Don't pass prefix_chars to add_argument_group unless given (Python 3.14 DeprecationWarning)

simple_parsing/parsing.py

@@ -370,12 +370,17 @@ def add_argument_group(
         argument_default=None,
         conflict_handler=None,
     ) -> argparse._ArgumentGroup:
+        kwargs = {}
+        if prefix_chars:
+            # Passing `prefix_chars` is deprecated since Python 3.14. When it is omitted, the group
+            # uses the parser's `prefix_chars` anyway.
+            kwargs["prefix_chars"] = prefix_chars
         return super().add_argument_group(
             title=title,
             description=description,
-            prefix_chars=prefix_chars or self.prefix_chars,
             argument_default=argument_default or self.argument_default,
             conflict_handler=conflict_handler or self.conflict_handler,
+            **kwargs,
         )
 
     def print_help(self, file=None, args: Sequence[str] | None = None):

test/test_issue_368.py

@@ -0,0 +1,36 @@
+"""Test for https://github.com/lebrice/SimpleParsing/issues/368."""
+
+from __future__ import annotations
+
+import warnings
+from dataclasses import dataclass
+
+import simple_parsing
+
+
+@dataclass
+class Config:
+    x: int = 1
+
+
+def test_add_argument_group_does_not_warn():
+    """Python 3.14 deprecates passing `prefix_chars` to `add_argument_group`."""
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", DeprecationWarning)
+        parser = simple_parsing.ArgumentParser()
+        parser.add_arguments(Config, dest="config")
+        args = parser.parse_args(["--x", "3"])
+    assert args.config == Config(x=3)
+
+
+def test_argument_group_inherits_parser_settings():
+    parser = simple_parsing.ArgumentParser(
+        prefix_chars="+", argument_default=123, conflict_handler="resolve"
+    )
+    group = parser.add_argument_group("group")
+    assert group.prefix_chars == "+"
+    assert group.argument_default == 123
+    assert group.conflict_handler == "resolve"
+
+    group.add_argument("+y", type=int)
+    assert parser.parse_args(["+y", "4"]).y == 4