Fix instruction durations in transpile() with BackendV2 (backport #8001)

qiskit/compiler/transpiler.py

@@ -938,11 +938,15 @@ def _parse_instruction_durations(backend, inst_durations, dt, circuits):
     take precedence over backend durations, but be superceded by ``inst_duration``s.
     """
     if not inst_durations:
-        backend_durations = InstructionDurations()
-        try:
-            backend_durations = InstructionDurations.from_backend(backend)
-        except AttributeError:
-            pass
+        backend_version = getattr(backend, "version", 0)
+        if backend_version <= 1:
+            backend_durations = InstructionDurations()
+            try:
+                backend_durations = InstructionDurations.from_backend(backend)
+            except AttributeError:
+                pass
+        else:
+            backend_durations = backend.instruction_durations
 
     durations = []
     for circ in circuits:

releasenotes/notes/fix-transpile-backendv2-durations-dbc85688564cc271.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :func:`~.transpile` function when run with a
+    :class:`~.BackendV2` based backend and setting the ``scheduling_method``
+    keyword argument. Previously, the function would not correctly process
+    the default durations of the instructions supported by the backend which
+    would lead to an error.

test/python/compiler/test_transpiler.py

@@ -35,7 +35,7 @@
 from qiskit.circuit.library import CXGate, U3Gate, U2Gate, U1Gate, RXGate, RYGate, RZGate, UGate
 from qiskit.circuit.measure import Measure
 from qiskit.test import QiskitTestCase
-from qiskit.test.mock import FakeMelbourne, FakeRueschlikon, FakeAlmaden
+from qiskit.test.mock import FakeMelbourne, FakeRueschlikon, FakeAlmaden, FakeMumbaiV2
 from qiskit.transpiler import Layout, CouplingMap
 from qiskit.transpiler import PassManager
 from qiskit.transpiler.target import Target
@@ -1242,6 +1242,18 @@ def test_delay_converts_to_dt(self):
         out = transpile(qc, dt=1e-9)
         self.assertEqual(out.data[0][0].unit, "dt")
 
+    def test_scheduling_backend_v2(self):
+        """Test that scheduling method works with Backendv2."""
+        qc = QuantumCircuit(2)
+        qc.h(0)
+        qc.cx(0, 1)
+        qc.measure_all()
+
+        backend = FakeMumbaiV2()
+        out = transpile([qc, qc], backend, scheduling_method="alap")
+        self.assertIn("delay", out[0].count_ops())
+        self.assertIn("delay", out[1].count_ops())
+
     @data(1, 2, 3)
     def test_no_infinite_loop(self, optimization_level):
         """Verify circuit cost always descends and optimization does not flip flop indefinitely."""