Fix merge_response template not mutate original object (#127865)

* Fix merge_response template not mutate original object

* Add comment

homeassistant/helpers/template.py

@@ -9,6 +9,7 @@ import collections.abc
 from collections.abc import Callable, Generator, Iterable
 from contextlib import AbstractContextManager
 from contextvars import ContextVar
+from copy import deepcopy
 from datetime import date, datetime, time, timedelta
 from functools import cache, cached_property, lru_cache, partial, wraps
 import json
@@ -2166,7 +2167,8 @@ def merge_response(value: ServiceResponse) -> list[Any]:
 
     is_single_list = False
     response_items: list = []
-    for entity_id, entity_response in value.items():  # pylint: disable=too-many-nested-blocks
+    input_service_response = deepcopy(value)
+    for entity_id, entity_response in input_service_response.items():  # pylint: disable=too-many-nested-blocks
         if not isinstance(entity_response, dict):
             raise TypeError("Response is not a dictionary")
         for value_key, type_response in entity_response.items():

tests/helpers/test_template.py

@@ -6564,3 +6564,21 @@ def test_warn_no_hass(hass: HomeAssistant, caplog: pytest.LogCaptureFixture) ->
     template.Template("blah", hass)
     assert message not in caplog.text
     caplog.clear()
+
+
+async def test_merge_response_not_mutate_original_object(
+    hass: HomeAssistant, snapshot: SnapshotAssertion
+) -> None:
+    """Test the merge_response does not mutate original service response value."""
+
+    value = '{"calendar.family": {"events": [{"summary": "An event"}]}'
+    _template = (
+        "{% set calendar_response = " + value + "} %}"
+        "{{ merge_response(calendar_response) }}"
+        # We should be able to merge the same response again
+        # as the merge is working on a copy of the original object (response)
+        "{{ merge_response(calendar_response) }}"
+    )
+
+    tpl = template.Template(_template, hass)
+    assert tpl.async_render()