Feature: able to parse Optional[Type] field to None from command-line

simple_parsing/wrappers/field_parsing.py

@@ -200,7 +200,12 @@ def parse_optional(t: type[T]) -> Callable[[Optional[Any]], Optional[T]]:
     parse = get_parsing_fn(t)
 
     def _parse_optional(val: Optional[Any]) -> Optional[T]:
-        return val if val is None else parse(val)
+        if val is None:
+            return val
+        elif isinstance(val, str) and val == "None":
+            return None
+        else:
+            return parse(val)
 
     return _parse_optional
 

test/test_optional.py

@@ -31,6 +31,26 @@ def test_optional_seed():
     assert config == Config(123)
 
 
+@dataclass
+class Config2:
+    var_with_default: Optional[int] = 42
+
+
+def test_optional_with_default_value():
+    """Test that a value marked as Optional with a default value can be overridden to None.
+    """
+    parser = ArgumentParser()
+    parser.add_arguments(Config2, dest="config")
+
+    args = parser.parse_args("".split())
+    config: Config2 = args.config
+    assert config == Config2(var_with_default=42)
+
+    args = parser.parse_args("--var_with_default".split())
+    config: Config2 = args.config
+    assert config == Config2(var_with_default=None)
+
+
 @dataclass
 class Child:
     name: str = "Kevin"