Improve parameter kind handling for argument requirement determination

CHANGELOG.rst

@@ -20,6 +20,12 @@ Added
 - Support for Python 3.14 (`#753
   <https://github.com/omni-us/jsonargparse/pull/753>`__).
 
+Fixed
+^^^^^
+- Improved parameter kind handling for argument requirement determination.
+  ``KEYWORD_ONLY`` parameters now correctly use ``--flag`` style (`#756
+  <https://github.com/omni-us/jsonargparse/pull/756>`__).
+
 Changed
 ^^^^^^^
 - Removed support for python 3.8 (`#752

jsonargparse/_signatures.py

@@ -344,7 +344,22 @@ def _add_signature_parameter(
                     default = None
                 elif get_typehint_origin(annotation) in not_required_types:
                     default = SUPPRESS
-        is_required = default == inspect_empty
+        # Determine argument characteristics based on parameter kind and default value
+        if kind == kinds.POSITIONAL_ONLY:
+            is_required = True  # Always required
+            is_non_positional = False  # Can be positional
+        elif kind == kinds.POSITIONAL_OR_KEYWORD:
+            is_required = default == inspect_empty  # Required if no default
+            is_non_positional = False  # Can be positional
+        elif kind == kinds.KEYWORD_ONLY:
+            is_required = default == inspect_empty  # Required if no default
+            is_non_positional = True  # Must use --flag style
+        elif kind is None:
+            # programmatically created parameters without kind
+            is_required = default == inspect_empty  # Required if no default
+            is_non_positional = False  # Can be positional (preserve old behavior)
+        else:
+            raise RuntimeError(f"The code should never reach here: kind={kind}")  # pragma: no cover
         src = get_parameter_origins(param.component, param.parent)
         skip_message = f'Skipping parameter "{name}" from "{src}" because of: '
         if not fail_untyped and annotation == inspect_empty:
@@ -356,7 +371,7 @@ def _add_signature_parameter(
             default = None
             is_required = False
             is_required_link_target = True
-        if kind in {kinds.VAR_POSITIONAL, kinds.VAR_KEYWORD} or (not is_required and name[0] == "_"):
+        if not is_required and name[0] == "_":
             return
         elif skip and name in skip:
             self.logger.debug(skip_message + "Parameter requested to be skipped.")
@@ -371,12 +386,12 @@ def _add_signature_parameter(
             kwargs["default"] = default
             if default is None and not is_optional(annotation, object) and not is_required_link_target:
                 annotation = Optional[annotation]
-        elif not as_positional:
+        elif not as_positional or is_non_positional:
             kwargs["required"] = True
         is_subclass_typehint = False
         is_dataclass_like_typehint = is_dataclass_like(annotation)
         dest = (nested_key + "." if nested_key else "") + name
-        args = [dest if is_required and as_positional else "--" + dest]
+        args = [dest if is_required and as_positional and not is_non_positional else "--" + dest]
         if param.origin:
             parser = container
             if not isinstance(container, ArgumentParser):

jsonargparse_tests/test_signatures.py

@@ -697,3 +697,19 @@ def test_add_function_param_conflict(parser):
     with pytest.raises(ValueError) as ctx:
         parser.add_function_arguments(func_param_conflict)
     ctx.match("Unable to add parameter 'cfg' from")
+
+
+def func_positional_and_keyword_only(a: int, /, b: int, *, c: int, d: int = 1):
+    pass
+
+
+def test_add_function_positional_and_keyword_only_parameters(parser):
+    parser.add_function_arguments(func_positional_and_keyword_only, as_positional=True)
+
+    # Test that we can parse with both parameters
+    cfg = parser.parse_args(["1", "2", "--c=3", "--d=4"])
+    assert cfg == Namespace(a=1, b=2, c=3, d=4)
+    with pytest.raises(ArgumentError, match="Unrecognized arguments: --b=2"):
+        parser.parse_args(["1", "--b=2", "--c=3", "--d=4"])
+    with pytest.raises(ArgumentError, match='Key "c" is required'):
+        parser.parse_args(["1", "2", "--d=4"])