Fixed mypy error and added tests for PrivateAttr defaults (#149)

- Used getattr() instead of direct attribute access to satisfy mypy's
  union-attr check on InstanceOrType.
- Added regression tests verifying PrivateAttr with default and
  default_factory work correctly on database-loaded instances.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: xr843

sqlmodel/_compat.py

@@ -98,8 +98,10 @@ def init_pydantic_private_attrs(new_object: InstanceOrType["SQLModel"]) -> None:
     # mirroring what Pydantic's own BaseModel.__init__ does. Previously this was
     # set to None, which caused AttributeError when accessing PrivateAttr fields
     # on instances reconstructed from the database (via __new__, bypassing __init__).
+    cls = new_object if isinstance(new_object, type) else new_object.__class__
+    private_attributes = getattr(cls, "__private_attributes__", {})
     pydantic_private = {}
-    for k, v in new_object.__class__.__private_attributes__.items():
+    for k, v in private_attributes.items():
         pydantic_private[k] = v.get_default(call_default_factory=True)
     object.__setattr__(
         new_object,

tests/test_private_attr.py

@@ -0,0 +1,52 @@
+from pydantic import PrivateAttr
+from sqlmodel import Field, Session, SQLModel, create_engine, select
+
+
+def test_private_attr_default_preserved_after_db_load(clear_sqlmodel):
+    """PrivateAttr defaults should be available on instances loaded from DB (#149)."""
+
+    class Item(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        _secret: str = PrivateAttr(default="default_secret")
+
+    engine = create_engine("sqlite:///:memory:")
+    SQLModel.metadata.create_all(engine)
+
+    with Session(engine) as db:
+        item = Item(name="test")
+        assert item._secret == "default_secret"
+        db.add(item)
+        db.commit()
+
+    with Session(engine) as db:
+        loaded_item = db.exec(select(Item)).one()
+        # Previously raised AttributeError because __pydantic_private__ was None.
+        assert loaded_item._secret == "default_secret"
+
+    SQLModel.metadata.clear()
+
+
+def test_private_attr_default_factory_preserved_after_db_load(clear_sqlmodel):
+    """PrivateAttr with default_factory should work on DB-loaded instances (#149)."""
+
+    class Item(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        _tags: list[str] = PrivateAttr(default_factory=list)
+
+    engine = create_engine("sqlite:///:memory:")
+    SQLModel.metadata.create_all(engine)
+
+    with Session(engine) as db:
+        item = Item(name="test")
+        item._tags.append("hello")
+        db.add(item)
+        db.commit()
+
+    with Session(engine) as db:
+        loaded_item = db.exec(select(Item)).one()
+        # Should get a fresh empty list from default_factory, not AttributeError.
+        assert loaded_item._tags == []
+
+    SQLModel.metadata.clear()