Add --use-default-factory-for-optional-nested-models option

Author: koxudaxi

src/datamodel_code_generator/__init__.py

@@ -470,6 +470,7 @@ def generate(  # noqa: PLR0912, PLR0913, PLR0914, PLR0915
     frozen_dataclasses: bool = False,
     no_alias: bool = False,
     use_frozen_field: bool = False,
+    use_default_factory_for_optional_nested_models: bool = False,
     formatters: list[Formatter] = DEFAULT_FORMATTERS,
     settings_path: Path | None = None,
     parent_scoped_naming: bool = False,
@@ -717,6 +718,7 @@ def get_header_and_first_line(csv_file: IO[str]) -> dict[str, Any]:
         frozen_dataclasses=frozen_dataclasses,
         no_alias=no_alias,
         use_frozen_field=use_frozen_field,
+        use_default_factory_for_optional_nested_models=use_default_factory_for_optional_nested_models,
         formatters=formatters,
         encoding=encoding,
         parent_scoped_naming=parent_scoped_naming,

src/datamodel_code_generator/__main__.py

@@ -459,6 +459,7 @@ def validate_all_exports_collision_strategy(cls, values: dict[str, Any]) -> dict
     dataclass_arguments: Optional[DataclassArguments] = None  # noqa: UP045
     no_alias: bool = False
     use_frozen_field: bool = False
+    use_default_factory_for_optional_nested_models: bool = False
     formatters: list[Formatter] = DEFAULT_FORMATTERS
     parent_scoped_naming: bool = False
     disable_future_imports: bool = False
@@ -761,6 +762,7 @@ def run_generate_from_config(  # noqa: PLR0913, PLR0917
         frozen_dataclasses=config.frozen_dataclasses,
         no_alias=config.no_alias,
         use_frozen_field=config.use_frozen_field,
+        use_default_factory_for_optional_nested_models=config.use_default_factory_for_optional_nested_models,
         formatters=config.formatters,
         settings_path=settings_path,
         parent_scoped_naming=config.parent_scoped_naming,

src/datamodel_code_generator/arguments.py

@@ -594,6 +594,13 @@ def start_section(self, heading: str | None) -> None:
     action="store_true",
     default=None,
 )
+field_options.add_argument(
+    "--use-default-factory-for-optional-nested-models",
+    help="Use default_factory for optional nested model fields instead of None default. "
+    "E.g., `field: Optional[Model] = Field(default_factory=Model)` instead of `field: Optional[Model] = None`",
+    action="store_true",
+    default=None,
+)
 
 # ======================================================================================
 # Options for templating output

src/datamodel_code_generator/cli_options.py

@@ -92,6 +92,9 @@ class CLIOptionMeta:
     "--strip-default-none": CLIOptionMeta(name="--strip-default-none", category=OptionCategory.MODEL),
     "--dataclass-arguments": CLIOptionMeta(name="--dataclass-arguments", category=OptionCategory.MODEL),
     "--use-frozen-field": CLIOptionMeta(name="--use-frozen-field", category=OptionCategory.MODEL),
+    "--use-default-factory-for-optional-nested-models": CLIOptionMeta(
+        name="--use-default-factory-for-optional-nested-models", category=OptionCategory.MODEL
+    ),
     "--union-mode": CLIOptionMeta(name="--union-mode", category=OptionCategory.MODEL),
     "--parent-scoped-naming": CLIOptionMeta(name="--parent-scoped-naming", category=OptionCategory.MODEL),
     "--use-one-literal-as-default": CLIOptionMeta(name="--use-one-literal-as-default", category=OptionCategory.MODEL),

src/datamodel_code_generator/model/base.py

@@ -166,6 +166,7 @@ class Config:
     read_only: bool = False
     write_only: bool = False
     use_frozen_field: bool = False
+    use_default_factory_for_optional_nested_models: bool = False
 
     if not TYPE_CHECKING:
         if not PYDANTIC_V2:

src/datamodel_code_generator/model/dataclass.py

@@ -143,6 +143,19 @@ def field(self) -> str | None:
             return None
         return result
 
+    def _get_default_factory_for_nested_model(self) -> str | None:
+        """Get default_factory for nested dataclass model fields.
+
+        Returns the class name if the field type references a DataClass,
+        otherwise returns None.
+        """
+        for data_type in self.data_type.data_types or (self.data_type,):
+            if data_type.is_dict:
+                continue
+            if data_type.reference and isinstance(data_type.reference.source, DataClass):
+                return data_type.alias or data_type.reference.source.class_name
+        return None
+
     def __str__(self) -> str:
         """Generate field() call or default value representation."""
         data: dict[str, Any] = {k: v for k, v in self.extras.items() if k in self._FIELD_KEYS}
@@ -161,6 +174,17 @@ def __str__(self) -> str:
                 }
             }
 
+        # Handle default_factory for optional nested models
+        if (
+            self.use_default_factory_for_optional_nested_models
+            and not self.required
+            and (self.default is None or self.default is UNDEFINED)
+            and "default_factory" not in data
+        ):
+            nested_model_name = self._get_default_factory_for_nested_model()
+            if nested_model_name:
+                data["default_factory"] = nested_model_name
+
         if not data:
             return ""
 

src/datamodel_code_generator/model/msgspec.py

@@ -247,7 +247,7 @@ def field(self) -> str | None:
             return None
         return result
 
