gh-144749: Default value check for descriptors in dataclass

Lib/dataclasses.py

@@ -831,6 +831,11 @@ def _get_field(cls, a_name, a_type, default_kw_only):
     # If the default value isn't derived from Field, then it's only a
     # normal default value.  Convert it to a Field().
     default = getattr(cls, a_name, MISSING)
+    # First checks if default is a descriptor,
+    # then checks if there is no default value returned by default.__get__.
+    if hasattr(default, "__get__") and default is cls.__dict__.get(a_name):
+            default = MISSING
+
     if isinstance(default, Field):
         f = default
     else:

Lib/test/test_dataclasses/__init__.py

@@ -4186,6 +4186,23 @@ class C:
         with self.assertRaisesRegex(TypeError, 'missing 1 required positional argument'):
             c = C()
 
+    def test_return_self_no_default_value(self):
+        class D:
+            def __get__(self, instance: Any, owner: object) -> Any:
+                if instance is None:
+                    return self
+                return instance._x
+
+            def __set__(self, instance: Any, value: int) -> None:
+                instance._x = value
+
+        @dataclass
+        class C:
+            i: D = D()
+
+        with self.assertRaisesRegex(TypeError, 'missing 1 required positional argument'):
+            c = C()
+
 class TestStringAnnotations(unittest.TestCase):
     def test_classvar(self):
         # Some expressions recognized as ClassVar really aren't.  But

Misc/NEWS.d/next/Library/2026-02-17-23-50-41.gh-issue-144749.sv3Ztr.rst

@@ -0,0 +1,4 @@
+Previously descriptors that return themselves from calls to  ``__get__`` were
+treated as default values in a dataclass. Now, a ``TypeError`` is raised if no
+initial value is provided to the dataclass constructor.
+