bugfix: substraction works with new opmath (#4441)

* bugfix: substraction works with new opmath

* changelog

* add test when not implemented

* use s_prod in Operator.__sub__

doc/releases/changelog-dev.md

@@ -311,6 +311,9 @@ array([False, False])
   and `var` measurements.
   [(#4426)](https://github.com/PennyLaneAI/pennylane/pull/4426)
 
+* Subtracting a `Prod` from another operator now works as expected.
+  [(#4441)](https://github.com/PennyLaneAI/pennylane/pull/4441)
+
 <h3>Contributors ✍️</h3>
 
 This release contains contributions from (in alphabetical order):

pennylane/operation.py

@@ -1511,7 +1511,9 @@ def __matmul__(self, other):
 
     def __sub__(self, other):
         """The subtraction operation of Operator-Operator objects and Operator-scalar."""
-        if isinstance(other, (Operator, TensorLike)):
+        if isinstance(other, Operator):
+            return self + qml.s_prod(-1, other)
+        if isinstance(other, TensorLike):
             return self + (qml.math.multiply(-1, other))
         return NotImplemented
 

tests/test_operation.py

@@ -1147,6 +1147,14 @@ def test_sub_rsub_and_neg_dunder_methods(self):
         neg_op = -qml.PauliX(0)
         assert np.allclose(a=neg_op.matrix(), b=np.array([[0, -1], [-1, 0]]), rtol=0)
 
+    def test_sub_obs_from_op(self):
+        """Test that __sub__ returns an SProd to be consistent with other Operator dunders."""
+        op = qml.S(0) - qml.PauliX(1)
+        assert isinstance(op, Sum)
+        assert isinstance(op[1], SProd)
+        assert qml.equal(op[0], qml.S(0))
+        assert qml.equal(op[1], SProd(-1, qml.PauliX(1)))
+
     def test_mul_with_scalar(self):
         """Test the __mul__ dunder method with a scalar value."""
         sprod_op = 4 * qml.RX(1, 0)
@@ -2489,6 +2497,7 @@ def test_composed(self):
     (qml.S(0), qml.PauliX(0)),
     (qml.PauliX(0), qml.S(0)),
     (qml.PauliX(0), qml.PauliY(0)),
+    (qml.PauliZ(0), qml.prod(qml.PauliX(0), qml.PauliY(1))),
 ]
 
 
@@ -2522,6 +2531,11 @@ def test_sub_operators(self, op0, op1):
             assert op[1].scalar == -1
             assert qml.equal(op[1].base, op1)
 
+        def test_sub_with_unknown_not_supported(self):
+            """Test subtracting an unexpected type from an Operator."""
+            with pytest.raises(TypeError, match="unsupported operand type"):
+                _ = qml.S(0) - "foo"
+
         def test_op_with_scalar(self):
             """Tests adding/subtracting ops with scalars."""
             x0 = qml.PauliX(0)