Init private attributes from supertypes in model_post_init

pydantic/_internal/_model_construction.py

@@ -93,7 +93,7 @@ def __new__(
             private_attributes = inspect_namespace(
                 namespace, config_wrapper.ignored_types, class_vars, base_field_names
             )
-            if private_attributes:
+            if private_attributes or base_private_attributes:
                 original_model_post_init = get_model_post_init(namespace, bases)
                 if original_model_post_init is not None:
                     # if there are private_attributes and a model_post_init function, we handle both

tests/test_main.py

@@ -2081,6 +2081,23 @@ class C(B):
     assert calls == ['C.model_post_init']
 
 
+def test_model_post_init_supertype_private_attrs():
+    """https://github.com/pydantic/pydantic/issues/9098"""
+
+    class Model(BaseModel):
+        _private: int = 12
+
+    class SubModel(Model):
+        def model_post_init(self, __context: Any) -> None:
+            if self._private == 12:
+                self._private = 13
+            super().model_post_init(__context)
+
+    m = SubModel()
+
+    assert m._private == 13
+
+
 def test_model_post_init_subclass_setting_private_attrs():
     """https://github.com/pydantic/pydantic/issues/7091"""