Prevent nesting sections in data entry flows (#124645)

homeassistant/helpers/config_validation.py

@@ -1090,6 +1090,11 @@ def key_dependency[_KT: Hashable, _VT](
 
 
 def custom_serializer(schema: Any) -> Any:
+    """Serialize additional types for voluptuous_serialize."""
+    return _custom_serializer(schema, allow_section=True)
+
+
+def _custom_serializer(schema: Any, *, allow_section: bool) -> Any:
     """Serialize additional types for voluptuous_serialize."""
     from .. import data_entry_flow  # pylint: disable=import-outside-toplevel
     from . import selector  # pylint: disable=import-outside-toplevel
@@ -1104,10 +1109,15 @@ def custom_serializer(schema: Any) -> Any:
         return {"type": "boolean"}
 
     if isinstance(schema, data_entry_flow.section):
+        if not allow_section:
+            raise ValueError("Nesting expandable sections is not supported")
         return {
             "type": "expandable",
             "schema": voluptuous_serialize.convert(
-                schema.schema, custom_serializer=custom_serializer
+                schema.schema,
+                custom_serializer=functools.partial(
+                    _custom_serializer, allow_section=False
+                ),
             ),
             "expanded": not schema.options["collapsed"],
         }

tests/test_data_entry_flow.py

@@ -1098,3 +1098,27 @@ def test_section_in_serializer() -> None:
         ],
         "type": "expandable",
     }
+
+
+def test_nested_section_in_serializer() -> None:
+    """Test section with custom_serializer."""
+    with pytest.raises(
+        ValueError, match="Nesting expandable sections is not supported"
+    ):
+        cv.custom_serializer(
+            data_entry_flow.section(
+                vol.Schema(
+                    {
+                        vol.Required("section_1"): data_entry_flow.section(
+                            vol.Schema(
+                                {
+                                    vol.Optional("option_1", default=False): bool,
+                                    vol.Required("option_2"): int,
+                                }
+                            )
+                        )
+                    }
+                ),
+                {"collapsed": False},
+            )
+        )