Fix PassManagerConfig.from_backend with BackendV1 and no CouplingMap (#…

…10172)

* Fix PassManagerConfig.from_backend with BackendV1 and no CouplingMap

This commit fixes an issue in the PassManagerConfig.from_backend
constructor method when using BackendV1 based simulator backends. The
pass was incorrectly handling the case when the backend configuration
didn't have a coupling map defined and incorrectly creating a coupling
map with 0 qubits instead of using None to indicate the lack of
connectivity. This has been fixed so the coupling map creation is
skipped if there is no coupling map attribute in the backend's
configuration.

Fixes #10171

* Fix tests

(cherry picked from commit 4762e26)

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