Fix empty dict/list defaults not generating default_factory for Pydantic models (#2655)

koxudaxi · web-flow · commit b6151d575cba · 2025-12-13T10:44:32.000+09:00
* Add support for empty dict and list default values in PodSpec models

* Refactor default values in PodSpec and related models to use empty list and literals

* Refactor default values in models to use Field with default_factory for better initialization

* Use STANDARD_LIST for empty list default in base_model
diff --git a/src/datamodel_code_generator/model/pydantic/base_model.py b/src/datamodel_code_generator/model/pydantic/base_model.py
@@ -23,7 +23,7 @@
     IMPORT_EXTRA,
     IMPORT_FIELD,
 )
-from datamodel_code_generator.types import UnionIntFloat, chain_as_tuple
+from datamodel_code_generator.types import STANDARD_LIST, UnionIntFloat, chain_as_tuple
 
 if TYPE_CHECKING:
     from collections import defaultdict
@@ -134,6 +134,8 @@ def _get_default_as_pydantic_model(self) -> str | None:
                     and isinstance(data_type_child.reference.source, BaseModelBase)
                     and isinstance(self.default, list)
                 ):  # pragma: no cover
+                    if not self.default:
+                        return STANDARD_LIST
                     return (
                         f"lambda :[{data_type_child.alias or data_type_child.reference.source.class_name}."
                         f"{self._PARSE_METHOD}(v) for v in {self.default!r}]"
@@ -197,7 +199,7 @@ def __str__(self) -> str:  # noqa: PLR0912
 
         if self.required:
             default_factory = None
-        elif self.default and "default_factory" not in data:
+        elif self.default is not UNDEFINED and self.default is not None and "default_factory" not in data:
             default_factory = self._get_default_as_pydantic_model()
         else:
             default_factory = data.pop("default_factory", None)
diff --git a/tests/data/expected/main/jsonschema/all_of_any_of_base_class_ref.py b/tests/data/expected/main/jsonschema/all_of_any_of_base_class_ref.py
@@ -16,7 +16,7 @@ class MapState1(BaseModel):
 class MapState2(BaseModel):
     latitude: Latitude
     longitude: Longitude
-    zoom: Optional[Zoom] = 0
+    zoom: Optional[Zoom] = Field(default_factory=lambda: Zoom.parse_obj(0))
     bearing: Optional[Bearing] = None
     pitch: Pitch
     drag_rotate: Optional[DragRotate] = Field(None, alias="dragRotate")
diff --git a/tests/data/expected/main/jsonschema/jsonschema_root_model_ordering.py b/tests/data/expected/main/jsonschema/jsonschema_root_model_ordering.py
@@ -10,22 +10,22 @@
 
 
 class Zoo(BaseModel):
-    animals: Optional[List[Animals]] = Field([], title='Animals')
+    animals: Optional[List[Animals]] = Field(default_factory=list, title='Animals')
 
 
 class Dog(BaseModel):
     name: Literal['dog'] = Field('dog', title='woof')
-    friends: Optional[List[Friends]] = Field([], title='Friends')
+    friends: Optional[List[Friends]] = Field(default_factory=list, title='Friends')
 
 
 class Cat(BaseModel):
     name: Literal['cat'] = Field('cat', title='meow')
-    friends: Optional[List[Friends]] = Field([], title='Friends')
+    friends: Optional[List[Friends]] = Field(default_factory=list, title='Friends')
 
 
 class Bird(BaseModel):
     name: Literal['bird'] = Field('bird', title='chirp')
-    friends: Optional[List[Friends]] = Field([], title='Friends')
+    friends: Optional[List[Friends]] = Field(default_factory=list, title='Friends')
 
 
 class Animals(RootModel[Union[Dog, Cat, Bird]]):
diff --git a/tests/data/expected/main/jsonschema/jsonschema_root_model_ordering_keep_model_order.py b/tests/data/expected/main/jsonschema/jsonschema_root_model_ordering_keep_model_order.py
@@ -11,25 +11,25 @@
 
 class Bird(BaseModel):
     name: Literal['bird'] = Field('bird', title='chirp')
-    friends: Optional[List[Friends]] = Field([], title='Friends')
+    friends: Optional[List[Friends]] = Field(default_factory=list, title='Friends')
 
 
 class Cat(BaseModel):
     name: Literal['cat'] = Field('cat', title='meow')
-    friends: Optional[List[Friends]] = Field([], title='Friends')
+    friends: Optional[List[Friends]] = Field(default_factory=list, title='Friends')
 
 
 class Dog(BaseModel):
     name: Literal['dog'] = Field('dog', title='woof')
-    friends: Optional[List[Friends]] = Field([], title='Friends')
+    friends: Optional[List[Friends]] = Field(default_factory=list, title='Friends')
 
 
 class Friends(RootModel[Union[Dog, Cat, Bird]]):
     root: Union[Dog, Cat, Bird] = Field(..., discriminator='name', title='Animal')
 
 
 class Zoo(BaseModel):
-    animals: Optional[List[Animals]] = Field([], title='Animals')
+    animals: Optional[List[Animals]] = Field(default_factory=list, title='Animals')
 
