Fix omegaconf parser mode failing on spawned processes

CHANGELOG.rst

@@ -44,6 +44,8 @@ Fixed
   values (`#773 <https://github.com/omni-us/jsonargparse/pull/773>`__).
 - ``save`` with ``multifile=True`` not saving separate subconfigs for items in a
   list (`#779 <https://github.com/omni-us/jsonargparse/pull/779>`__).
+- ``omegaconf`` parser mode failing on spawned processes (`#784
+  <https://github.com/omni-us/jsonargparse/pull/784>`__).
 
 
 v4.41.0 (2025-09-04)

jsonargparse/_core.py

@@ -64,7 +64,6 @@
     get_loader_exceptions,
     load_value,
     loaders,
-    set_omegaconf_loader,
 )
 from ._namespace import (
     Namespace,
@@ -1613,10 +1612,9 @@ def parser_mode(self) -> str:
 
     @parser_mode.setter
     def parser_mode(self, parser_mode: str):
-        if parser_mode in {"omegaconf", "omegaconf+"}:
-            set_omegaconf_loader(parser_mode)
-        if parser_mode not in loaders:
-            raise ValueError(f"The only accepted values for parser_mode are {set(loaders)}.")
+        accepted = set(loaders).union({"omegaconf", "omegaconf+"})
+        if parser_mode not in accepted:
+            raise ValueError(f"The only accepted values for parser_mode are {accepted}.")
         if parser_mode == "jsonnet":
             import_jsonnet("parser_mode=jsonnet")
         self._parser_mode = parser_mode

jsonargparse/_loaders_dumpers.py

@@ -196,10 +196,12 @@ def load_value(value: str, simple_types: bool = False, **kwargs):
     loaded_value = load_basic(value)
 
     mode = get_load_value_mode()
-    if loaded_value is not_loaded and not loader_json_superset[mode]:
+    if loaded_value is not_loaded and not loader_json_superset.get(mode, True):
         loaded_value = load_list_or_dict(value)
 
     if loaded_value is not_loaded:
+        if mode not in loaders and mode in {"omegaconf", "omegaconf+"}:
+            set_omegaconf_loader(mode)
         loader = loaders[mode]
         load_kwargs = {}
         if kwargs and mode in loader_params:

jsonargparse_tests/test_omegaconf.py

@@ -2,6 +2,7 @@
 
 import json
 import math
+import multiprocessing
 import os
 from dataclasses import dataclass
 from pathlib import Path
@@ -224,3 +225,27 @@ def test_omegaconf_absolute_to_relative_paths():
         "h": "${.f[0]}",
     }
     assert omegaconf_absolute_to_relative_paths(data) == expected
+
+
+def parse_in_spawned_process(queue, parser, args):
+    try:
+        cfg = parser.parse_args(args)
+        queue.put(cfg)
+    except Exception as ex:
+        queue.put(ex)
+
+
+@skip_if_omegaconf_unavailable
+def test_omegaconf_in_spawned_process(parser):
+    parser.parser_mode = "omegaconf"
+    parser.add_argument("--dict", type=dict)
+    assert parser.parse_args(['--dict={"x":1}']).dict == {"x": 1}
+
+    ctx = multiprocessing.get_context("spawn")
+    queue = ctx.Queue()
+    process = ctx.Process(target=parse_in_spawned_process, args=(queue, parser, ['--dict={"x":2}']))
+    process.start()
+    process.join()
+
+    cfg = queue.get()
+    assert cfg.dict == {"x": 2}