Added test for new observable stopping condition

pennylane/devices/default_qutrit_mixed.py

@@ -58,12 +58,14 @@
 
 def observable_stopping_condition(obs: qml.operation.Operator) -> bool:
     """Specifies whether an observable is accepted by DefaultQutritMixed."""
-    if obs.name in {"SProd", "Prod", "Sum"}:
+    if isinstance(obs, qml.operation.Tensor):
+        return all(observable_stopping_condition(observable) for observable in obs.obs)
+    if obs.name in {"Prod", "Sum"}:
         return all(observable_stopping_condition(observable) for observable in obs.operands)
     if obs.name in {"LinearCombination", "Hamiltonian"}:
         return all(observable_stopping_condition(observable) for observable in obs.terms()[1])
-    if obs.name == "Tensor":
-        return all(observable_stopping_condition(observable) for observable in obs.obs)
+    if obs.name == "SProd":
+        return observable_stopping_condition(obs.base)
 
     return obs.name in observables
 

tests/devices/qutrit_mixed/test_qutrit_mixed_preprocessing.py

@@ -18,7 +18,11 @@
 
 import pennylane as qml
 from pennylane.devices import ExecutionConfig
-from pennylane.devices.default_qutrit_mixed import DefaultQutritMixed, stopping_condition
+from pennylane.devices.default_qutrit_mixed import (
+    DefaultQutritMixed,
+    stopping_condition,
+    observable_stopping_condition,
+)
 
 
 class NoMatOp(qml.operation.Operation):
@@ -124,11 +128,28 @@ def test_measurement_is_swapped_out(self, mp_fn, mp_cls, shots):
             (qml.TRX(1.1, 0), True),
         ],
     )
-    def test_accepted_operator(self, op, expected):  # TODO: Add channel ops once added.
-        """Test that _accepted_operator works correctly"""
+    def test_accepted_observables(self, op, expected):
+        """Test that stopping_condition works correctly"""
         res = stopping_condition(op)
         assert res == expected
 
+    @pytest.mark.parametrize(
+        "obs, expected",
+        [
+            (qml.TShift(0), False),
+            (qml.GellMann(0, 1), True),
+            (qml.Snapshot(), False),
+            (qml.operation.Tensor(qml.GellMann(0, 1), qml.GellMann(3, 3)), True),
+            (qml.ops.op_math.SProd(1.2, qml.GellMann(0, 1)), True),
+            (qml.sum(qml.ops.op_math.SProd(1.2, qml.GellMann(0, 1)), qml.GellMann(1, 3)), True),
+            (qml.ops.op_math.Prod(qml.GellMann(0, 1), qml.GellMann(3, 3)), True),
+        ],
+    )
+    def test_accepted_operator(self, obs, expected):
+        """Test that observable_stopping_condition works correctly"""
+        res = observable_stopping_condition(obs)
+        assert res == expected
+
 
 class TestPreprocessingIntegration:
     """Test preprocess produces output that can be executed by the device."""