add_argument of a positional and providing default now raises an error (#694)

Author: mauvilsa

CHANGELOG.rst

@@ -25,6 +25,8 @@ Changed
 - ``validate`` now checks values before required so that errors related to wrong
   level in a config are easier to understand (`#681
   <https://github.com/omni-us/jsonargparse/pull/681>`__).
+- ``add_argument`` of a positional and providing ``default`` now raises an error
+  (`#694 <https://github.com/omni-us/jsonargparse/pull/694>`__).
 
 Fixed
 ^^^^^

jsonargparse/_core.py

@@ -147,6 +147,8 @@ def add_argument(self, *args, enable_path: bool = False, **kwargs):
         ActionJsonnet._check_ext_vars_action(parser, action)
         if is_meta_key(action.dest):
             raise ValueError(f'Argument with destination name "{action.dest}" not allowed.')
+        if action.option_strings == [] and "default" in kwargs:
+            raise ValueError("Positional arguments not allowed to have a default value.")
         if action.help is None:
             action.help = empty_help
         if action.required:

jsonargparse_tests/test_core.py

@@ -47,6 +47,11 @@
 )
 
 
+def test_add_argument_positional_with_default(parser):
+    with pytest.raises(ValueError, match="Positional arguments not allowed to have a default value"):
+        parser.add_argument("pos", default="abc")
+
+
 def test_parse_args_simple(parser):
     parser.add_argument("--op", type=int)
     assert parser.parse_args(["--op=1"]) == Namespace(op=1)