Add warning if no backend provided on cloud channel

qiskit_ibm_runtime/session.py

@@ -12,6 +12,7 @@
 
 """Qiskit Runtime flexible session."""
 
+import warnings
 from typing import Dict, Optional, Type, Union, Callable, Any
 from types import TracebackType
 from functools import wraps
@@ -112,8 +113,15 @@ 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.')
+            warnings.warn(
+                "In a future release no sooner than 3 months after the release date of "
+                "qiskit-ibm-runtime version 0.21.0, "
+                "``backend`` will no longer be optional when using primitives. "
+                "Please provide a backend."
+            )
 
         self._instance = None
 

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

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    In a future release, ``backend`` will be a required argument to :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(UserWarning):
+            Session(service=service)
+
     def test_passing_ibm_backend(self):
         """Test passing in IBMBackend instance."""
         backend = MagicMock(spec=IBMBackend)