Add as_mutable for Jinja templating

homeassistant/helpers/template.py

@@ -2034,6 +2034,23 @@ def to_json(value, ensure_ascii=True):
     return json.dumps(value, ensure_ascii=ensure_ascii)
 
 
+class _MutableDict(dict):
+    pass
+
+
+class _MutableList(list):
+    pass
+
+
+def as_mutable(value):
+    """Create a mutable copy of a dict or list."""
+    if isinstance(value, dict):
+        return _MutableDict(value)
+    if isinstance(value, list):
+        return _MutableList(value)
+    raise ValueError("Can only create mutable lists or dicts.")
+
+
 @pass_context
 def random_every_time(context, values):
     """Choose a random value.
@@ -2241,6 +2258,7 @@ def __init__(self, hass, limited=False, strict=False):
             str | jinja2.nodes.Template, CodeType | str | None
         ] = weakref.WeakValueDictionary()
         self.add_extension("jinja2.ext.loopcontrols")
+        self.add_extension("jinja2.ext.do")
         self.filters["round"] = forgiving_round
         self.filters["multiply"] = multiply
         self.filters["log"] = logarithm
@@ -2261,6 +2279,7 @@ def __init__(self, hass, limited=False, strict=False):
         self.filters["timestamp_utc"] = timestamp_utc
         self.filters["to_json"] = to_json
         self.filters["from_json"] = from_json
+        self.filters["as_mutable"] = as_mutable
         self.filters["is_defined"] = fail_when_undefined
         self.filters["average"] = average
         self.filters["random"] = random_every_time
@@ -2463,6 +2482,10 @@ def is_safe_callable(self, obj):
 
     def is_safe_attribute(self, obj, attr, value):
         """Test if attribute is safe."""
+
+        if isinstance(obj, (_MutableDict, _MutableList)):
+            return True
+
         if isinstance(
             obj, (AllStates, DomainStates, TemplateState, LoopContext, AsyncLoopContext)
         ):

tests/helpers/test_template.py

@@ -1075,6 +1075,58 @@ def test_from_json(hass: HomeAssistant) -> None:
     assert actual_result == expected_result
 
 
+def test_mutables(hass: HomeAssistant) -> None:
+    """Test as_mutable and mutability of those objects."""
+    assert (
+        template.Template(
+            """
+            {% set d = {} | as_mutable %}
+            {% do d.update({'foo': 'bar'}) %}
+            {{ d.foo }}
+            """,
+            hass,
+        ).async_render()
+        == "bar"
+    )
+
+    assert (
+        template.Template(
+            """
+            {% set l = [] | as_mutable %}
+            {% do l.append('bar') %}
+            {{ l[0] }}
+            """,
+            hass,
+        ).async_render()
+        == "bar"
+    )
+
+    # Ensure that without as_mutable, updates still don't work.
+    with pytest.raises(TemplateError):
+        template.Template(
+            """
+            {% set d = {} %}
+            {% do d.update({'foo': 'bar'}) %}
+            {{ d.foo }}
+            """,
+            hass,
+        ).async_render()
+
+    with pytest.raises(TemplateError):
+        template.Template(
+            """
+            {% set l = [] %}
+            {% do l.append('bar') %}
+            {{ l[0] }}
+            """,
+            hass,
+        ).async_render()
+
+    # Ensure that as_mutable raises an error when non-dicts/lists are passed
+    with pytest.raises(TemplateError):
+        template.Template("{{ 5 | as_mutable }}", hass).async_render()
+
+
 def test_average(hass: HomeAssistant) -> None:
     """Test the average filter."""
     assert template.Template("{{ [1, 2, 3] | average }}", hass).async_render() == 2