Fix MRO indexing issue

The indexing only worked correctly for one base dataclass.

Author: Zahari Kassabov

validobj/tests/test_delayed_dataclasses.py

@@ -11,7 +11,11 @@ class Linked:
     parents: Linked | list[Linked] | None = None
 
 @dataclasses.dataclass
-class Parent:
+class Grandparent:
+    grandpa_field: int = 3
+
+@dataclasses.dataclass
+class Parent(Grandparent):
     parent_field: int = 1
     override_field: str = 'parent'
 
@@ -26,11 +30,12 @@ def test_delayed_annotations():
     assert parse_input(inp, Linked).parents.value == 2
 
 def test_derived_dataclasses():
-    inp = {'child_field': 3, 'parent_field': 4, 'override_field': 5}
+    inp = {'child_field': 3, 'parent_field': 4, 'override_field': 5, 'grandpa_field': 10}
     res = parse_input(inp, Child)
     assert res.child_field == 3
     assert res.parent_field == 4
     assert res.override_field == 5
+    assert res.grandpa_field == 10
 
     with pytest.raises(ValidationError):
         parse_input({'override_field': 'x'}, Child)

validobj/validation.py

@@ -190,7 +190,7 @@ def _dataclass_types(cls):
     res = {}
 
     # Base classes, including current one, from parent to child, excluding object
-    for base in cls.__mro__[1::-1]:
+    for base in cls.__mro__[-2::-1]:
         if dataclasses.is_dataclass(base):
             res.update(base.__annotations__)