Add warning if no backend provided on cloud channel

qiskit_ibm_runtime/base_primitive.py

@@ -29,6 +29,7 @@
 from .runtime_job import RuntimeJob
 from .ibm_backend import IBMBackend
 from .utils.default_session import get_cm_session
+from .utils.deprecation import issue_deprecation_msg
 from .utils.utils import validate_isa_circuits
 from .constants import DEFAULT_DECODERS
 from .qiskit_runtime_service import QiskitRuntimeService
@@ -122,6 +123,12 @@ def __init__(
                 raise ValueError(
                     "A backend or session must be specified when not using ibm_cloud channel."
                 )
+            issue_deprecation_msg(
+                "Not providing a backend is deprecated",
+                "0.21.0",
+                "Passing in a backend will be required, please provide a backend.",
+                3,
+            )
         # Check if initialized within a IBMBackend session. If so, issue a warning.
         if get_cm_provider_session():
             warnings.warn(

qiskit_ibm_runtime/session.py

@@ -21,7 +21,7 @@
 from .utils.result_decoder import ResultDecoder
 from .ibm_backend import IBMBackend
 from .utils.default_session import set_cm_session
-from .utils.deprecation import deprecate_arguments
+from .utils.deprecation import deprecate_arguments, issue_deprecation_msg
 from .utils.converters import hms_to_seconds
 
 
@@ -112,8 +112,14 @@ def __init__(
         else:
             self._service = service
 
-        if self._service.channel == "ibm_quantum" and not backend:
-            raise ValueError('"backend" is required for ``ibm_quantum`` channel.')
+        if not backend:
+            if self._service.channel == "ibm_quantum":
+                raise ValueError('"backend" is required for ``ibm_quantum`` channel.')
+            issue_deprecation_msg(
+                "Not providing a backend is deprecated",
+                "0.21.0",
+                "Passing in a backend will be required, please provide a backend.",
+            )
 
         self._instance = None
 

releasenotes/notes/require-cloud-backend-38a74144347c735d.yaml

@@ -0,0 +1,6 @@
+---
+deprecations:
+  - |
+    In a future release, ``backend`` will be a required parameter for :class:`qiskit_ibm_runtime.Sampler`, 
+    :class:`qiskit_ibm_runtime.Estimator`, :class:`qiskit_ibm_runtime.Session` and 
+    :class:`qiskit_ibm_runtime.Batch` when using the ``ibm_cloud`` channel.

test/unit/test_session.py

@@ -44,6 +44,13 @@ def test_missing_backend(self):
         with self.assertRaises(ValueError):
             Session(service=service)
 
+    def test_missing_backend_cloud_warning(self):
+        """Test warning if no backend provided on cloud channel."""
+        service = MagicMock()
+        service.channel = "ibm_cloud"
+        with self.assertWarns(DeprecationWarning):
+            Session(service=service)
+
     def test_passing_ibm_backend(self):
         """Test passing in IBMBackend instance."""
         backend = MagicMock(spec=IBMBackend)