-    def __str__(self) -> str:
+    def __str__(self) -> str:  # noqa: PLR0912
         """Generate field() call or default value representation."""
         data: dict[str, Any] = {k: v for k, v in self.extras.items() if k in self._FIELD_KEYS}
         if self.alias:
@@ -284,6 +284,18 @@ def __str__(self) -> str:
             else:
                 data["default_factory"] = type(default_value).__name__
 
+        # Handle default_factory for optional nested models
+        if (
+            self.use_default_factory_for_optional_nested_models
+            and not self.required
+            and (self.default is None or self.default is UNDEFINED)
+            and "default_factory" not in data
+        ):
+            nested_model_name = self._get_default_factory_for_optional_nested_model()
+            if nested_model_name:
+                data["default_factory"] = nested_model_name
+                data.pop("default", None)
+
         if not data:
             return ""
 
@@ -412,6 +424,19 @@ def _get_default_as_struct_model(self) -> str | None:
                 )
         return None
 
+    def _get_default_factory_for_optional_nested_model(self) -> str | None:
+        """Get default_factory for optional nested Struct model fields.
+
+        Returns the class name if the field type references a Struct,
+        otherwise returns None.
+        """
+        for data_type in self.data_type.data_types or (self.data_type,):
+            if data_type.is_dict:
+                continue
+            if data_type.reference and isinstance(data_type.reference.source, Struct):
+                return data_type.alias or data_type.reference.source.class_name
+        return None
+
 
 class DataTypeManager(_DataTypeManager):
     """Type manager for msgspec Struct models."""

src/datamodel_code_generator/model/pydantic/base_model.py

@@ -152,6 +152,19 @@ def _get_default_as_pydantic_model(self) -> str | None:
                 )
         return None
 
+    def _get_default_factory_for_optional_nested_model(self) -> str | None:
+        """Get default_factory for optional nested Pydantic model fields.
+
+        Returns the class name if the field type references a BaseModel,
+        otherwise returns None.
+        """
+        for data_type in self.data_type.data_types or (self.data_type,):
+            if data_type.is_dict:
+                continue
+            if data_type.reference and isinstance(data_type.reference.source, BaseModelBase):
+                return data_type.alias or data_type.reference.source.class_name
+        return None
+
     def _process_data_in_str(self, data: dict[str, Any]) -> None:
         if self.const:
             data["const"] = True
@@ -204,6 +217,15 @@ def __str__(self) -> str:  # noqa: PLR0912
         else:
             default_factory = data.pop("default_factory", None)
 
+        # Handle default_factory for optional nested models
+        if (
+            default_factory is None
+            and self.use_default_factory_for_optional_nested_models
+            and not self.required
+            and (self.default is None or self.default is UNDEFINED)
+        ):
+            default_factory = self._get_default_factory_for_optional_nested_model()
+
         self.__dict__["_computed_default_factory"] = default_factory
 
         field_arguments = sorted(f"{k}={v!r}" for k, v in data.items() if v is not None)

src/datamodel_code_generator/parser/base.py

@@ -734,6 +734,7 @@ def __init__(  # noqa: PLR0913, PLR0915
         frozen_dataclasses: bool = False,
         no_alias: bool = False,
         use_frozen_field: bool = False,
+        use_default_factory_for_optional_nested_models: bool = False,
         formatters: list[Formatter] = DEFAULT_FORMATTERS,
         parent_scoped_naming: bool = False,
         dataclass_arguments: DataclassArguments | None = None,
@@ -875,6 +876,7 @@ def __init__(  # noqa: PLR0913, PLR0915
         self.type_mappings: dict[tuple[str, str], str] = Parser._parse_type_mappings(type_mappings)
         self.read_only_write_only_model_type: ReadOnlyWriteOnlyModelType | None = read_only_write_only_model_type
         self.use_frozen_field: bool = use_frozen_field
+        self.use_default_factory_for_optional_nested_models: bool = use_default_factory_for_optional_nested_models
 
     @property
     def field_name_model_type(self) -> ModelType:

src/datamodel_code_generator/parser/graphql.py

@@ -190,6 +190,7 @@ def __init__(  # noqa: PLR0913
         read_only_write_only_model_type: ReadOnlyWriteOnlyModelType | None = None,
         use_serialize_as_any: bool = False,
         use_frozen_field: bool = False,
+        use_default_factory_for_optional_nested_models: bool = False,
     ) -> None:
         """Initialize the GraphQL parser with configuration options."""
         super().__init__(
@@ -284,6 +285,7 @@ def __init__(  # noqa: PLR0913
             read_only_write_only_model_type=read_only_write_only_model_type,
             use_serialize_as_any=use_serialize_as_any,
             use_frozen_field=use_frozen_field,
+            use_default_factory_for_optional_nested_models=use_default_factory_for_optional_nested_models,
         )
 
         self.data_model_scalar_type = data_model_scalar_type