Log deprecation warning when template.Template is created without hass (#125142)

* Log deprecation warning when template.Template is created without hass

* Improve docstring

homeassistant/helpers/template.py

@@ -507,10 +507,26 @@ class Template:
     )
 
     def __init__(self, template: str, hass: HomeAssistant | None = None) -> None:
-        """Instantiate a template."""
+        """Instantiate a template.
+
+        Note: A valid hass instance should always be passed in. The hass parameter
+        will be non optional in Home Assistant Core 2025.10.
+        """
+        # pylint: disable-next=import-outside-toplevel
+        from .frame import report
+
         if not isinstance(template, str):
             raise TypeError("Expected template to be a string")
 
+        if not hass:
+            report(
+                (
+                    "creates a template object without passing hass, "
+                    "which will stop working in HA Core 2025.10"
+                ),
+                error_if_core=False,
+            )
+
         self.template: str = template.strip()
         self._compiled_code: CodeType | None = None
         self._compiled: jinja2.Template | None = None

tests/helpers/test_template.py

@@ -6547,3 +6547,20 @@ async def test_merge_response_with_incorrect_response(hass: HomeAssistant) -> No
     tpl = template.Template(_template, hass)
     with pytest.raises(TemplateError, match="TypeError: Response is not a dictionary"):
         tpl.async_render()
+
+
+def test_warn_no_hass(hass: HomeAssistant, caplog: pytest.LogCaptureFixture) -> None:
+    """Test deprecation warning when instantiating Template without hass."""
+
+    message = "Detected code that creates a template object without passing hass"
+    template.Template("blah")
+    assert message in caplog.text
+    caplog.clear()
+
+    template.Template("blah", None)
+    assert message in caplog.text
+    caplog.clear()
+
+    template.Template("blah", hass)
+    assert message not in caplog.text
+    caplog.clear()