Fix handling of empty enums to return null type when not nullable and add tests for null-only enum generation (#2674)

Author: koxudaxi

src/datamodel_code_generator/parser/jsonschema.py

@@ -2567,6 +2567,46 @@ def parse_enum(
                 )
             )
 
+        if not enum_fields:
+            if not nullable:
+                return self.data_type_manager.get_data_type(Types.null)
+            name = self._apply_title_as_name(name, obj)
+            reference = self.model_resolver.add(
+                path,
+                name,
+                class_name=True,
+                singular_name=singular_name,
+                singular_name_suffix="Enum",
+                loaded=True,
+            )
+            data_model_root_type = self.data_model_root_type(
+                reference=reference,
+                fields=[
+                    self.data_model_field_type(
+                        data_type=self.data_type_manager.get_data_type(Types.null),
+                        default=obj.default,
+                        required=False,
+                        nullable=True,
+                        strip_default_none=self.strip_default_none,
+                        extras=self.get_field_extras(obj),
+                        use_annotated=self.use_annotated,
+                        has_default=obj.has_default,
+                        use_field_description=self.use_field_description,
+                        use_inline_field_description=self.use_inline_field_description,
+                        original_name=None,
+                    )
+                ],
+                custom_base_class=obj.custom_base_path or self.base_class,
+                custom_template_dir=self.custom_template_dir,
+                extra_template_data=self.extra_template_data,
+                path=self.current_source_path,
+                default=obj.default if obj.has_default else UNDEFINED,
+                nullable=obj.type_has_null,
+                treat_dot_as_module=self.treat_dot_as_module,
+            )
+            self.results.append(data_model_root_type)
+            return self.data_type(reference=reference)
+
         def create_enum(reference_: Reference) -> DataType:
             type_: Types | None = (
                 self._get_type_with_mappings(obj.type, obj.format) if isinstance(obj.type, str) else None

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

@@ -0,0 +1,11 @@
+# generated by datamodel-codegen:
+#   filename:  null_only_enum.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+
+class NullEnum(BaseModel):
+    __root__: None = None

tests/data/openapi/null_only_enum.yaml

@@ -0,0 +1,12 @@
+openapi: 3.0.1
+info:
+  title: Null Only Enum Test
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    NullEnum:
+      type: string
+      enum:
+        - null
+      nullable: true

tests/main/openapi/test_main_openapi.py

@@ -4172,3 +4172,14 @@ def test_main_unique_items_default_set(output_model: str, expected_output: str,
         expected_file=expected_output,
         extra_args=["--output-model-type", output_model, "--use-unique-items-as-set"],
     )
+
+
+def test_main_openapi_null_only_enum(output_file: Path) -> None:
+    """Test OpenAPI generation with enum containing only null value."""
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "null_only_enum.yaml",
+        output_path=output_file,
+        input_file_type="openapi",
+        assert_func=assert_file_content,
+        expected_file="null_only_enum.py",
+    )

tests/parser/test_jsonschema.py

@@ -921,3 +921,11 @@ def test_parse_combined_schema_anyof_with_ref_and_schema_keywords() -> None:
     parser.parse_raw_obj("Model", schema, [])
     results = list(parser.results)
     assert len(results) >= 1
+
+
+def test_parse_enum_empty_enum_not_nullable() -> None:
+    """Test parse_enum returns null type when enum_fields is empty and not nullable."""
+    parser = JsonSchemaParser("")
+    obj = JsonSchemaObject.parse_obj({"type": "integer", "enum": []})
+    result = parser.parse_enum("EmptyEnum", obj, ["EmptyEnum"])
+    assert result.type == "None"