Fix PassManagerConfig.from_backend with BackendV1 and no CouplingMap

qiskit/transpiler/passmanager_config.py

@@ -144,7 +144,9 @@ def from_backend(cls, backend, **pass_manager_options):
                 res.inst_map = backend.instruction_schedule_map
         if res.coupling_map is None:
             if backend_version < 2:
-                res.coupling_map = CouplingMap(getattr(config, "coupling_map", None))
+                cmap_edge_list = getattr(config, "coupling_map", None)
+                if cmap_edge_list is not None:
+                    res.coupling_map = CouplingMap(cmap_edge_list)
             else:
                 res.coupling_map = backend.coupling_map
         if res.instruction_durations is None:

releasenotes/notes/fix-pm-config-from-backend-f3b71b11858b4f08.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :meth:`.PassManagerConfig.from_backend` constructor
+    when building a :class:`~.PassManagerConfig` object from a :class:`~.BackendV1`
+    instance that didn't have a coupling map attribute defined. Previously, the
+    constructor would incorrectly create a :class:`~.CouplingMap` object with
+    0 qubits instead of using ``None``.
+    Fixed `#10171 <https://github.com/Qiskit/qiskit-terra/issues/10171>`__

test/python/transpiler/test_passmanager_config.py

@@ -86,6 +86,7 @@ def test_simulator_backend_v1(self):
         config = PassManagerConfig.from_backend(backend)
         self.assertIsInstance(config, PassManagerConfig)
         self.assertIsNone(config.inst_map)
+        self.assertIsNone(config.coupling_map)
 
     def test_invalid_user_option(self):
         """Test from_backend() with an invalid user option."""
@@ -103,7 +104,7 @@ def test_str(self):
 	initial_layout: None
 	basis_gates: ['id', 'rz', 'sx', 'x']
 	inst_map: None
-	coupling_map: 
+	coupling_map: None
 	layout_method: None
 	routing_method: None
 	translation_method: None