Add support for Unset sentinel and deprecate skip_none/skip_null in favor skip_unset

CHANGELOG.rst

@@ -19,6 +19,10 @@ Added
 ^^^^^
 - Support ``Deque`` and ``FrozenSet`` in type hints (`#905
   <https://github.com/omni-us/jsonargparse/pull/905>`__).
+- Add :obj:`.Unset` sentinel and ``unset_sentinel`` setting in
+  ``.set_parsing_settings`` to distinguish between arguments not provided and
+  those explicitly set to ``null`` (`#909
+  <https://github.com/omni-us/jsonargparse/pull/909>`__).
 
 Changed
 ^^^^^^^
@@ -56,6 +60,13 @@ Deprecated
 - ``enable_path`` parameter of :class:`.ActionJsonSchema` was deprecated and
   will be removed in v5.0.0. Use ``sub_config`` instead (`#907
   <https://github.com/omni-us/jsonargparse/pull/907>`__).
+- ``skip_none`` parameter of :meth:`dump <.ArgumentParser.dump>`, :meth:`save
+  <.ArgumentParser.save>`, and :meth:`validate <.ArgumentParser.validate>` was
+  deprecated and will be removed in v5.0.0. Use ``skip_unset`` instead (`#909
+  <https://github.com/omni-us/jsonargparse/pull/909>`__).
+- ``skip_null`` flag for ``--print_config`` was deprecated and will be removed
+  in v5.0.0. Use ``skip_unset`` instead (`#909
+  <https://github.com/omni-us/jsonargparse/pull/909>`__).
 
 
 v4.48.0 (2026-04-10)

DOCUMENTATION.rst

@@ -414,6 +414,71 @@ form like ``--module.child=...``, the parsing will fail and display the
 configured error message.
 
 
+.. _unset-values:
+
+Unset values
+------------
+
+By default, jsonargparse follows argparse behavior: an argument that is not
+provided on the command line is given the value ``None`` in the parsed
+namespace. This makes it impossible to distinguish between an argument that was
+explicitly set to ``None`` (e.g. ``--opt=null``) and one that was simply
+omitted.
+
+The :obj:`.Unset` sentinel (enabled via
+``set_parsing_settings(unset_sentinel=True)``) addresses this by using a
+dedicated sentinel object as the default for arguments that have no explicitly
+provided ``default`` value. The three possible states for an argument then
+become:
+
+- :obj:`.Unset` – the argument was not provided and no ``default`` was given in
+  ``add_argument``.
+- ``None`` – the argument was either explicitly set to ``null``, or its
+  ``add_argument`` call included ``default=None``.
+- Any other value – the argument was provided with that value (or defaults to
+  it).
+
+Example:
+
+.. testcode:: unset-values
+
+    from jsonargparse import ArgumentParser, Unset, set_parsing_settings
+    from typing import Optional
+
+    set_parsing_settings(unset_sentinel=True)
+
+    parser = ArgumentParser()
+    parser.add_argument("--num", type=Optional[int])           # no default given
+    parser.add_argument("--flag", type=Optional[int], default=None)  # explicit None
+
+    cfg = parser.parse_args([])
+    assert cfg.num is Unset   # no default → Unset
+    assert cfg.flag is None   # explicit default=None → None
+
+    cfg = parser.parse_args(["--num=null"])
+    assert cfg.num is None    # explicitly set to null
+
+    cfg = parser.parse_args(["--num=5"])
+    assert cfg.num == 5       # provided value
+
+.. testcleanup:: docstrings
+
+    set_parsing_settings(unset_sentinel=False)
+
+The ``skip_unset`` parameter of :meth:`dump <.ArgumentParser.dump>`, :meth:`save
+<.ArgumentParser.save>`, and :meth:`validate <.ArgumentParser.validate>`
+controls whether :obj:`.Unset` entries are excluded. The
+``--print_config=skip_unset`` flag does the same for command-line use.
+
+**Relation to** ``argument_default=SUPPRESS``
+
+Argparse's ``argument_default=SUPPRESS`` (and per-argument ``default=SUPPRESS``)
+is a complementary mechanism: it causes an unprovided argument to be
+**completely absent** from the parsed namespace, i.e. it has no key at all.
+These two features play well together and represent different levels of
+"absence".
+
+
 .. _type-hints:
 
 Type hints
@@ -1283,7 +1348,8 @@ with a large set of options to create an initial config file including all
 default values. If the `ruamel.yaml <https://pypi.org/project/ruamel.yaml>`__
 package is installed, the config can be printed having the help descriptions
 content as YAML comments by using ``--print_config=comments``. Another option is
-``--print_config=skip_null`` which skips entries whose value is ``null``.
+``--print_config=skip_unset`` which skips entries whose value is the configured
+unset value (see :ref:`unset-values`).
 
