Keep values of private attributes set within model_post_init in sub…

…classes (#7775)

pydantic/_internal/_model_construction.py

@@ -259,12 +259,13 @@ def init_private_attributes(self: BaseModel, __context: Any) -> None:
         self: The BaseModel instance.
         __context: The context.
     """
-    pydantic_private = {}
-    for name, private_attr in self.__private_attributes__.items():
-        default = private_attr.get_default()
-        if default is not PydanticUndefined:
-            pydantic_private[name] = default
-    object_setattr(self, '__pydantic_private__', pydantic_private)
+    if getattr(self, '__pydantic_private__', None) is None:
+        pydantic_private = {}
+        for name, private_attr in self.__private_attributes__.items():
+            default = private_attr.get_default()
+            if default is not PydanticUndefined:
+                pydantic_private[name] = default
+        object_setattr(self, '__pydantic_private__', pydantic_private)
 
 
 def get_model_post_init(namespace: dict[str, Any], bases: tuple[type[Any], ...]) -> Callable[..., Any] | None:

tests/test_main.py

@@ -2021,6 +2021,38 @@ class C(B):
     assert calls == ['C.model_post_init']
 
 
+def test_model_post_init_subclass_setting_private_attrs():
+    """https://github.com/pydantic/pydantic/issues/7091"""
+
+    class Model(BaseModel):
+        _priv1: int = PrivateAttr(91)
+        _priv2: int = PrivateAttr(92)
+
+        def model_post_init(self, __context) -> None:
+            self._priv1 = 100
+
+    class SubModel(Model):
+        _priv3: int = PrivateAttr(93)
+        _priv4: int = PrivateAttr(94)
+        _priv5: int = PrivateAttr()
+        _priv6: int = PrivateAttr()
+
+        def model_post_init(self, __context) -> None:
+            self._priv3 = 200
+            self._priv5 = 300
+            super().model_post_init(__context)
+
+    m = SubModel()
+
+    assert m._priv1 == 100
+    assert m._priv2 == 92
+    assert m._priv3 == 200
+    assert m._priv4 == 94
+    assert m._priv5 == 300
+    with pytest.raises(AttributeError):
+        assert m._priv6 == 94
+
+
 def test_model_post_init_correct_mro():
     """https://github.com/pydantic/pydantic/issues/7293"""
     calls = []