bpo-46014: Add ability to use typing.Union with singledispatch (GH-30017)

Lib/test/test_functools.py

@@ -2684,6 +2684,17 @@ class TestSingleDispatch(unittest.TestCase):
             'typing.Iterable[str] is not a class.'
         ))
 
+        with self.assertRaises(TypeError) as exc:
+            @i.register
+            def _(arg: typing.Union[int, typing.Iterable[str]]):
+                return "Invalid Union"
+        self.assertTrue(str(exc.exception).startswith(
+            "Invalid annotation for 'arg'."
+        ))
+        self.assertTrue(str(exc.exception).endswith(
+            'typing.Union[int, typing.Iterable[str]] not all arguments are classes.'
+        ))
+
     def test_invalid_positional_argument(self):
         @functools.singledispatch
         def f(*args):
@@ -2692,6 +2703,25 @@ class TestSingleDispatch(unittest.TestCase):
         with self.assertRaisesRegex(TypeError, msg):
             f()
 
+    def test_union(self):
+        @functools.singledispatch
+        def f(arg):
+            return "default"
+
+        @f.register
+        def _(arg: typing.Union[str, bytes]):
+            return "typing.Union"
+
+        @f.register
+        def _(arg: int | float):
+            return "types.UnionType"
+
+        self.assertEqual(f([]), "default")
+        self.assertEqual(f(""), "typing.Union")
+        self.assertEqual(f(b""), "typing.Union")
+        self.assertEqual(f(1), "types.UnionType")
+        self.assertEqual(f(1.0), "types.UnionType")
+
 
 class CachedCostItem:
     _cost = 1