Fix dataclass field ordering conflict when inheriting from parent with default fields

pyproject.toml

@@ -209,6 +209,7 @@ filterwarnings = [
   "ignore:^.*jsonschema.exceptions.RefResolutionError is deprecated as of version 4.18.0. If you wish to catch potential reference resolution errors, directly catch referencing.exceptions.Unresolvable..*",
   "ignore:^.*`experimental string processing` has been included in `preview` and deprecated. Use `preview` instead..*",
   "ignore:^.*No schemas found in components/schemas.*",
+  "ignore:^.*Dataclass .* has a field ordering conflict due to inheritance.*:UserWarning",
 ]
 norecursedirs = "tests/data/*"
 verbosity_assertions = 2

src/datamodel_code_generator/model/dataclass.py

@@ -31,7 +31,8 @@
     from datamodel_code_generator.reference import Reference
 
 
-def _has_field_assignment(field: DataModelFieldBase) -> bool:
+def has_field_assignment(field: DataModelFieldBase) -> bool:
+    """Check if a dataclass field has a default value or field() assignment."""
     return bool(field.field) or not (
         field.required or (field.represented_default == "None" and field.strip_default_none)
     )
@@ -66,7 +67,7 @@ def __init__(  # noqa: PLR0913
         """Initialize dataclass with fields sorted by field assignment requirement."""
         super().__init__(
             reference=reference,
-            fields=sorted(fields, key=_has_field_assignment),
+            fields=sorted(fields, key=has_field_assignment),
             decorators=decorators,
             base_classes=base_classes,
             custom_base_class=custom_base_class,

src/datamodel_code_generator/parser/base.py

@@ -18,6 +18,7 @@
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Callable, NamedTuple, Optional, Protocol, TypeVar, cast, runtime_checkable
 from urllib.parse import ParseResult
+from warnings import warn
 
 from pydantic import BaseModel
 from typing_extensions import TypeAlias
@@ -1798,6 +1799,65 @@ def __set_one_literal_on_default(self, models: list[DataModel]) -> None:
                 if model_field.nullable is not True:  # pragma: no cover
                     model_field.nullable = False
 
+    def __fix_dataclass_field_ordering(self, models: list[DataModel]) -> None:
+        """Fix field ordering for dataclasses with inheritance after defaults are set."""
+        for model in models:
+            if (inherited := self.__get_dataclass_inherited_info(model)) is None:
+                continue
+            inherited_names, has_default = inherited
+            if not has_default or not any(self.__is_new_required_field(f, inherited_names) for f in model.fields):
+                continue
+
+            if self.target_python_version.has_kw_only_dataclass:
+                for field in model.fields:
+                    if self.__is_new_required_field(field, inherited_names):
+                        field.extras["kw_only"] = True
+            else:
+                warn(
+                    f"Dataclass '{model.class_name}' has a field ordering conflict due to inheritance. "
+                    f"An inherited field has a default value, but new required fields are added. "
+                    f"This will cause a TypeError at runtime. Consider using --target-python-version 3.10 "
+                    f"or higher to enable automatic field(kw_only=True) fix.",
+                    category=UserWarning,
+                    stacklevel=2,
+                )
+            model.fields = sorted(model.fields, key=dataclass_model.has_field_assignment)
+
+    @classmethod
+    def __get_dataclass_inherited_info(cls, model: DataModel) -> tuple[set[str], bool] | None:
+        """Get inherited field names and whether any has default. Returns None if not applicable."""
+        if not isinstance(model, dataclass_model.DataClass):
+            return None
+        if not model.base_classes or model.dataclass_arguments.get("kw_only"):
+            return None
+
+        inherited_names: set[str] = set()
+        has_default = False
+        for base in model.base_classes:
+            if not base.reference or not isinstance(base.reference.source, DataModel):
+                continue  # pragma: no cover
+            for f in base.reference.source.iter_all_fields():
+                if not f.name or f.extras.get("init") is False:
+                    continue  # pragma: no cover
+                inherited_names.add(f.name)
+                if dataclass_model.has_field_assignment(f):
+                    has_default = True
+
+        for f in model.fields:
+            if f.name not in inherited_names or f.extras.get("init") is False:
+                continue
+            if dataclass_model.has_field_assignment(f):  # pragma: no branch
+                has_default = True
+        return (inherited_names, has_default) if inherited_names else None
+
+    def __is_new_required_field(self, field: DataModelFieldBase, inherited: set[str]) -> bool:  # noqa: PLR6301
+        """Check if field is a new required init field."""
+        return (
+            field.name not in inherited
+            and field.extras.get("init") is not False
+            and not dataclass_model.has_field_assignment(field)
+        )
+
     @classmethod
     def __update_type_aliases(cls, models: list[DataModel]) -> None:
         """Update type aliases to properly handle forward references per PEP 484."""
@@ -2463,6 +2523,7 @@ class Processed(NamedTuple):
             self.__change_field_name(models)
             self.__apply_discriminator_type(models, imports)
             self.__set_one_literal_on_default(models)
+            self.__fix_dataclass_field_ordering(models)
 
