pydantic
diff --git a/‎pydantic/main.py
Lines changed: 11 additions & 1 deletion b/‎pydantic/main.py
Lines changed: 11 additions & 1 deletion
diff --git a/‎tests/test_main.py
Lines changed: 28 additions & 0 deletions b/‎tests/test_main.py
Lines changed: 28 additions & 0 deletions
@@ -99,10 +99,20 @@ def _model_field_setattr_handler(model: BaseModel, name: str, val: Any) -> None:
     model.__pydantic_fields_set__.add(name)
 
 
+def _private_setattr_handler(model: BaseModel, name: str, val: Any) -> None:
+    if getattr(model, '__pydantic_private__', None) is None:
+        # While the attribute should be present at this point, this may not be the case if
+        # users do unusual stuff with `model_post_init()` (which is where the  `__pydantic_private__`
+        # is initialized, by wrapping the user-defined `model_post_init()`), e.g. if they mock
+        # the `model_post_init()` call. Ideally we should find a better way to init private attrs.
+        object.__setattr__(model, '__pydantic_private__', {})
+    model.__pydanti
10000
c_private__[name] = val  # pyright: ignore[reportOptionalSubscript]
+
+
 _SIMPLE_SETATTR_HANDLERS: Mapping[str, Callable[[BaseModel, str, Any], None]] = {
     'model_field': _model_field_setattr_handler,
     'validate_assignment': lambda model, name, val: model.__pydantic_validator__.validate_assignment(model, name, val),  # pyright: ignore[reportAssignmentType]
-    'private': lambda model, name, val: model.__pydantic_private__.__setitem__(name, val),  # pyright: ignore[reportOptionalMemberAccess]
+    'private': _private_setattr_handler,
     'cached_property': lambda model, name, val: model.__dict__.__setitem__(name, val),
     'extra_known': lambda model, name, val: _object_setattr(model, name, val),
 }
 
@@ -2307,6 +2307,34 @@ class C(A, B):
     assert calls == ['C.model_post_init']
 
 
+def test_model_post_init_mocked_setattr() -> None:
+    """https://github.com/pydantic/pydantic/issues/11646
+
+    Fixes a small regression in 2.11. To instantiate private attributes on model instances
+    (and as such the `__pydantic_private__` instance attribute), Pydantic defines its own
+    `model_post_init()` (and wraps the user-defined one if it exists). In tests, some users
+    can mock their `model_post_init()` if they want to avoid unwanted side-effects (meaning
+    `__pydantic_private__` won't be instantiated).
+    In 2.11, the `BaseModel.__setattr__` logic was tweaked and required the `__pydantic_private__`
+    attribute to be present, resulting in attribute errors.
+    """
+
+    class Model(BaseModel):
+        _a: int
+
+        def model_post_init(self, context: Any, /) -> None:
+            """Do some stuff"""
+
+    # This reproduces `patch.object(Model, 'model_post_init')`:
+    Model.model_post_init = lambda *args, **kwargs: None
+
+    m = Model()
+    assert m.__pydantic_private__ is None
+
+    m._a = 2
+    assert m._a == 2
+
+
 def test_del_model_attr():
     class Model(BaseModel):
         some_field: str