 From within Python it is also possible to serialize a config object by using
 either the :meth:`dump <.ArgumentParser.dump>` or :meth:`save

jsonargparse/_actions.py

@@ -170,14 +170,16 @@ def __init__(
             help=(
                 "Print the configuration after applying all other arguments and exit. The optional "
                 "flags customizes the output and are one or more keywords separated by comma. The "
-                "supported flags are:%s skip_default, skip_null."
+                "supported flags are:%s skip_default, skip_unset."
             )
             % (" comments," if ruamel_support else ""),
         )
 
     def __call__(self, parser, namespace, value, option_string=None):
-        kwargs = {"subparser": parser, "key": None, "skip_none": False, "skip_validation": False}
-        valid_flags = {"": None, "skip_default": "skip_default", "skip_null": "skip_none"}
+        from ._deprecated import deprecated_skip_null, deprecated_valid_flags
+
+        kwargs = {"subparser": parser, "key": None, "skip_unset": False, "skip_validation": False}
+        valid_flags = {"": None, "skip_default": "skip_default", "skip_unset": "skip_unset"} | deprecated_valid_flags
         if ruamel_support:
             valid_flags["comments"] = "with_comments"
         if value is not None:
@@ -186,7 +188,11 @@ def __call__(self, parser, namespace, value, option_string=None):
             if len(invalid_flags) > 0:
                 raise argument_error(f'Invalid option "{invalid_flags[0]}" for {option_string}')
             for flag in [f for f in flags if f != ""]:
-                kwargs[valid_flags[flag]] = True
+                mapped = valid_flags[flag]
+                if deprecated_skip_null(flag):
+                    kwargs["skip_unset"] = True
+                else:
+                    kwargs[mapped] = True
         while hasattr(parser, "parent_parser"):
             kwargs["key"] = parser.subcommand if kwargs["key"] is None else parser.subcommand + "." + kwargs["key"]
             parser = parser.parent_parser

jsonargparse/_common.py

@@ -31,6 +31,7 @@
 from ._type_checking import ActionsContainer, ArgumentParser, docstring_parser
 
 __all__ = [
+    "Unset",
     "set_parsing_settings",
 ]
 
@@ -44,6 +45,27 @@
 capture_typing_extension_shadows(_UnpackGenericAlias, "_UnpackGenericAlias", unpack_meta_types)
 
 
+class _UnsetType:
+    """Sentinel class for unset argument values."""
+
+    _instance = None
+    _SERIALIZED = "==UNSET=="
+
+    def __new__(cls):
+        if cls._instance is None:
+            cls._instance = super().__new__(cls)
+        return cls._instance
+
+    def __repr__(self):
+        return "Unset"
+
+    def __bool__(self):
+        return False
+
+
+Unset = _UnsetType()
+
+
 class InstantiatorCallable(Protocol):
     def __call__(self, class_type: type[ClassType], *args, **kwargs) -> ClassType:
         pass  # pragma: no cover
@@ -96,12 +118,13 @@ def parser_context(**kwargs):
             context_var.reset(token)
 
 
-parsing_settings = {
+parsing_settings: dict = {
     "validate_defaults": False,
     "parse_optionals_as_positionals": False,
     "add_print_completion_argument": False,
     "stubs_resolver_allow_py_files": False,
     "omegaconf_absolute_to_relative_paths": False,
+    "unset_sentinel": None,
 }
 
 
@@ -124,6 +147,7 @@ def set_parsing_settings(
     add_print_completion_argument: Optional[bool] = None,
     stubs_resolver_allow_py_files: Optional[bool] = None,
     omegaconf_absolute_to_relative_paths: Optional[bool] = None,
+    unset_sentinel: Optional[bool] = None,
     subclasses_disabled: Optional[list[Union[type, Callable[[type], bool]]]] = None,
     subclasses_enabled: Optional[list[Union[type, str]]] = None,
 ) -> None:
@@ -157,6 +181,12 @@ def set_parsing_settings(
             with ``omegaconf+`` parser mode, absolute interpolation paths are
             converted to relative. This is only intended for backward
             compatibility with ``omegaconf`` parser mode.
+        unset_sentinel: If ``True``, parsers will use the :obj:`.Unset` sentinel
+            for arguments that have not been given a value (instead of
+            ``None``). This allows distinguishing between ``None`` as an
+            explicitly given value and an argument that was not provided at
+            all. If ``False``, uses ``None`` (the default, argparse-compatible
+            behavior) unless overridden by ``argument_default``.
         subclasses_disabled: List of types or functions, so that when parsing
             only the exact type hints (not their subclasses) are accepted.
             Descendants of the configured types are also disabled. Functions
@@ -205,6 +235,11 @@ def set_parsing_settings(
         raise ValueError(
             f"omegaconf_absolute_to_relative_paths must be a boolean, but got {omegaconf_absolute_to_relative_paths}."
         )
+    # unset_sentinel
+    if isinstance(unset_sentinel, bool):
+        parsing_settings["unset_sentinel"] = Unset if unset_sentinel else None
+    elif unset_sentinel is not None:
+        raise ValueError(f"unset_sentinel must be a boolean, but got {unset_sentinel}.")
     # subclass behavior
     if subclasses_disabled or subclasses_enabled:
         subclass_type_behavior(
@@ -224,7 +259,11 @@ def get_parsing_setting(name: str):
 
 
 def validate_default(container: ActionsContainer, action: argparse.Action):
-    if action.default is None or not get_parsing_setting("validate_defaults") or not hasattr(action, "_check_type"):
+    if (
+        action.default is get_parsing_setting("unset_sentinel")
+        or not get_parsing_setting("validate_defaults")
+        or not hasattr(action, "_check_type")
+    ):
         return
     try:
         from ._core import ArgumentGroup