             processed_models.append(Processed(module, module_, models, init, imports, scoped_model_resolver))
 

tests/data/expected/main/openapi/dataclass_inheritance_field_ordering_py310.py

@@ -0,0 +1,19 @@
+# generated by datamodel-codegen:
+#   filename:  dataclass_inheritance_field_ordering.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from typing import Optional
+
+
+@dataclass
+class ParentWithDefault:
+    name: Optional[str] = 'default_name'
+    read_only_field: Optional[str] = None
+
+
+@dataclass
+class ChildWithRequired(ParentWithDefault):
+    child_id: str = field(kw_only=True)

tests/data/expected/main/openapi/discriminator/dataclass_enum_one_literal_as_default_py310.py

@@ -0,0 +1,33 @@
+# generated by datamodel-codegen:
+#   filename:  discriminator_enum.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from enum import Enum
+from typing import Literal, TypeAlias, Union
+
+
+class RequestVersionEnum(Enum):
+    v1 = 'v1'
+    v2 = 'v2'
+
+
+@dataclass
+class RequestBase:
+    version: RequestVersionEnum
+
+
+@dataclass
+class RequestV1(RequestBase):
+    request_id: str = field(kw_only=True)
+    version: Literal['v1'] = 'v1'
+
+
+@dataclass
+class RequestV2(RequestBase):
+    version: Literal['v2'] = 'v2'
+
+
+Request: TypeAlias = Union[RequestV1, RequestV2]

tests/data/openapi/dataclass_inheritance_field_ordering.yaml

@@ -0,0 +1,25 @@
+openapi: "3.0.0"
+info:
+  title: Dataclass Inheritance Field Ordering Test
+  version: "1.0"
+components:
+  schemas:
+    ParentWithDefault:
+      type: object
+      properties:
+        name:
+          type: string
+          default: "default_name"
+        read_only_field:
+          type: string
+          readOnly: true
+
+    ChildWithRequired:
+      allOf:
+        - $ref: '#/components/schemas/ParentWithDefault'
+        - type: object
+          properties:
+            child_id:
+              type: string
+          required:
+            - child_id

tests/main/openapi/test_main_openapi.py

@@ -2413,6 +2413,72 @@ def test_main_openapi_discriminator_one_literal_as_default_dataclass(output_file
     )
 
 
+@pytest.mark.skipif(
+    black.__version__.split(".")[0] == "19",
+    reason="Installed black doesn't support the old style",
+)
+def test_main_openapi_discriminator_one_literal_as_default_dataclass_py310(output_file: Path) -> None:
+    """Test OpenAPI generation with discriminator one literal as default for dataclass with Python 3.10+."""
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "discriminator_enum.yaml",
+        output_path=output_file,
+        input_file_type="openapi",
+        assert_func=assert_file_content,
+        expected_file=EXPECTED_OPENAPI_PATH / "discriminator" / "dataclass_enum_one_literal_as_default_py310.py",
+        extra_args=[
+            "--output-model-type",
+            "dataclasses.dataclass",
+            "--use-one-literal-as-default",
+            "--target-python-version",
+            "3.10",
+        ],
+    )
+
+
+@pytest.mark.skipif(
+    black.__version__.split(".")[0] == "19",
+    reason="Installed black doesn't support the old style",
+)
+def test_main_openapi_discriminator_one_literal_as_default_dataclass_py39_warning(output_file: Path) -> None:
+    """Test that Python 3.9 emits warning for dataclass field ordering conflict."""
+    with pytest.warns(UserWarning, match=r"Dataclass .* has a field ordering conflict due to inheritance"):
+        run_main_and_assert(
+            input_path=OPEN_API_DATA_PATH / "discriminator_enum.yaml",
+            output_path=output_file,
+            input_file_type="openapi",
+            assert_func=assert_file_content,
+            expected_file=EXPECTED_OPENAPI_PATH / "discriminator" / "dataclass_enum_one_literal_as_default.py",
+            extra_args=[
+                "--output-model-type",
+                "dataclasses.dataclass",
+                "--use-one-literal-as-default",
+                "--target-python-version",
+                "3.9",
+            ],
+        )
+
+
+@pytest.mark.skipif(
+    black.__version__.split(".")[0] == "19",
+    reason="Installed black doesn't support the old style",
+)
+def test_main_openapi_dataclass_inheritance_parent_default(output_file: Path) -> None:
+    """Test dataclass field ordering fix when parent has default field."""
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "dataclass_inheritance_field_ordering.yaml",
+        output_path=output_file,
+        input_file_type="openapi",
+        assert_func=assert_file_content,
+        expected_file=EXPECTED_OPENAPI_PATH / "dataclass_inheritance_field_ordering_py310.py",
+        extra_args=[
+            "--output-model-type",
+            "dataclasses.dataclass",
+            "--target-python-version",
+            "3.10",
+        ],
+    )
+
+
 @pytest.mark.skipif(
     black.__version__.split(".")[0] == "19",
     reason="Installed black doesn't support the old style",