✨ Implement simple generic mixin for saving a single type argument

tests/test__util.py

@@ -0,0 +1,53 @@
+from typing import Generic, TypeVar
+from unittest import TestCase
+from unittest.mock import MagicMock, patch
+
+from marshmallow_generic import _util
+
+
+class GenericInsightMixinTestCase(TestCase):
+    @patch.object(_util, "super")
+    def test___init_subclass__(self, mock_super: MagicMock) -> None:
+        mock_super_meth = MagicMock()
+        mock_super.return_value = MagicMock(__init_subclass__=mock_super_meth)
+
+        # Should be `None` by default:
+        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:
+        t = TypeVar("t")
+
+        class Foo:
+            pass
+
+        class Bar(Generic[t]):
+            pass
+
+        class TestSchema1(Bar[str], _util.GenericInsightMixin[t]):
+            pass
+
+        self.assertIsNone(TestSchema1._type_arg)  # type: ignore[misc]
+        mock_super.assert_called_once()
+        mock_super_meth.assert_called_once_with()
+
+        mock_super.reset_mock()
+        mock_super_meth.reset_mock()
+
+        # If the mixin type argument was specified,
+        # ensure it was assigned to the attribute on the child class:
+
+        class TestSchema2(Bar[str], _util.GenericInsightMixin[Foo]):
+            pass
+
+        self.assertIs(Foo, TestSchema2._type_arg)  # type: ignore[misc]
+        mock_super.assert_called_once()
+        mock_super_meth.assert_called_once_with()
+
+    def test__get_type_arg(self) -> None:
+        with self.assertRaises(AttributeError):
+            _util.GenericInsightMixin._get_type_arg()
+
+        _type = object()
+        with patch.object(_util.GenericInsightMixin, "_type_arg", new=_type):
+            self.assertIs(_type, _util.GenericInsightMixin._get_type_arg())