Add warning when entity used in template doesn't exist (#57316)

homeassistant/helpers/template.py

@@ -831,7 +831,12 @@ def _get_state_if_valid(hass: HomeAssistant, entity_id: str) -> TemplateState |
 
 
 def _get_state(hass: HomeAssistant, entity_id: str) -> TemplateState | None:
-    return _get_template_state_from_state(hass, entity_id, hass.states.get(entity_id))
+    state_obj = _get_template_state_from_state(
+        hass, entity_id, hass.states.get(entity_id)
+    )
+    if state_obj is None:
+        _LOGGER.warning("Template warning: entity '%s' doesn't exist", entity_id)
+    return state_obj
 
 
 def _get_template_state_from_state(

tests/helpers/test_template.py

@@ -3206,3 +3206,21 @@ async def test_undefined_variable(hass, caplog):
         "Template variable warning: 'no_such_variable' is undefined when rendering '{{ no_such_variable }}'"
         in caplog.text
     )
+
+
+async def test_missing_entity(hass, caplog):
+    """Test a warning is logged on missing entity."""
+    hass.states.async_set("binary_sensor.active", "on")
+    valid_template = template.Template(
+        "{{ is_state('binary_sensor.active', 'on') }}", hass
+    )
+    invalid_template = template.Template(
+        "{{ is_state('binary_sensor.abcde', 'on') }}", hass
+    )
+    assert valid_template.async_render() is True
+    assert invalid_template.async_render() is False
+    assert (
+        "Template warning: entity 'binary_sensor.active' doesn't exist"
+        not in caplog.text
+    )
+    assert "Template warning: entity 'binary_sensor.abcde' doesn't exist" in caplog.text