gh-91330: Tests and docs for dataclass descriptor-typed fields (GH-94424) (GH-94576)

Co-authored-by: Erik De Bonte <erikd@microsoft.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
(cherry picked from commit 5f319308a8)

Lib/test/test_dataclasses.py

@@ -3229,6 +3229,115 @@ class TestDescriptors(unittest.TestCase):
 
         self.assertEqual(D.__set_name__.call_count, 1)
 
+    def test_init_calls_set(self):
+        class D:
+            pass
+
+        D.__set__ = Mock()
+
+        @dataclass
+        class C:
+            i: D = D()
+
+        # Make sure D.__set__ is called.
+        D.__set__.reset_mock()
+        c = C(5)
+        self.assertEqual(D.__set__.call_count, 1)
+
+    def test_getting_field_calls_get(self):
+        class D:
+            pass
+
+        D.__set__ = Mock()
+        D.__get__ = Mock()
+
+        @dataclass
+        class C:
+            i: D = D()
+
+        c = C(5)
+
+        # Make sure D.__get__ is called.
+        D.__get__.reset_mock()
+        value = c.i
+        self.assertEqual(D.__get__.call_count, 1)
+
+    def test_setting_field_calls_set(self):
+        class D:
+            pass
+
+        D.__set__ = Mock()
+
+        @dataclass
+        class C:
+            i: D = D()
+
+        c = C(5)
+
+        # Make sure D.__set__ is called.
+        D.__set__.reset_mock()
+        c.i = 10
+        self.assertEqual(D.__set__.call_count, 1)
+
+    def test_setting_uninitialized_descriptor_field(self):
+        class D:
+            pass
+
+        D.__set__ = Mock()
+
+        @dataclass
+        class C:
+            i: D
+
+        # D.__set__ is not called because there's no D instance to call it on
+        D.__set__.reset_mock()
+        c = C(5)
+        self.assertEqual(D.__set__.call_count, 0)
+
+        # D.__set__ still isn't called after setting i to an instance of D
+        # because descriptors don't behave like that when stored as instance vars
+        c.i = D()
+        c.i = 5
+        self.assertEqual(D.__set__.call_count, 0)
+
+    def test_default_value(self):
+        class D:
+            def __get__(self, instance: Any, owner: object) -> int:
+                if instance is None:
+                    return 100
+
+                return instance._x
+
+            def __set__(self, instance: Any, value: int) -> None:
+                instance._x = value
+
+        @dataclass
+        class C:
+            i: D = D()
+
+        c = C()
+        self.assertEqual(c.i, 100)
+
+        c = C(5)
+        self.assertEqual(c.i, 5)
+
+    def test_no_default_value(self):
+        class D:
+            def __get__(self, instance: Any, owner: object) -> int:
+                if instance is None:
+                    raise AttributeError()
+
+                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):