Fix BlendSearch OptunaSearch warning for non-hierarchical spaces with Ray Tune domains (#1477)

* Initial plan

* Fix BlendSearch OptunaSearch warning for non-hierarchical spaces

Co-authored-by: thinkall <3197038+thinkall@users.noreply.github.com>

* Clean up test file

Co-authored-by: thinkall <3197038+thinkall@users.noreply.github.com>

* Add regression test for BlendSearch UDF mode warning fix

Co-authored-by: thinkall <3197038+thinkall@users.noreply.github.com>

* Improve the fix and tests

* Fix Define-by-run function passed in  argument is not yet supported when using

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: thinkall <3197038+thinkall@users.noreply.github.com>
Co-authored-by: Li Jiang <bnujli@gmail.com>
Co-authored-by: Li Jiang <lijiang1@microsoft.com>

Author: Copilot

flaml/tune/searcher/blendsearch.py

@@ -217,7 +217,24 @@ def __init__(
         if global_search_alg is not None:
             self._gs = global_search_alg
         elif getattr(self, "__name__", None) != "CFO":
-            if space and self._ls.hierarchical:
+            # Use define-by-run for OptunaSearch when needed:
+            # - Hierarchical/conditional spaces are best supported via define-by-run.
+            # - Ray Tune domain/grid specs can trigger an "unresolved search space" warning
+            #   unless we switch to define-by-run.
+            use_define_by_run = bool(getattr(self._ls, "hierarchical", False))
+            if (not use_define_by_run) and isinstance(space, dict) and space:
+                try:
+                    from .variant_generator import parse_spec_vars
+
+                    _, domain_vars, grid_vars = parse_spec_vars(space)
+                    use_define_by_run = bool(domain_vars or grid_vars)
+                except Exception:
+                    # Be conservative: if we can't determine whether the space is
+                    # unresolved, fall back to the original behavior.
+                    use_define_by_run = False
+
+            self._use_define_by_run = use_define_by_run
+            if use_define_by_run:
                 from functools import partial
 
                 gs_space = partial(define_by_run_func, space=space)
@@ -487,7 +504,7 @@ def on_trial_complete(self, trial_id: str, result: Optional[Dict] = None, error:
                             self._ls_bound_max,
                             self._subspace.get(trial_id, self._ls.space),
                         )
-                    if self._gs is not None and self._experimental and (not self._ls.hierarchical):
+                    if self._gs is not None and self._experimental and (not getattr(self, "_use_define_by_run", False)):
                         self._gs.add_evaluated_point(flatten_dict(config), objective)
                         # TODO: recover when supported
                         # converted = convert_key(config, self._gs.space)

test/tune/test_searcher.py

@@ -324,3 +324,26 @@ def test_no_optuna():
     import flaml.tune.searcher.suggestion
 
     subprocess.check_call([sys.executable, "-m", "pip", "install", "optuna==2.8.0"])
+
+
+def test_unresolved_search_space(caplog):
+    import logging
+
+    from flaml import tune
+    from flaml.tune.searcher.blendsearch import BlendSearch
+
+    if caplog is not None:
+        caplog.set_level(logging.INFO)
+
+    BlendSearch(metric="loss", mode="min", space={"lr": tune.uniform(0.001, 0.1), "depth": tune.randint(1, 10)})
+    try:
+        text = caplog.text
+    except AttributeError:
+        text = ""
+    assert (
+        "unresolved search space" not in text and text
+    ), "BlendSearch should not produce warning about unresolved search space"
+
+
+if __name__ == "__main__":
+    test_unresolved_search_space(None)