test (Operator): 🧪 Increase coverage.

tests/test_operation.py

@@ -1169,6 +1169,19 @@ def test_multiply_tensor_in_place(self):
         assert isinstance(t, Tensor)
         assert t.obs == [X, Y, Z, H]
 
+    def test_operation_multiply_invalid(self):
+        """Test that an exception is raised if an observable
+        is multiplied by an operation"""
+        X = qml.PauliX(0)
+        Y = qml.CNOT(wires=[0, 1])
+        Z = qml.PauliZ(0)
+
+        with pytest.raises(
+            ValueError, match="Can only perform tensor products between observables"
+        ):
+            T = X @ Z
+            T @ Y
+
     def test_eigvals(self):
         """Test that the correct eigenvalues are returned for the Tensor"""
         X = qml.PauliX(0)