Dict override syntax to allow standard form: --cfg.dict.key val

CHANGELOG.rst

@@ -25,6 +25,8 @@ Added
 
 Fixed
 ^^^^^
+- Dict override syntax to allow standard form: --cfg.dict.key val (`#824
+  <https://github.com/omni-us/jsonargparse/pull/824>`__).
 - Evaluation of postponed annotations for dataclass inheritance across modules
   not working correctly (`#814
   <https://github.com/omni-us/jsonargparse/pull/814>`__).

jsonargparse/_typehints.py

@@ -919,7 +919,11 @@ def adapt_typehints(
     elif typehint_origin in mapping_origin_types:
         if isinstance(val, NestedArg):
             if isinstance(prev_val, dict):
-                val = {**prev_val, val.key: val.val}
+                if isinstance(val.key, str) and "." in val.key:
+                    key_prefix, key_suffix = val.key.split(".", 1)
+                    val = {**prev_val, key_prefix: {key_suffix: val.val}}
+                else:
+                    val = {**prev_val, val.key: val.val}
             else:
                 val = {val.key: val.val}
         elif isinstance(val, MappingProxyType):

jsonargparse_tests/test_typehints.py

@@ -8,7 +8,7 @@
 import uuid
 from calendar import Calendar, TextCalendar
 from collections import OrderedDict
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from enum import Enum
 from pathlib import Path
 from types import MappingProxyType
@@ -580,6 +580,32 @@
     assert cfg.dict == {"one": 1, "two": 2}
 
 
+@dataclass
+class _Cfg:
+    """Needs to be defined outside of test_nested_dict_command_line_set_items"""
+
+    @dataclass
+    class _Vals:
+        val_0: int = 0
+        val_1: int = 0
+
+    vals: dict[str, _Vals] = field(default_factory=dict)
+
+
+def test_nested_dict_command_line_set_items(parser):
+    parser.add_class_arguments(_Cfg, nested_key="cfg")
+
+    # works before #824
+    args = ["--cfg", '{"vals": {"a": {"val_0": 0, "val_1": 1}}}', "--cfg.vals.a", '{"val_0": 100}']
+    cfg = parser.parse_args(args).cfg
+
+    # does not work before #824
+    args = ["--cfg", '{"vals": {"a": {"val_0": 0, "val_1": 1}}}', "--cfg.vals.a.val_0", "100"]
+    cfg = parser.parse_args(args).cfg
+
+    assert (cfg.vals["a"].val_0, cfg.vals["a"].val_1) == (100, 1)
+
+
 def test_dict_command_line_set_items_with_space(parser):
     parser.add_argument("--dict", type=dict)
     cfg = parser.parse_args(["--dict.a=x y"])