Set hass when adding template attribute (#49094)

homeassistant/components/template/template_entity.py

@@ -211,6 +211,8 @@ class TemplateEntity(Entity):
             if the template or validator resulted in an error.
 
         """
+        assert self.hass is not None, "hass cannot be None"
+        template.hass = self.hass
         attribute = _TemplateAttribute(
             self, attribute, template, validator, on_update, none_on_template_error
         )

tests/components/template/test_template_entity.py

@@ -0,0 +1,22 @@
+"""Test template entity."""
+import pytest
+
+from homeassistant.components.template import template_entity
+from homeassistant.helpers import template
+
+
+async def test_template_entity_requires_hass_set():
+    """Test template entity requires hass to be set before accepting templates."""
+    entity = template_entity.TemplateEntity()
+
+    with pytest.raises(AssertionError):
+        entity.add_template_attribute("_hello", template.Template("Hello"))
+
+    entity.hass = object()
+    entity.add_template_attribute("_hello", template.Template("Hello", None))
+
+    tpl_with_hass = template.Template("Hello", entity.hass)
+    entity.add_template_attribute("_hello", tpl_with_hass)
+
+    # Because hass is set in `add_template_attribute`, both templates match `tpl_with_hass`
+    assert len(entity._template_attrs.get(tpl_with_hass, [])) == 2