Fail when Namespace is used as a type to make it clear about being un…

…supported (#656)

CHANGELOG.rst

@@ -31,6 +31,8 @@ Changed
   parameter of ``ArgumentParser.{dump, save, get_defaults}`` renamed to
   ``skip_validation`` (`#639
   <https://github.com/omni-us/jsonargparse/pull/639>`__).
+- Fail when ``Namespace`` is used as a type to make it clear about being
+  unsupported (`#656 <https://github.com/omni-us/jsonargparse/pull/656>`__).
 
 Fixed
 ^^^^^

jsonargparse/_typehints.py

@@ -299,6 +299,9 @@ def is_supported_typehint(typehint, full=False):
         """Whether the given type hint is supported."""
         typehint = get_unaliased_type(typehint)
 
+        if is_subclass(typehint, Namespace):
+            raise ValueError("jsonargparse.Namespace is only intended for parsing results and not supported as a type.")
+
         supported = (
             typehint in root_types
             or get_typehint_origin(typehint) in root_types

jsonargparse_tests/test_typehints.py

@@ -67,6 +67,12 @@ def test_add_argument_failure_given_type_and_action(parser):
     ctx.match("Providing both type and action not allowed")
 
 
+@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"):
+        parser.add_argument("--ns", type=typehint)
+
+
 # basic types tests