use named constants instead of string literals when serializing Statu…

…s values for storage (microsoft#921)

# Pull Request

## Title

Use constants instead of string literals when serializing `Status`
values in MLOS storage

---

## Description

Replace all string literals for `Status` values in storage with
corresponding `Status.*.name` constants.

- **Issue link**: Closes: microsoft#920 

---

## Type of Change

- 🔄 Refactor

mlos_bench/mlos_bench/storage/sql/experiment.py

@@ -127,7 +127,7 @@ def _setup(self) -> None:
 
     def merge(self, experiment_ids: list[str]) -> None:
         _LOG.info("Merge: %s <- %s", self._experiment_id, experiment_ids)
-        raise NotImplementedError("TODO")
+        raise NotImplementedError("TODO: Merging experiments not implemented yet.")
 
     def load_tunable_config(self, config_id: int) -> dict[str, Any]:
         with self._engine.connect() as conn:
@@ -169,7 +169,13 @@ def load(
                 .where(
                     self._schema.trial.c.exp_id == self._experiment_id,
                     self._schema.trial.c.trial_id > last_trial_id,
-                    self._schema.trial.c.status.in_(["SUCCEEDED", "FAILED", "TIMED_OUT"]),
+                    self._schema.trial.c.status.in_(
+                        [
+                            Status.SUCCEEDED.name,
+                            Status.FAILED.name,
+                            Status.TIMED_OUT.name,
+                        ]
+                    ),
                 )
                 .order_by(
                     self._schema.trial.c.trial_id.asc(),
@@ -233,9 +239,9 @@ def pending_trials(self, timestamp: datetime, *, running: bool) -> Iterator[Stor
         timestamp = utcify_timestamp(timestamp, origin="local")
         _LOG.info("Retrieve pending trials for: %s @ %s", self._experiment_id, timestamp)
         if running:
-            pending_status = ["PENDING", "READY", "RUNNING"]
+            pending_status = [Status.PENDING.name, Status.READY.name, Status.RUNNING.name]
         else:
-            pending_status = ["PENDING"]
+            pending_status = [Status.PENDING.name]
         with self._engine.connect() as conn:
             cur_trials = conn.execute(
                 self._schema.trial.select().where(
@@ -319,7 +325,7 @@ def _new_trial(
                         trial_id=self._trial_id,
                         config_id=config_id,
                         ts_start=ts_start,
-                        status="PENDING",
+                        status=Status.PENDING.name,
                     )
                 )
 

mlos_bench/mlos_bench/storage/sql/trial.py

@@ -122,7 +122,12 @@ def update(
                             self._schema.trial.c.trial_id == self._trial_id,
                             self._schema.trial.c.ts_end.is_(None),
                             self._schema.trial.c.status.notin_(
-                                ["SUCCEEDED", "CANCELED", "FAILED", "TIMED_OUT"]
+                                [
+                                    Status.SUCCEEDED.name,
+                                    Status.CANCELED.name,
+                                    Status.FAILED.name,
+                                    Status.TIMED_OUT.name,
+                                ]
                             ),
                         )
                         .values(
@@ -160,7 +165,13 @@ def update(
                             self._schema.trial.c.trial_id == self._trial_id,
                             self._schema.trial.c.ts_end.is_(None),
                             self._schema.trial.c.status.notin_(
-                                ["RUNNING", "SUCCEEDED", "CANCELED", "FAILED", "TIMED_OUT"]
+                                [
+                                    Status.RUNNING.name,
+                                    Status.SUCCEEDED.name,
+                                    Status.CANCELED.name,
+                                    Status.FAILED.name,
+                                    Status.TIMED_OUT.name,
+                                ]
                             ),
                         )
                         .values(