✨ Implement simple generic mixin for saving a single type argument

src/marshmallow_generic/_util.py

@@ -0,0 +1,31 @@
+from typing import Any, Generic, Optional, TypeVar, get_args, get_origin
+
+
+_T = TypeVar("_T")
+
+
+class GenericInsightMixin(Generic[_T]):
+    _type_arg: Optional[type[_T]] = None
+
+    @classmethod
+    def __init_subclass__(cls, **kwargs: Any) -> None:
+        """Saves the type argument in the `_type_arg` class attribute."""
+        super().__init_subclass__(**kwargs)
+        for base in cls.__orig_bases__:  # type: ignore[attr-defined]
+            origin = get_origin(base)
+            if origin is None or not issubclass(origin, GenericInsightMixin):
+                continue
+            type_arg = get_args(base)[0]
+            # Do not set the attribute for GENERIC subclasses!
+            if not isinstance(type_arg, TypeVar):
+                cls._type_arg = type_arg
+                return
+
+    @classmethod
+    def _get_type_arg(cls) -> type[_T]:
+        """Returns the type argument of the class (if specified)."""
+        if cls._type_arg is None:
+            raise AttributeError(
+                f"{cls.__name__} is generic; type argument unspecified"
+            )
+        return cls._type_arg