Added option to enable validation of default values

CHANGELOG.rst

@@ -25,6 +25,8 @@ Added
   <https://github.com/omni-us/jsonargparse/pull/701>`__).
 - Resolve parameters completely from stubs when ``inspect.signature`` fails
   (`#698 <https://github.com/omni-us/jsonargparse/pull/698>`__).
+- Option to enable validation of default values (`#711
+  <https://github.com/omni-us/jsonargparse/pull/711>`__).
 
 Changed
 ^^^^^^^

jsonargparse/_common.py

@@ -29,7 +29,7 @@
     reconplogger_support,
     typing_extensions_import,
 )
-from ._type_checking import ArgumentParser
+from ._type_checking import ActionsContainer, ArgumentParser
 
 __all__ = [
     "LoggerProperty",
@@ -55,7 +55,7 @@ def __call__(self, class_type: Type[ClassType], *args, **kwargs) -> ClassType:
 InstantiatorsDictType = Dict[Tuple[type, bool], InstantiatorCallable]
 
 
-parent_parser: ContextVar[Optional["ArgumentParser"]] = ContextVar("parent_parser", default=None)
+parent_parser: ContextVar[Optional[ArgumentParser]] = ContextVar("parent_parser", default=None)
 parser_capture: ContextVar[bool] = ContextVar("parser_capture", default=False)
 defaults_cache: ContextVar[Optional[Namespace]] = ContextVar("defaults_cache", default=None)
 lenient_check: ContextVar[Union[bool, str]] = ContextVar("lenient_check", default=False)
@@ -90,22 +90,34 @@ def parser_context(**kwargs):
 
 
 parsing_settings = dict(
+    validate_defaults=False,
     parse_optionals_as_positionals=False,
 )
 
 
-def set_parsing_settings(*, parse_optionals_as_positionals: Optional[bool] = None) -> None:
+def set_parsing_settings(
+    *,
+    validate_defaults: Optional[bool] = None,
+    parse_optionals_as_positionals: Optional[bool] = None,
+) -> None:
     """
     Modify settings that affect the parsing behavior.
 
     Args:
+        validate_defaults: Whether default values must be valid according to the
+            argument type. The default is False, meaning no default validation,
+            like in argparse.
         parse_optionals_as_positionals: [EXPERIMENTAL] If True, the parser will
             take extra positional command line arguments as values for optional
             arguments. This means that optional arguments can be given by name
             --key=value as usual, but also as positional. The extra positionals
             are applied to optionals in the order that they were added to the
-            parser.
+            parser. By default, this is False.
     """
+    if isinstance(validate_defaults, bool):
+        parsing_settings["validate_defaults"] = validate_defaults
+    elif validate_defaults is not None:
+        raise ValueError(f"validate_defaults must be a boolean, but got {validate_defaults}.")
     if isinstance(parse_optionals_as_positionals, bool):
         parsing_settings["parse_optionals_as_positionals"] = parse_optionals_as_positionals
     elif parse_optionals_as_positionals is not None:
@@ -118,6 +130,18 @@ def get_parsing_setting(name: str):
     return parsing_settings[name]
 
 
+def validate_default(container: ActionsContainer, action: argparse.Action):
+    if not isinstance(action, Action) or not get_parsing_setting("validate_defaults") or action.default is None:
+        return
+    try:
+        with parser_context(parent_parser=container):
+            default = action.default
+            action.default = None
+            action.default = action._check_type_(default)
+    except Exception as ex:
+        raise ValueError(f"Default value is not valid: {ex}") from ex
+
+
 def get_optionals_as_positionals_actions(parser, include_positionals=False):
     from jsonargparse._actions import ActionConfigFile, _ActionConfigLoad, filter_default_actions
     from jsonargparse._completions import ShtabAction

jsonargparse/_core.py

@@ -46,6 +46,7 @@
     lenient_check,
     parser_context,
     supports_optionals_as_positionals,
+    validate_default,
 )
 from ._completions import (
     argcomplete_namespace,
@@ -151,6 +152,7 @@ def add_argument(self, *args, enable_path: bool = False, **kwargs):
             raise ValueError(f'Argument with destination name "{action.dest}" not allowed.')
         if action.option_strings == [] and "default" in kwargs:
             raise ValueError("Positional arguments not allowed to have a default value.")
+        validate_default(self, action)
         if action.help is None:
             action.help = empty_help
         if action.required:

jsonargparse_tests/test_parsing_settings.py

@@ -22,6 +22,28 @@ def test_get_parsing_setting_failure():
         get_parsing_setting("unknown_setting")
 
 
+# validate_defaults
+
+
+def test_set_validate_defaults_failure():
+    with pytest.raises(ValueError, match="validate_defaults must be a boolean"):
+        set_parsing_settings(validate_defaults="invalid")
+
+
+def test_validate_defaults_success(parser):
+    set_parsing_settings(validate_defaults=True)
+
+    parser.add_argument("--num", type=int, default=1)
+    parser.add_argument("--untyped", default=2)
+
+
+def test_validate_defaults_failure(parser):
+    set_parsing_settings(validate_defaults=True)
+
+    with pytest.raises(ValueError, match="Default value is not valid:"):
+        parser.add_argument("--num", type=int, default="x")
+
+
 # parse_optionals_as_positionals