do not add type of default if it is already in type

dargs/dargs.py

@@ -224,7 +224,7 @@ def _reorg_dtype(self):
         # check conner cases
         if self.sub_fields or self.sub_variants:
             self.dtype.add(list if self.repeat else dict)
-        if self.optional and self.default is not _Flags.NONE:
+        if self.optional and self.default is not _Flags.NONE and all([not isinstance_annotation(self.default, tt) for tt in self.dtype]):
             self.dtype.add(type(self.default))
         # and make it compatible with `isinstance`
         self.dtype = tuple(self.dtype)
@@ -968,6 +968,19 @@ def trim_by_pattern(
         argdict.pop(key)
 
 
+def isinstance_annotation(value, dtype) -> bool:
+    """Same as isinstance(), but supports arbitrary type annotations."""
+    try:
+        typeguard.check_type(
+            value,
+            dtype,
+            collection_check_strategy=typeguard.CollectionCheckStrategy.ALL_ITEMS,
+        )
+    except typeguard.TypeCheckError as e:
+        return False
+    return True
+
+
 class ArgumentEncoder(json.JSONEncoder):
     """Extended JSON Encoder to encode Argument object:
 

dargs/sphinx.py

@@ -192,5 +192,5 @@ def _test_arguments() -> List[Argument]:
     return [
         Argument(name="test1", dtype=int, doc="Argument 1"),
         Argument(name="test2", dtype=[float, None], doc="Argument 2"),
-        Argument(name="test3", dtype=List[str], doc="Argument 3"),
+        Argument(name="test3", dtype=List[str], default=["test"], doc="Argument 3"),
     ]