Fix BackendV2 support for PassManagerConfig.from_backend() (Qiskit#8109)

This commit fixes the PassManagerConfig.from_backend() constructor
method when running with a BackendV2 object. Previously this was not
correctly handled and it always assumed the input backend was a
BackendV1 object. This would cause an error if you ran the constructor
with a BackendV2.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/transpiler/passmanager_config.py

@@ -106,25 +106,38 @@ def from_backend(cls, backend, **pass_manager_options):
             AttributeError: If the backend does not support a `configuration()` method.
         """
         res = cls(**pass_manager_options)
-        config = backend.configuration()
+        backend_version = getattr(backend, "version", 0)
+        if not isinstance(backend_version, int):
+            backend_version = 0
+        if backend_version < 2:
+            config = backend.configuration()
 
         if res.basis_gates is None:
-            res.basis_gates = getattr(config, "basis_gates", None)
-        if res.inst_map is None and hasattr(backend, "defaults"):
-            res.inst_map = backend.defaults().instruction_schedule_map
+            if backend_version < 2:
+                res.basis_gates = getattr(config, "basis_gates", None)
+            else:
+                res.basis_gates = backend.operation_names
+        if res.inst_map is None:
+            if backend_version < 2:
+                if hasattr(backend, "defaults"):
+                    res.inst_map = backend.defaults().instruction_schedule_map
+            else:
+                res.inst_map = backend.instruction_schedule_map
         if res.coupling_map is None:
-            res.coupling_map = CouplingMap(getattr(config, "coupling_map", None))
+            if backend_version < 2:
+                res.coupling_map = CouplingMap(getattr(config, "coupling_map", None))
+            else:
+                res.coupling_map = backend.coupling_map
         if res.instruction_durations is None:
-            res.instruction_durations = InstructionDurations.from_backend(backend)
-        if res.backend_properties is None:
+            if backend_version < 2:
+                res.instruction_durations = InstructionDurations.from_backend(backend)
+            else:
+                res.instruction_durations = backend.instruction_durations
+        if res.backend_properties is None and backend_version < 2:
             res.backend_properties = backend.properties()
         if res.target is None:
-            backend_version = getattr(backend, "version", 0)
-            if not isinstance(backend_version, int):
-                backend_version = 0
             if backend_version >= 2:
                 res.target = backend.target
-
         return res
 
     def __str__(self):

releasenotes/notes/fix-pm-config-backend_v2-ac8b83267105a47d.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed support in the :meth:`.PassManagerConfig.from_backend` constructor
+    method for building a :class:`~.PassManagerConfig` object from a
+    :class:`~.BackendV2` instance. Previously this wasn't handled correctly
+    and would fail when running with a :class:`~.BackendV2` object.

test/python/transpiler/test_passmanager_config.py

@@ -16,7 +16,7 @@
 from qiskit.providers.backend import Backend
 from qiskit.test import QiskitTestCase
 from qiskit.test.mock import FakeMelbourne
-from qiskit.test.mock.backends.almaden.fake_almaden import FakeAlmaden
+from qiskit.test.mock.backends.almaden.fake_almaden import FakeAlmaden, FakeAlmadenV2
 from qiskit.test.mock.backends import FakeArmonk
 from qiskit.transpiler.coupling import CouplingMap
 from qiskit.transpiler.passmanager_config import PassManagerConfig
@@ -39,6 +39,14 @@ def test_config_from_backend(self):
             str(config.coupling_map), str(CouplingMap(backend.configuration().coupling_map))
         )
 
+    def test_config_from_backend_v2(self):
+        """Test from_backend() with a BackendV2 instance."""
+        backend = FakeAlmadenV2()
+        config = PassManagerConfig.from_backend(backend)
+        self.assertEqual(config.basis_gates, backend.operation_names)
+        self.assertEqual(config.inst_map, backend.instruction_schedule_map)
+        self.assertEqual(config.coupling_map.get_edges(), backend.coupling_map.get_edges())
+
     def test_invalid_backend(self):
         """Test from_backend() with an invalid backend."""
         with self.assertRaises(AttributeError):