The CLASS_PATH_OR_NAME for subclass help is now optional and if not g…

…iven the help of the base class is printed (#628)

CHANGELOG.rst

@@ -20,6 +20,9 @@ Changed
 - Argument groups created from dataclass-like that have zero configurable
   arguments no longer adds a config loader (`#634
   <https://github.com/omni-us/jsonargparse/pull/634>`__).
+- The ``CLASS_PATH_OR_NAME`` for subclass help is now optional and if not given
+  the help of the base class is printed (`#628
+  <https://github.com/omni-us/jsonargparse/pull/628>`__).
 
 Deprecated
 ^^^^^^^^^^
@@ -29,6 +32,7 @@ Deprecated
 
 
 v4.34.1 (2024-12-02)
+--------------------
 
 Fixed
 ^^^^^

jsonargparse/_actions.py

@@ -20,6 +20,7 @@
     change_to_path_dir,
     default_config_option_help,
     get_import_path,
+    get_typehint_origin,
     import_object,
     indent_text,
     iter_to_set_str,
@@ -346,8 +347,12 @@ def __init__(self, typehint=None, **kwargs):
             super().__init__(**kwargs)
 
     def update_init_kwargs(self, kwargs):
-        from ._typehints import get_subclass_names
+        from ._typehints import get_optional_arg, get_subclass_names, get_unaliased_type
 
+        typehint = get_unaliased_type(get_optional_arg(self._typehint))
+        if get_typehint_origin(typehint) is not Union:
+            assert "nargs" not in kwargs
+            kwargs["nargs"] = "?"
         self._basename = iter_to_set_str(get_subclass_names(self._typehint, callable_return=True))
         kwargs.update(
             {
@@ -376,7 +381,10 @@ def print_help(self, call_args):
         try:
             typehint = get_unaliased_type(get_optional_arg(self._typehint))
             baseclasses = get_subclass_types(typehint, callable_return=True)
-            val_class = import_object(resolve_class_path_by_name(typehint, value))
+            if self.nargs == "?" and value is None:
+                val_class = typehint
+            else:
+                val_class = import_object(resolve_class_path_by_name(typehint, value))
         except Exception as ex:
             raise TypeError(f"{option_string}: {ex}") from ex
         if not any(is_subclass(val_class, b) for b in baseclasses):

jsonargparse/_link_arguments.py

@@ -204,7 +204,7 @@ def __init__(
         help_str: Optional[str]
         if is_target_subclass and not valid_target_leaf:
             type_attr = None
-            help_str = f"Use --{self.target[1].dest}.help CLASS_PATH for details."
+            help_str = f"Use --{self.target[1].dest}.help for details."
         else:
             type_attr = getattr(self.target[1], "_typehint", self.target[1].type)
             help_str = self.target[1].help

jsonargparse_tests/test_subclasses.py

@@ -548,6 +548,9 @@ def test_subclass_nested_help(parser):
     help_str = get_parse_args_stdout(parser, [f"--op.help={__name__}.Nested", "--op.cal.help=TextCalendar"])
     assert "Help for --op.cal.help=calendar.TextCalendar" in help_str
     assert "--op.cal.firstweekday" in help_str
+    help_str = get_parse_args_stdout(parser, [f"--op.help={__name__}.Nested", "--op.cal.help"])
+    assert "Help for --op.cal.help=calendar.Calendar" in help_str
+    assert "--op.cal.firstweekday" in help_str
 
     with pytest.raises(ArgumentError) as ctx:
         parser.parse_args([f"--op.help={__name__}.Nested", "--op.p1=1"])
@@ -1346,7 +1349,7 @@ def test_add_subclass_required_group(parser):
     parser.add_subclass_arguments(Calendar, "cal", required=True)
     pytest.raises(ArgumentError, lambda: parser.parse_args([]))
     help_str = get_parser_help(parser)
-    assert "[-h] [--cal.help CLASS_PATH_OR_NAME] --cal " in help_str
+    assert "[-h] [--cal.help [CLASS_PATH_OR_NAME]] --cal " in help_str
 
 
 def test_add_subclass_not_required_group(parser):