fix: improve implicit qubit permutations warning

pytket/extensions/qiskit/qiskit_convert.py

@@ -15,6 +15,7 @@
 
 """Methods to allow conversion between Qiskit and pytket circuit classes
 """
+import warnings
 from collections import defaultdict
 from collections.abc import Iterable
 from inspect import signature
@@ -861,8 +862,13 @@ def append_tk_command_to_qiskit(
 supported_gate_rebase = AutoRebase(_protected_tket_gates)
 
 
+def _has_implicit_permutation(circ: Circuit) -> bool:
+    """Returns True if a Circuit has a non-trivial permutation of qubits, false otherwise."""
+    return any(q0 != q1 for q0, q1 in circ.implicit_qubit_permutation().items())
+
+
 def tk_to_qiskit(
-    tkcirc: Circuit, replace_implicit_swaps: bool = False
+    tkcirc: Circuit, replace_implicit_swaps: bool = False, perm_warning: bool = True,
 ) -> QuantumCircuit:
     """
     Converts a pytket :py:class:`Circuit` to a qiskit :py:class:`qiskit.QuantumCircuit`.
@@ -871,19 +877,29 @@ def tk_to_qiskit(
     If no exact replacement can be found for a part of the circuit then an equivalent
     circuit will be returned using the tket gates which are supported in qiskit.
 
-    Please note that implicit swaps in a pytket Circuit are not handled by default.
+    Note that implicit swaps in a pytket Circuit are not handled by default.
     Consider using the replace_implicit_swaps flag to replace these implicit swaps with
     SWAP gates.
 
     :param tkcirc: A :py:class:`Circuit` to be converted
     :param replace_implicit_swaps: Implement implicit permutation by adding SWAPs
         to the end of the circuit.
+    :param perm_warnings: Warn on if input circuit has implicit qubit permutations, True by
+        default.
     :return: The converted circuit
     """
     tkc = tkcirc.copy()  # Make a local copy of tkcirc
     if replace_implicit_swaps:
         tkc.replace_implicit_wire_swaps()
 
+    if _has_implicit_permutation(tkcirc) and perm_warning and not replace_implicit_swaps:
+        warnings.warn(
+            "The pytket Circuit contains implicit qubit permutations"
+            + " which aren't handled by default."
+            + " Consider using the replace_implicit_swaps flag in tk_to_qiskit or"
+            + " replacing them using Circuit.replace_implicit_swaps()."
+        )
+
     qcirc = QuantumCircuit(name=tkc.name)
     qreg_sizes: dict[str, int] = {}
     for qb in tkc.qubits:

tests/qiskit_convert_test.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import os
+import warnings
 from collections import Counter
 from math import pi
 
@@ -62,7 +63,7 @@
     reg_eq,
 )
 from pytket.extensions.qiskit import IBMQBackend, qiskit_to_tk, tk_to_qiskit
-from pytket.extensions.qiskit.backends import qiskit_aer_backend
+from pytket.extensions.qiskit.backends import qiskit_aer_backend, AerStateBackend, AerBackend
 from pytket.extensions.qiskit.qiskit_convert import _gate_str_2_optype
 from pytket.extensions.qiskit.result_convert import qiskit_result_to_backendresult
 from pytket.extensions.qiskit.tket_pass import TketAutoPass, TketPass
@@ -1165,6 +1166,23 @@ def test_symbolic_param_conv() -> None:
         }
     )
 
+def test_implicit_swap_warning() -> None:
+    c = Circuit(2).H(0).SWAP(0, 1)
+    c.replace_SWAPs()
+    with pytest.warns(UserWarning, match="The pytket Circuit contains implicit qubit"):
+        tk_to_qiskit(c)
+
+    state_backend = AerStateBackend()
+    shots_backend = AerBackend()
+    with pytest.warns(UserWarning) as record:
+        state_backend.run_circuit(c)
+        c.measure_all()
+        shots_backend.run_circuit(c)
+        assert len(record) == 0
+
+
+
+
 
 # https://github.com/CQCL/pytket-qiskit/issues/337
 def test_nonregister_bits() -> None: