✨ Overload @post_load decorator to retain the function type

tests/test__util.py

@@ -12,7 +12,9 @@ class GenericInsightMixinTestCase(TestCase):
         mock_super.return_value = MagicMock(__init_subclass__=mock_super_meth)
 
         # Should be `None` by default:
-        self.assertIsNone(_util.GenericInsightMixin._type_arg)  # type: ignore[misc]
+        self.assertIsNone(
+            _util.GenericInsightMixin._type_arg  # type: ignore[misc]
+        )
 
         # If the mixin type argument was not specified (still generic),
         # ensure that the attribute remains `None` on the subclass:

tests/test_decorators.py

@@ -0,0 +1,28 @@
+from collections.abc import Callable
+from unittest import TestCase
+from unittest.mock import MagicMock, patch
+
+from marshmallow_generic import decorators
+
+
+class DecoratorsTestCase(TestCase):
+    @patch.object(decorators, "_post_load")
+    def test_post_load(self, mock_original_post_load: MagicMock) -> None:
+        mock_original_post_load.return_value = expected_output = object()
+
+        def test_function(x: int) -> str:
+            return str(x)
+
+        pass_many, pass_original = MagicMock(), MagicMock()
+        # Explicit annotation to possibly catch mypy errors:
+        output: Callable[[int], str] = decorators.post_load(
+            test_function,
+            pass_many=pass_many,
+            pass_original=pass_original,
+        )
+        self.assertIs(expected_output, output)
+        mock_original_post_load.assert_called_once_with(
+            test_function,
+            pass_many=pass_many,
+            pass_original=pass_original,
+        )