Fix/type action restriction

CHANGELOG.rst

@@ -28,7 +28,8 @@ Fixed
 - Regression parsing strings with omegaconf as the parser mode (`#686
   <https://github.com/omni-us/jsonargparse/pull/686>`__).
 - Help incorrectly showing environment variable name for ``--print_shtab``.
-
+- ``add_argument`` raises error when type is assigned with ``action=None``
+  (`#687 <https://github.com/omni-us/jsonargparse/issues/687>`__).
 
 v4.37.0 (2025-02-14)
 --------------------

jsonargparse/_core.py

@@ -122,7 +122,7 @@ def add_argument(self, *args, enable_path: bool = False, **kwargs):
             enable_path: Whether to try parsing path/subconfig when argument is a complex type.
         """
         parser = self.parser if hasattr(self, "parser") else self
-        if "action" in kwargs:
+        if kwargs.get("action") is not None:
             if ActionParser._is_valid_action_parser(parser, kwargs["action"]):
                 return ActionParser._move_parser_actions(parser, args, kwargs)
             ActionConfigFile._ensure_single_config_argument(self, kwargs["action"])

jsonargparse/_typehints.py

@@ -280,7 +280,7 @@ def normalize_default(self, default):
 
     @staticmethod
     def prepare_add_argument(args, kwargs, enable_path, container, logger, sub_add_kwargs=None):
-        if "action" in kwargs:
+        if kwargs.get("action") is not None:
             raise ValueError("Providing both type and action not allowed.")
         typehint = kwargs.pop("type")
         if args[0].startswith("--") and ActionTypeHint.supports_append(typehint):

jsonargparse_tests/test_typehints.py

@@ -68,6 +68,11 @@ def test_add_argument_failure_given_type_and_action(parser):
     ctx.match("Providing both type and action not allowed")
 
 
+def test_add_argument_given_type_and_null_action(parser):
+    parser.add_argument("--op1", type=Optional[bool], action=None)
+    assert parser.get_defaults().op1 is None
+
+
 @pytest.mark.parametrize("typehint", [Namespace, Optional[Namespace], Union[int, Namespace], List[Namespace]])
 def test_namespace_unsupported_as_type(parser, typehint):
     with pytest.raises(ValueError, match="Namespace .* not supported as a type"):