Fix grover operator work wires (#4668)

The default to `GroverOperator` for work wires was `None`. Unfortunately this was being interpreted as `work_wires = Wires([None,])`. This PR makes it so that `work_wires=None` is interpreted as `work_wires = Wires([])`. Fixes PennyLaneAI/pennylane-qiskit#339 --------- Co-authored-by: Matthew Silverman <[email protected]>
PennyLaneAI · Oct 13, 2023 · eafabe4 · eafabe4
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diff --git a/doc/releases/changelog-dev.md b/doc/releases/changelog-dev.md
@@ -340,6 +340,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* Providing `work_wires=None` to `qml.GroverOperator` no longer interprets `None` as a wire.
+  [(#4668)](https://github.com/PennyLaneAI/pennylane/pull/4668)
+
 * Fixed issue where `__copy__` method of the `qml.Select()` operator attempted to access un-initialized data.
 [(#4551)](https://github.com/PennyLaneAI/pennylane/pull/4551)
 

diff --git a/pennylane/templates/subroutines/grover.py b/pennylane/templates/subroutines/grover.py
@@ -113,7 +113,10 @@ def __init__(self, wires=None, work_wires=None, id=None):
         if (not hasattr(wires, "__len__")) or (len(wires) < 2):
             raise ValueError("GroverOperator must have at least two wires provided.")
 
-        self._hyperparameters = {"n_wires": len(wires), "work_wires": Wires(work_wires)}
+        self._hyperparameters = {
+            "n_wires": len(wires),
+            "work_wires": Wires(work_wires) if work_wires is not None else Wires([]),
+        }
 
         super().__init__(wires=wires, id=id)
 

diff --git a/tests/templates/test_subroutines/test_grover.py b/tests/templates/test_subroutines/test_grover.py
@@ -62,6 +62,12 @@ def test_work_wires():
     assert ops[2].hyperparameters["work_wires"] == work_wire
 
 
+def test_work_wires_None():
+    """Test that work wires of None are not inpreted as work wires."""
+    op = qml.GroverOperator(wires=(0, 1, 2, 3), work_wires=None)
+    assert op.hyperparameters["work_wires"] == qml.wires.Wires([])
+
+
 @pytest.mark.parametrize("bad_wires", [0, (0,), tuple()])
 def test_single_wire_error(bad_wires):
     """Assert error raised when called with only a single wire"""