Don't create new session in from_id() (#1896)

* Don't create new session in `from_id()`

* Add release note

* change new_session to private

* add unit test

* Use class variable

---------

Co-authored-by: ptristan3 <44805021+ptristan3@users.noreply.github.com>

qiskit_ibm_runtime/batch.py

@@ -83,6 +83,8 @@ class Batch(Session):
  <https://docs.quantum.ibm.com/guides/run-jobs-batch>`_" page.
  """
 
+ _create_new_session = True
+
  def __init__(
  self,
  service: Optional[QiskitRuntimeService] = None,
@@ -131,7 +133,7 @@ def __init__(
 
  def _create_session(self) -> Optional[str]:
  """Create a session."""
- if isinstance(self._service, QiskitRuntimeService):
+ if isinstance(self._service, QiskitRuntimeService) and Batch._create_new_session:
  session = self._service._api_client.create_session(
  self.backend(), self._instance, self._max_time, self._service.channel, "batch"
  )

qiskit_ibm_runtime/session.py

@@ -82,6 +82,8 @@ class Session:
  print(f"Counts: {pub_result.data.cr.get_counts()}")
  """
 
+ _create_new_session = True
+
  def __init__(
  self,
  service: Optional[QiskitRuntimeService] = None,
@@ -104,7 +106,6 @@ def __init__(
  This value must be less than the
  `system imposed maximum
  <https://docs.quantum.ibm.com/guides/max-execution-time>`_.
-
  Raises:
  ValueError: If an input value is invalid.
  """
@@ -165,7 +166,7 @@ def __init__(
 
  def _create_session(self) -> Optional[str]:
  """Create a session."""
- if isinstance(self._service, QiskitRuntimeService):
+ if isinstance(self._service, QiskitRuntimeService) and Session._create_new_session:
  session = self._service._api_client.create_session(
  self.backend(), self._instance, self._max_time, self._service.channel, "dedicated"
  )
@@ -367,7 +368,7 @@ def from_id(cls, session_id: str, service: QiskitRuntimeService) -> "Session":
  """
 
  response = service._api_client.session_details(session_id)
- backend = response.get("backend_name")
+ backend = service.backend(response.get("backend_name"))
  mode = response.get("mode")
  state = response.get("state")
  class_name = "dedicated" if cls.__name__.lower() == "session" else cls.__name__.lower()
@@ -376,7 +377,9 @@ def from_id(cls, session_id: str, service: QiskitRuntimeService) -> "Session":
  f"Input ID {session_id} has execution mode {mode} instead of {class_name}."
  )
 
+ cls._create_new_session = False
  session = cls(service, backend)
+ cls._create_new_session = True
  if state == "closed":
  session._active = False
  session._session_id = session_id

release-notes/unreleased/1896.bug.rst

@@ -0,0 +1,2 @@
+Fixed an issue where ``Session.from_id()`` would create 
+a new empty session. 

test/unit/test_session.py

@@ -151,6 +151,8 @@ def test_session_from_id(self):
  session_id = "123"
  session = Session.from_id(session_id=session_id, service=service)
  session.run(program_id="foo", inputs={})
+ session._create_session = MagicMock()
+ self.assertTrue(session._create_session.assert_not_called)
  self.assertEqual(session.session_id, session_id)
 
  def test_correct_execution_mode(self):