Use backend service in session if known

qiskit_ibm_runtime/session.py

@@ -73,8 +73,9 @@ def __init__(
         """Session constructor.
 
         Args:
-            service: Optional instance of the ``QiskitRuntimeService`` class,
-                defaults to ``QiskitRuntimeService()`` which tries to initialize
+            service: Optional instance of the ``QiskitRuntimeService`` class.
+                If ``None``, the service associated with the backend, if known, is used.
+                Otherwise ``QiskitRuntimeService()`` is used to initialize
                 your default saved account.
             backend: Optional instance of :class:`qiskit_ibm_runtime.IBMBackend` class or
                 string name of backend. If not specified, a backend will be selected
@@ -90,7 +91,14 @@ def __init__(
             ValueError: If an input value is invalid.
         """
 
-        self._service = service or QiskitRuntimeService()
+        if service is None:
+            self._service = (
+                backend.service
+                if isinstance(backend, IBMBackend)
+                else QiskitRuntimeService()
+            )
+        else:
+            self._service = service
 
         if self._service.channel == "ibm_quantum" and not backend:
             raise ValueError('"backend" is required for ``ibm_quantum`` channel.')

releasenotes/notes/use-backend-service-020daf6e4a4d044e.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    If a :class:`qiskit_ibm_runtime.IBMBackend` instance is passed to
+    the :class:`qiskit_ibm_runtime.Session` constructor, the service used
+    to initialize the ``IBMBackend`` instance is used for the session
+    instead of the default account service.

test/unit/test_session.py

@@ -49,6 +49,13 @@ def test_passing_ibm_backend(self):
         session = Session(service=MagicMock(), backend=backend)
         self.assertEqual(session.backend(), "ibm_gotham")
 
+    def test_using_ibm_backend_service(self):
+        """Test using service from an IBMBackend instance."""
+        backend = MagicMock(spec=IBMBackend)
+        backend.name = "ibm_gotham"
+        session = Session(backend=backend)
+        self.assertEqual(session.service, backend.service)
+
     def test_max_time(self):
         """Test max time."""
         max_times = [