slots=True``
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Forbid setting :term:`Data descriptors <descriptor>` in
		:func:`dataclasses.dataclass` with ``slots=True``