 
 class Animals(RootModel[Union[Dog, Cat, Bird]]):
diff --git a/tests/data/expected/main/openapi/empty_dict_default.py b/tests/data/expected/main/openapi/empty_dict_default.py
@@ -0,0 +1,20 @@
+# generated by datamodel-codegen:
+#   filename:  empty_dict_default.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel, Field
+
+
+class ObjectMeta(BaseModel):
+    name: Optional[str] = None
+    namespace: Optional[str] = None
+
+
+class PodSpec(BaseModel):
+    metadata: Optional[ObjectMeta] = Field(
+        default_factory=lambda: ObjectMeta.parse_obj({})
+    )
diff --git a/tests/data/expected/main/openapi/empty_list_default.py b/tests/data/expected/main/openapi/empty_list_default.py
@@ -0,0 +1,17 @@
+# generated by datamodel-codegen:
+#   filename:  empty_list_default.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import List, Optional
+
+from pydantic import BaseModel, Field
+
+
+class Container(BaseModel):
+    name: Optional[str] = None
+
+
+class PodSpec(BaseModel):
+    containers: Optional[List[Container]] = Field(default_factory=list)
diff --git a/tests/data/expected/main/openapi/pydantic_v2_empty_dict_default.py b/tests/data/expected/main/openapi/pydantic_v2_empty_dict_default.py
@@ -0,0 +1,20 @@
+# generated by datamodel-codegen:
+#   filename:  empty_dict_default.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel, Field
+
+
+class ObjectMeta(BaseModel):
+    name: Optional[str] = None
+    namespace: Optional[str] = None
+
+
+class PodSpec(BaseModel):
+    metadata: Optional[ObjectMeta] = Field(
+        default_factory=lambda: ObjectMeta.model_validate({})
+    )
diff --git a/tests/data/expected/main/openapi/pydantic_v2_empty_list_default.py b/tests/data/expected/main/openapi/pydantic_v2_empty_list_default.py
@@ -0,0 +1,17 @@
+# generated by datamodel-codegen:
+#   filename:  empty_list_default.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import List, Optional
+
+from pydantic import BaseModel, Field
+
+
+class Container(BaseModel):
+    name: Optional[str] = None
+
+
+class PodSpec(BaseModel):
+    containers: Optional[List[Container]] = Field(default_factory=list)
diff --git a/tests/data/openapi/empty_dict_default.yaml b/tests/data/openapi/empty_dict_default.yaml
@@ -0,0 +1,28 @@
+openapi: 3.1.0
+info:
+  title: Empty Dict Default Test
+  version: 0.1.0
+servers:
+- url: http://example.com
+paths:
+  /test:
+    get:
+      responses:
+        '200':
+          description: OK
+components:
+  schemas:
+    ObjectMeta:
+      type: object
+      properties:
+        name:
+          type: string
+        namespace:
+          type: string
+    PodSpec:
+      type: object
+      properties:
+        metadata:
+          allOf:
+            - $ref: '#/components/schemas/ObjectMeta'
+          default: {}
diff --git a/tests/data/openapi/empty_list_default.yaml b/tests/data/openapi/empty_list_default.yaml
@@ -0,0 +1,27 @@
+openapi: 3.1.0
+info:
+  title: Empty List Default Test
+  version: 0.1.0
+servers:
+- url: http://example.com
+paths:
+  /test:
+    get:
+      responses:
+        '200':
+          description: OK
+components:
+  schemas:
+    Container:
+      type: object
+      properties:
+        name:
+          type: string
+    PodSpec:
+      type: object
+      properties:
+        containers:
+          type: array
+          items:
+            $ref: '#/components/schemas/Container'
+          default: []
diff --git a/tests/main/openapi/test_main_openapi.py b/tests/main/openapi/test_main_openapi.py
@@ -1889,6 +1889,62 @@ def test_main_openapi_union_default_object(output_model: str, expected_output: s
     )
 
 
+@pytest.mark.parametrize(
+    ("output_model", "expected_output"),
+    [
+        (
+            "pydantic.BaseModel",
+            "empty_dict_default.py",
+        ),
+        (
+            "pydantic_v2.BaseModel",
+            "pydantic_v2_empty_dict_default.py",
+        ),
+    ],
+)
+@pytest.mark.skipif(
+    black.__version__.split(".")[0] == "19",
+    reason="Installed black doesn't support the old style",
+)
+def test_main_openapi_empty_dict_default(output_model: str, expected_output: str, output_file: Path) -> None:
+    """Test OpenAPI generation with empty dict default values."""
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "empty_dict_default.yaml",
+        output_path=output_file,
+        expected_file=EXPECTED_OPENAPI_PATH / expected_output,
+        input_file_type="openapi",
+        extra_args=["--output-model", output_model, "--target-python-version", "3.9", "--openapi-scopes", "schemas"],
+    )
+
+
+@pytest.mark.parametrize(
+    ("output_model", "expected_output"),
+    [
+        (
+            "pydantic.BaseModel",
+            "empty_list_default.py",
+        ),
+        (
+            "pydantic_v2.BaseModel",
+            "pydantic_v2_empty_list_default.py",
+        ),
+    ],
+)
+@pytest.mark.skipif(
+    black.__version__.split(".")[0] == "19",
+    reason="Installed black doesn't support the old style",
+)
+def test_main_openapi_empty_list_default(output_model: str, expected_output: str, output_file: Path) -> None:
+    """Test OpenAPI generation with empty list default values."""
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "empty_list_default.yaml",
+        output_path=output_file,
+        expected_file=EXPECTED_OPENAPI_PATH / expected_output,
+        input_file_type="openapi",
+        extra_args=["--output-model", output_model, "--target-python-version", "3.9", "--openapi-scopes", "schemas"],
+    )
+
+
 def test_main_dataclass(output_file: Path) -> None:
     """Test OpenAPI generation with dataclass output."""
     run_main_and_assert(
