python-adaptive
diff --git a/‎adaptive/learner/balancing_learner.py
Copy file name to clipboardExpand all lines: adaptive/learner/balancing_learner.py
+35-23 b/‎adaptive/learner/balancing_learner.py
Copy file name to clipboardExpand all lines: adaptive/learner/balancing_learner.py
+35-23
@@ -3,7 +3,7 @@
 import itertools
 import sys
 from collections import defaultdict
-from collections.abc import Iterable, Sequence
+from collections.abc import Generator, Iterable, Sequence
 from contextlib import suppress
 from functools import partial
 from operator import itemgetter
@@ -126,11 +126,10 @@ def __init__(
         self._cdims_default = cdims
 
         if len({learner.__class__ for learner in self.learners}) > 1:
-            raise TypeError(
-                "A BalacingLearner can handle only one type" " of learners."
-            )
+            raise TypeError("A BalacingLearner can handle only one type of learners.")
 
         self.strategy: STRATEGY_TYPE = strategy
+        self._gen: Generator | None = None
 
     def new(self) -> BalancingLearner:
         """Create a new `BalancingLearner` with the same parameters."""
@@ -288,27 +287,16 @@ def _ask_and_tell_based_on_cycle(
     def _ask_and_tell_based_on_sequential(
         self, n: int
     ) -> tuple[list[tuple[Int, Any]], list[float]]:
+        if self._gen is None:
+            self._gen = _sequential_generator(self.learners)
         points: list[tuple[Int, Any]] = []
         loss_improvements: list[float] = []
-        learner_index = 0
-
-        while len(points) < n:
-            learner = self.learners[learner_index]
-            if learner.done():  # type: ignore[attr-defined]
-                if learner_index == len(self.learners) - 1:
-                    break
-                learner_index += 1
-                continue
-
-            point, loss_improvement = learner.ask(n=1)
-            if not point:  # if learner is exhausted, we don't get points
-                if learner_index == len(self.learners) - 1:
-                    break
-                learner_index += 1
-                continue
-            points.append((learner_index, point[0]))
-            loss_improvements.append(loss_improvement[0])
-            self.tell_pending((learner_index, point[0]))
+        for learner_index, point, loss_improvement in self._gen:
+            points.append((learner_index, point))
+            loss_improvements.append(loss_improvement)
+            self.tell_pending((learner_index, point))
+            if len(points) >= n:
+                break
 
         return points, loss_improvements
 
@@ -629,3 +617,27 @@ def __getstate__(self) -> tuple[list[BaseLearner], CDIMS_TYPE, STRATEGY_TYPE]:
     def __setstate__(self, state: tuple[list[BaseLearner], CDIMS_TYPE, STRATEGY_TYPE]):
         learners, cdims, strategy = state
         self.__init__(learners, cdims=cdims, strategy=strategy)  # type: ignore[misc]
+
+
+def _sequential_generator(
+    learners: list[BaseLearner],
+) -> Generator[tuple[int, Any, float], None, None]:
+    learner_index = 0
+    if not hasattr(learners[0], "done"):
+        msg = "All learners must have a `done` method to use the 'sequential' strategy."
+        raise ValueError(msg)
+    while True:
+        learner = learners[learner_index]
+        if learner.done():  # type: ignore[attr-defined]
+            if learner_index == len(learners) - 1:
+                return
+            learner_index += 1
+            continue
+
+        point, loss_improvement = learner.ask(n=1)
+        if not point:  # if learner is exhausted, we don't get points
+            if learner_index == len(learners) - 1:
+                return
+            learner_index += 1
+            continue
+        yield learner_index, point[0], loss_improvement[0]