Raise error if backend retrieved not in current instance

qiskit_ibm_runtime/qiskit_runtime_service.py

@@ -585,11 +585,11 @@ def backends(
             if name:
                 if name not in self._backends:
                     raise QiskitBackendNotFoundError("No backend matches the criteria.")
-                if not self._backends[name] or instance != self._backends[name]._instance:
+                if not self._backends[name] or instance_filter != self._backends[name]._instance:
                     self._set_backend_config(name)
                     self._backends[name] = self._create_backend_obj(
                         self._backend_configs[name],
-                        instance,
+                        instance_filter,
                     )
                 if self._backends[name]:
                     backends.append(self._backends[name])
@@ -664,9 +664,17 @@ def _create_backend_obj(
                         break
 
             elif config.backend_name not in self._get_hgp(instance=instance).backends:
+                hgps_with_backend = []
+                for hgp in list(self._hgps.values()):
+                    if config.backend_name in hgp.backends:
+                        hgps_with_backend.append(
+                            to_instance_format(hgp._hub, hgp._group, hgp._project)
+                        )
                 raise QiskitBackendNotFoundError(
                     f"Backend {config.backend_name} is not in "
-                    f"{instance}: please try a different hub/group/project."
+                    f"{instance}. Please try a different instance. "
+                    f"{config.backend_name} is in the following instances you have access to: "
+                    f"{hgps_with_backend}"
                 )
 
             return ibm_backend.IBMBackend(

releasenotes/notes/backend-instance-filter-20d69b3951437f19.yaml

@@ -0,0 +1,16 @@
+---
+fixes:
+  - |
+    When a single backend is retrieved with the ``instance`` parameter, 
+
+    .. code-block::
+
+        service.backend('ibm_torino', instance='ibm-q/open/main')
+        # raises error if torino is not in ibm-q/open/main but in a different instance
+        # the user has access to
+        service = QiskitRuntimeService(channel="ibm_quantum", instance="ibm-q/open/main")
+        service.backend('ibm_torino') # raises the same error
+
+    if the backend is not in the instance but in a different one the user has access to, an error 
+    will be raised. The same error will now be raised if an instance is passed in at initialization
+    and then a backend not in that instance is retrieved. 

test/integration/test_backend.py

@@ -45,6 +45,25 @@ def test_backends(self, service):
             f"backend_names={backend_names}",
         )
 
+    @run_integration_test
+    def test_backend_wrong_instance(self, service):
+        """Test getting a backend with wrong instance."""
+        hgps = list(service._hgps.keys())
+        if len(hgps) < 2:
+            raise SkipTest("Skipping test, not enough instances")
+
+        hgp_1 = hgps[0]
+        hgp_2 = hgps[1]
+        hgp_1_backends = service.backends(instance=hgp_1)
+        hgp_2_backends = service.backends(instance=hgp_2)
+        unique_hgp = list(
+            set(hgp_2_backends) - set(hgp_1_backends)
+        )  # get differences between the two lists
+        if unique_hgp:
+            unqiue_backned = unique_hgp[0]
+            with self.assertRaises(QiskitBackendNotFoundError):
+                service.backend(unqiue_backned.name, instance=hgp_1)
+
     @run_integration_test
     @quantum_only
     def test_backends_no_config(self, service):