fix(PydanticDTO): enforce explicit specification for Optional fields and ensure default factory usage for list fields

litestar/plugins/pydantic/dto.py

@@ -137,12 +137,7 @@ def generate_field_definitions(
                         stacklevel=2,
                     )
 
-                if not is_pydantic_undefined(field_info.default):
-                    default = field_info.default
-                elif field_definition.is_optional:
-                    default = None
-                else:
-                    default = Empty
+                default = field_info.default if not is_pydantic_undefined(field_info.default) else Empty
 
                 default_factory = (
                     field_info.default_factory

tests/unit/test_plugins/test_pydantic/test_dto.py

@@ -57,7 +57,7 @@ class Model(base_model):  # type: ignore[misc, valid-type]
     dto_type = PydanticDTO[Model]
     field_defs = list(dto_type.generate_field_definitions(Model))
     assert len(field_defs) == 1
-    assert field_defs[0].default is None
+    assert field_defs[0].default is Empty
 
 
 def test_detect_nested_field_pydantic_v1(monkeypatch: pytest.MonkeyPatch) -> None:

tests/unit/test_plugins/test_pydantic/test_pydantic_dto_factory.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from dataclasses import replace
-from typing import TYPE_CHECKING, Callable, Optional
+from typing import TYPE_CHECKING, Callable, List, Optional, Union
 from unittest.mock import ANY
 
 import pydantic as pydantic_v2
@@ -11,6 +11,7 @@
 
 from litestar.dto import DTOField, DTOFieldDefinition, Mark, dto_field
 from litestar.plugins.pydantic import PydanticDTO
+from litestar.types.builtin_types import NoneType
 from litestar.typing import FieldDefinition
 
 from . import PydanticVersion
@@ -91,13 +92,41 @@ def expected_field_defs(int_factory: Callable[[], int]) -> list[DTOFieldDefiniti
                 default_factory=int_factory,
                 dto_field=DTOField(),
             ),
-            default=None,
             metadata=ANY,
             type_wrappers=ANY,
             raw=ANY,
             kwarg_definition=ANY,
             passthrough_constraints=False,
         ),
+        DTOFieldDefinition.from_field_definition(
+            field_definition=FieldDefinition.from_kwarg(
+                annotation=Optional[int],
+                name="f",
+            ),
+            model_name=ANY,
+            default_factory=None,
+            dto_field=DTOField(),
+            passthrough_constraints=False,
+        ),
+        replace(
+            DTOFieldDefinition.from_field_definition(
+                field_definition=FieldDefinition.from_kwarg(
+                    annotation=List[str],
+                    name="g",
+                ),
+                model_name=ANY,
+                default_factory=list,
+                dto_field=DTOField(),
+                passthrough_constraints=False,
+            ),
+            annotation=Optional[List[str]],
+            origin=Union,
+            instantiable_origin=Union,
+            safe_generic_origin=Union,
+            inner_types=ANY,
+            args=(List[str], NoneType),
+            raw=Optional[List[str]],
+        ),
     ]
 
 
@@ -111,6 +140,8 @@ class TestModel(pydantic_v1.BaseModel):
         c: Annotated[int, pydantic_v1.Field(gt=1)]
         d: int = pydantic_v1.Field(default=1)
         e: int = pydantic_v1.Field(default_factory=int_factory)
+        f: Optional[int]  # noqa: UP007
+        g: List[str] = pydantic_v1.Field(default_factory=list)  # noqa: UP006
 
     field_defs = list(PydanticDTO.generate_field_definitions(TestModel))
     assert field_defs[0].model_name == "TestModel"
@@ -128,6 +159,8 @@ class TestModel(pydantic_v2.BaseModel):
         c: Annotated[int, pydantic_v2.Field(gt=1)]
         d: int = pydantic_v2.Field(default=1)
         e: int = pydantic_v2.Field(default_factory=int_factory)
+        f: Optional[int]  # noqa: UP007
+        g: List[str] = pydantic_v2.Field(default_factory=list)  # noqa: UP006
 
     field_defs = list(PydanticDTO.generate_field_definitions(TestModel))
     assert field_defs[0].model_name == "TestModel"