Add warning when circ is mixed + open

lambeq/backend/pennylane.py

@@ -42,6 +42,7 @@
 from __future__ import annotations
 
 from itertools import product
+import sys
 from typing import TYPE_CHECKING
 
 import pennylane as qml
@@ -221,6 +222,12 @@ def to_pennylane(lambeq_circuit: Diagram, probabilities=False,
         raise ValueError('Only pure circuits, or circuits with discards'
                          ' are currently supported.')
 
+    if lambeq_circuit.is_mixed and lambeq_circuit.cod:
+        # Some qubits discarded, some left open
+        print('Warning: Circuit includes both discards and open codomain'
+              ' wires. All open wires will be discarded during conversion',
+              file=sys.stderr)
+
     tk_circ = lambeq_circuit.to_tk()
     op_list, params_list, wires_list, symbols_set = (
         extract_ops_from_tk(tk_circ)

tests/backend/test_pennylane_interface.py

@@ -98,6 +98,16 @@ def test_pennylane_circuit_mixed_error():
         snake.to_pennylane()
 
 
+def test_pennylane_circuit_mixed_warning(capsys):
+    bell_state = Diagram.caps(qubit, qubit)
+    bell_discarded = bell_state >> Discard() @ Id(qubit)
+    _ = bell_discarded.to_pennylane()
+    captured = capsys.readouterr()
+    assert captured.err == ('Warning: Circuit includes both discards and open '
+                            'codomain wires. All open wires will be discarded '
+                            'during conversion\n')
+
+
 def test_pennylane_circuit_draw(capsys):
     bell_state = Diagram.caps(qubit, qubit)
     bell_effect = bell_state[::-1]