BugFix: draw_mpl fails with adjointed CNOT

doc/releases/changelog-dev.md

@@ -354,6 +354,9 @@
   [(#5125)](https://github.com/PennyLaneAI/pennylane/pull/5125/) 
   [(#5148)](https://github.com/PennyLaneAI/pennylane/pull/5148)
 
+* `draw_mpl` no longer raises an error when drawing a circuit containing an adjoint of a controlled operation.
+  [(#5149)](https://github.com/PennyLaneAI/pennylane/pull/5149)
+
 <h3>Contributors ✍️</h3>
 
 This release contains contributions from (in alphabetical order):

pennylane/drawer/utils.py

@@ -14,6 +14,7 @@
 """
 This module contains some useful utility functions for circuit drawing.
 """
+import pennylane as qml
 from pennylane.ops import Controlled, Conditional
 from pennylane.measurements import MeasurementProcess, MidMeasureMP, MeasurementValue
 
@@ -95,6 +96,12 @@ def convert_wire_order(tape, wire_order=None, show_all_wires=False):
     return {wire: ind for ind, wire in enumerate(wire_order)}
 
 
+def _is_controlled(op):
+    """Checks whether an operation is a controlled operation."""
+    # TODO: remove the other ones once they all inherit from Controlled [sc-37951]
+    return isinstance(op, (Controlled, qml.CCZ, qml.CSWAP, qml.CH, qml.CNOT, qml.Toffoli))
+
+
 def unwrap_controls(op):
     """Unwraps nested controlled operations for drawing.
 
@@ -121,15 +128,17 @@ def unwrap_controls(op):
         next_ctrl = op
 
         while hasattr(next_ctrl, "base"):
-            base_control_wires = getattr(next_ctrl.base, "control_wires", [])
-            control_wires += base_control_wires
 
-            base_control_values = next_ctrl.base.hyperparameters.get(
-                "control_values", [True] * len(base_control_wires)
-            )
+            if _is_controlled(next_ctrl.base):
+                base_control_wires = getattr(next_ctrl.base, "control_wires", [])
+                control_wires += base_control_wires
+
+                base_control_values = next_ctrl.base.hyperparameters.get(
+                    "control_values", [True] * len(base_control_wires)
+                )
 
-            if control_values is not None:
-                control_values.extend(base_control_values)
+                if control_values is not None:
+                    control_values.extend(base_control_values)
 
             next_ctrl = next_ctrl.base
 

tests/drawer/test_draw_mpl.py

@@ -383,3 +383,18 @@ def circuit():
 
     _ = draw_qnode()
     spy.assert_called_once()
+
+
+def test_draw_mpl_with_control_in_adjoint():
+    def U(wires):
+        qml.adjoint(qml.CNOT)(wires=wires)
+
+    @qml.qnode(dev)
+    def circuit():
+        qml.ctrl(U, control=0)(wires=["a", 1.23])
+        return qml.state()
+
+    _, ax = qml.draw_mpl(circuit)()
+    assert len(ax.lines) == 4  # three wires, one control
+    assert len(ax.texts) == 4  # three wire labels, one gate label
+    assert ax.texts[-1].get_text() == "X†"