ActionParser not updating dest of groups for instantiation (#707)

Author: mauvilsa

CHANGELOG.rst

@@ -30,6 +30,11 @@ Changed
   ``fail_untyped=True`` (`#697
   <https://github.com/omni-us/jsonargparse/pull/697>`__).
 
+Fixed
+^^^^^
+- ``ActionParser`` not updating ``dest`` of groups for instantiation (`#707
+  <https://github.com/omni-us/jsonargparse/pull/707>`__).
+
 
 v4.38.0 (2025-03-26)
 --------------------

jsonargparse/_actions.py

@@ -578,6 +578,9 @@ def add_prefix(key):
         base_action_group._actions = filter_default_actions(base_action_group._actions)
         base_action_group._group_actions = filter_default_actions(base_action_group._group_actions)
         extra_action_groups = subparser._action_groups[2:]
+        for group in extra_action_groups:
+            if group.dest is not None:
+                group.dest = dest + "." + group.dest
 
         parser.add_argument(args[0], action=_ActionConfigLoad)
         parser.required_args.update(required_args)

jsonargparse_tests/test_signatures.py

@@ -8,6 +8,7 @@
 import pytest
 
 from jsonargparse import (
+    ActionParser,
     ArgumentError,
     Namespace,
     lazy_instance,
@@ -434,6 +435,36 @@ def test_add_class_unmatched_default_type(parser):
     assert json_or_yaml_load(dump) == {"cls": {"p1": "x", "p2": "deprecated"}}
 
 
+class LeafClass:
+    def __init__(self, p1: int = 1, p2: str = "-"):
+        self.p1 = p1
+        self.p2 = p2
+
+
+def test_add_class_and_action_parser(parser, subparser):
+    subparser.add_class_arguments(LeafClass, "deep.leaf")
+    parser.add_argument("--nested", action=ActionParser(parser=subparser))
+    parser.add_argument("--config", action="config")
+
+    config = {
+        "nested": {
+            "deep": {
+                "leaf": {
+                    "p1": 2,
+                    "p2": "x",
+                }
+            }
+        }
+    }
+    cfg = parser.parse_args([f"--config={json.dumps(config)}"])
+    init = parser.instantiate_classes(cfg)
+    assert list(cfg.keys()) == ["nested.deep.leaf.p1", "nested.deep.leaf.p2", "config"]
+    assert list(init.keys()) == ["nested.deep.leaf", "config"]
+    assert isinstance(init.nested.deep.leaf, LeafClass)
+    assert init.nested.deep.leaf.p1 == 2
+    assert init.nested.deep.leaf.p2 == "x"
+
+
 # add_method_arguments tests