Add keyword-only fields to dataclasses. (GH=25608)

Lib/test/test_dataclasses.py

@@ -61,6 +61,7 @@ class TestCase(unittest.TestCase):
                            f"default=1,default_factory={MISSING!r}," \
                            "init=True,repr=False,hash=None," \
                            "compare=True,metadata=mappingproxy({})," \
+                           f"kw_only={MISSING!r}," \
                            "_field_type=None)"
 
         self.assertEqual(repr_output, expected_output)
@@ -3501,5 +3502,163 @@ class TestMatchArgs(unittest.TestCase):
         self.assertEqual(C.__match_args__, ('z',))
 
 
+class TestKwArgs(unittest.TestCase):
+    def test_no_classvar_kwarg(self):
+        msg = 'field a is a ClassVar but specifies kw_only'
+        with self.assertRaisesRegex(TypeError, msg):
+            @dataclass
+            class A:
+                a: ClassVar[int] = field(kw_only=True)
+
+        with self.assertRaisesRegex(TypeError, msg):
+            @dataclass
+            class A:
+                a: ClassVar[int] = field(kw_only=False)
+
+        with self.assertRaisesRegex(TypeError, msg):
+            @dataclass(kw_only=True)
+            class A:
+                a: ClassVar[int] = field(kw_only=False)
+
+    def test_field_marked_as_kwonly(self):
+        #######################
+        # Using dataclass(kw_only=True)
+        @dataclass(kw_only=True)
+        class A:
+            a: int
+        self.assertTrue(fields(A)[0].kw_only)
+
+        @dataclass(kw_only=True)
+        class A:
+            a: int = field(kw_only=True)
+        self.assertTrue(fields(A)[0].kw_only)
+
+        @dataclass(kw_only=True)
+        class A:
+            a: int = field(kw_only=False)
+        self.assertFalse(fields(A)[0].kw_only)
+
+        #######################
+        # Using dataclass(kw_only=False)
+        @dataclass(kw_only=False)
+        class A:
+            a: int
+        self.assertFalse(fields(A)[0].kw_only)
+
+        @dataclass(kw_only=False)
+        class A:
+            a: int = field(kw_only=True)
+        self.assertTrue(fields(A)[0].kw_only)
+
+        @dataclass(kw_only=False)
+        class A:
+            a: int = field(kw_only=False)
+        self.assertFalse(fields(A)[0].kw_only)
+
+        #######################
+        # Not specifying dataclass(kw_only)
+        @dataclass
+        class A:
+            a: int
+        self.assertFalse(fields(A)[0].kw_only)
+
+        @dataclass
+        class A:
+            a: int = field(kw_only=True)
+        self.assertTrue(fields(A)[0].kw_only)
+
+        @dataclass
+        class A:
+            a: int = field(kw_only=False)
+        self.assertFalse(fields(A)[0].kw_only)
+
+    def test_match_args(self):
+        # kw fields don't show up in __match_args__.
+        @dataclass(kw_only=True)
+        class C:
+            a: int
+        self.assertEqual(C(a=42).__match_args__, ())
+
+        @dataclass
+        class C:
+            a: int
+            b: int = field(kw_only=True)
+        self.assertEqual(C(42, b=10).__match_args__, ('a',))
+
+    def test_KW_ONLY(self):
+        @dataclass
+        class A:
+            a: int
+            _: KW_ONLY
+            b: int
+            c: int
+        A(3, c=5, b=4)
+        msg = "takes 2 positional arguments but 4 were given"
+        with self.assertRaisesRegex(TypeError, msg):
+            A(3, 4, 5)
+
+
+        @dataclass(kw_only=True)
+        class B:
+            a: int
+            _: KW_ONLY
+            b: int
+            c: int
+        B(a=3, b=4, c=5)
+        msg = "takes 1 positional argument but 4 were given"
+        with self.assertRaisesRegex(TypeError, msg):
+            B(3, 4, 5)
+
+        # Explicitely make a field that follows KW_ONLY be non-keyword-only.
+        @dataclass
+        class C:
+            a: int
+            _: KW_ONLY
+            b: int
+            c: int = field(kw_only=False)
+        c = C(1, 2, b=3)
+        self.assertEqual(c.a, 1)
+        self.assertEqual(c.b, 3)
+        self.assertEqual(c.c, 2)
+        c = C(1, b=3, c=2)
+        self.assertEqual(c.a, 1)
+        self.assertEqual(c.b, 3)
+        self.assertEqual(c.c, 2)
+        c = C(1, b=3, c=2)
+        self.assertEqual(c.a, 1)
+        self.assertEqual(c.b, 3)
+        self.assertEqual(c.c, 2)
+        c = C(c=2, b=3, a=1)
+        self.assertEqual(c.a, 1)
+        self.assertEqual(c.b, 3)
+        self.assertEqual(c.c, 2)
+
+    def test_post_init(self):
+        @dataclass
+        class A:
+            a: int
+            _: KW_ONLY
+            b: InitVar[int]
+            c: int
+            d: InitVar[int]
+            def __post_init__(self, b, d):
+                raise CustomError(f'{b=} {d=}')
+        with self.assertRaisesRegex(CustomError, 'b=3 d=4'):
+            A(1, c=2, b=3, d=4)
+
+        @dataclass
+        class B:
+            a: int
+            _: KW_ONLY
+            b: InitVar[int]
+            c: int
+            d: InitVar[int]
+            def __post_init__(self, b, d):
+                self.a = b
+                self.c = d
+        b = B(1, c=2, b=3, d=4)
+        self.assertEqual(asdict(b), {'a': 3, 'c': 4})
+
+
 if __name__ == '__main__':
     unittest.main()