Support for x-enum-fallback-value field in enums.

datamodel_code_generator/model/enum.py

@@ -46,6 +46,7 @@ def __init__(
         description: Optional[str] = None,
         type_: Optional[Types] = None,
         default: Any = UNDEFINED,
+        fallback: Any = UNDEFINED,
     ):
 
         super().__init__(
@@ -70,6 +71,9 @@ def __init__(
                     *self.base_classes,
                 ]
 
+        if fallback != UNDEFINED:
+            self.extra_template_data.update({"fallback": fallback})
+
     @classmethod
     def get_data_type(cls, types: Types, **kwargs: Any) -> DataType:
         raise NotImplementedError

datamodel_code_generator/model/template/Enum.jinja2

@@ -15,3 +15,8 @@ class {{ class_name }}({{ base_class }}):
     """
     {%- endif %}
 {%- endfor -%}
+{%- if fallback %}
+    @classmethod
+    def _missing_(cls, value):
+      return {{ class_name }}.{{ fallback }}
+{%- endif %}

datamodel_code_generator/parser/jsonschema.py

@@ -189,6 +189,7 @@ def validate_ref(cls, value: Any) -> Any:
     ref: Optional[str] = Field(default=None, alias='$ref')
     nullable: Optional[bool] = False
     x_enum_varnames: List[str] = Field(default=[], alias='x-enum-varnames')
+    x_enum_fallback_value: Optional[str] = Field(default=None, alias='x-enum-fallback-value')
     description: Optional[str]
     title: Optional[str]
     example: Any
@@ -1015,6 +1016,27 @@ def parse_enum(
                 )
             )
 
+        if obj.x_enum_fallback_value is not None:
+            field_name = str(obj.x_enum_fallback_value)
+            default = f"'{field_name.translate(escape_characters)}'"
+            if field_name not in exclude_field_names:
+                field_name = self.model_resolver.get_valid_field_name(
+                    field_name, model_type=ModelType.ENUM
+                )
+                enum_fields.append(
+                    self.data_model_field_type(
+                        name=field_name,
+                        default=default,
+                        data_type=self.data_type_manager.get_data_type(
+                            Types.any,
+                        ),
+                        required=True,
+                        strip_default_none=self.strip_default_none,
+                        has_default=obj.has_default,
+                        use_field_description=self.use_field_description,
+                    )
+                )
+
         def create_enum(reference_: Reference) -> DataType:
             enum = Enum(
                 reference=reference_,
@@ -1026,6 +1048,7 @@ def create_enum(reference_: Reference) -> DataType:
                 if self.use_subclass_enum and isinstance(obj.type, str)
                 else None,
                 default=obj.default if obj.has_default else UNDEFINED,
+                fallback=obj.x_enum_fallback_value if obj.x_enum_fallback_value is not None else UNDEFINED,
             )
             self.results.append(enum)
             return self.data_type(reference=reference_)

tests/data/expected/parser/openapi/openapi_parser_x_enum_fallback_value/output.py

@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+from enum import Enum
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class EnumTypeWithFallback(Enum):
+    a = 'a'
+    b = 'b'
+    unknown = 'unknown'
+
+    @classmethod
+    def _missing_(cls, value):
+        return EnumTypeWithFallback.unknown
+
+
+class EnumTypeWithFallbackExistingField(Enum):
+    a = 'a'
+    b = 'b'
+    unknown = 'unknown'
+
+    @classmethod
+    def _missing_(cls, value):
+        return EnumTypeWithFallbackExistingField.unknown
+
+
+class EnumTypeWithFallbackAndDifferentDefault(Enum):
+    a = 'a'
+    b = 'b'
+    unknown = 'unknown'
+
+    @classmethod
+    def _missing_(cls, value):
+        return EnumTypeWithFallbackAndDifferentDefault.unknown
+
+
+class EnumTypeWithFallbackAndSameDefault(Enum):
+    a = 'a'
+    b = 'b'
+    unknown = 'unknown'
+
+    @classmethod
+    def _missing_(cls, value):
+        return EnumTypeWithFallbackAndSameDefault.unknown
+
+
+class TopLevelModel(BaseModel):
+    enum_field: EnumTypeWithFallback
+    enum_field_with_default: Optional[EnumTypeWithFallbackAndDifferentDefault] = 'a'
+    enum_field_with_default_fallback: Optional[
+        EnumTypeWithFallbackAndSameDefault
+    ] = 'unknown'

tests/data/openapi/x_enum_fallback_value.yaml

@@ -0,0 +1,50 @@
+openapi: 3.0
+components:
+  schemas:
+    enum_type_with_fallback:
+      type: string
+      description: Enum field with fallback value 'unknown'.
+      enum:
+        - 'a'
+        - 'b'
+      x-enum-fallback-value: 'unknown'
+
+    enum_type_with_fallback_existing_field:
+      type: string
+      description: Enum field with fallback value 'unknown'.
+      enum:
+        - 'a'
+        - 'b'
+        - 'unknown'
+      x-enum-fallback-value: 'unknown'
+
+    enum_type_with_fallback_and_different_default:
+      type: string
+      description: Enum field with default value 'a' and fallback value 'unknown'.
+      enum:
+        - 'a'
+        - 'b'
+        - 'unknown'
+      default: 'a'
+      x-enum-fallback-value: 'unknown'
+
+    enum_type_with_fallback_and_same_default:
+      type: string
+      description: Enum field with default value and fallback value 'unknown'.
+      enum:
+        - 'a'
+        - 'b'
+        - 'unknown'
+      default: 'unknown'
+      x-enum-fallback-value: 'unknown'
+
+    TopLevelModel:
+      required:
+        - enum_field
+      properties:
+        enum_field:
+          $ref: "#/components/schemas/enum_type_with_fallback"
+        enum_field_with_default:
+          $ref: "#/components/schemas/enum_type_with_fallback_and_different_default"
+        enum_field_with_default_fallback:
+          $ref: "#/components/schemas/enum_type_with_fallback_and_same_default"

tests/parser/test_openapi.py

@@ -379,6 +379,14 @@ class UnknownTypeNumber(Enum):
     )
 
 
+def test_openapi_parser_parse_x_enum_fallback_value():
+    parser = OpenAPIParser(
+        Path(DATA_PATH / 'x_enum_fallback_value.yaml'),
+    )
+    expected_dir = EXPECTED_OPEN_API_PATH / 'openapi_parser_x_enum_fallback_value'
+    assert parser.parse() == (expected_dir / 'output.py').read_text()
+
+
 @pytest.mark.skipif(
     pydantic.VERSION < '1.9.0', reason='Require Pydantic version 1.9.0 or later '
 )