ExplicitEnum subclass str (JSON dump compatible)

src/transformers/hf_argparser.py

@@ -84,7 +84,9 @@ def _parse_dataclass_field(parser: ArgumentParser, field: dataclasses.Field):
 
         origin_type = getattr(field.type, "__origin__", field.type)
         if origin_type is Union:
-            if len(field.type.__args__) != 2 or type(None) not in field.type.__args__:
+            if str not in field.type.__args__ and (
+                len(field.type.__args__) != 2 or type(None) not in field.type.__args__
+            ):
                 raise ValueError(
                     "Only `Union[X, NoneType]` (i.e., `Optional[X]`) is allowed for `Union` because"
                     " the argument parser only supports one type per argument."

src/transformers/utils/generic.py

@@ -240,7 +240,7 @@ def to_tuple(self) -> Tuple[Any]:
         return tuple(self[k] for k in self.keys())
 
 
-class ExplicitEnum(Enum):
+class ExplicitEnum(str, Enum):
     """
     Enum with more explicit error message for missing values.
     """