fix: use island-aware inspiration sampling in sample_from_island (#471)

sample_from_island() used plain random.sample() for inspirations when the island was non-empty, while only the empty-island fallback used _sample_inspirations() (island best + elites + feature-cell diversity). Add an explicit island_id parameter to _sample_inspirations() and reuse it in sample_from_island(). This keeps parallel workers isolated even when archive fallback returns a parent whose metadata belongs to a different island, and avoids the shared current_island fallback.

Closes #437

Author: pathakanshika144-afk

openevolve/database.py

@@ -467,21 +467,13 @@ def sample_from_island(
             parent = self._sample_from_island_weighted(island_id)
             sampling_mode = "weighted"
 
-        # Select inspirations from the same island
+        # Select inspirations using the same elite/diversity-aware strategy as sample().
+        # Pass the requested island explicitly so parallel workers remain isolated even
+        # if an archive fallback returns a parent whose metadata points elsewhere.
         if num_inspirations is None:
             num_inspirations = 5  # Default for backward compatibility
 
-        # Get other programs from the island for inspirations
-        other_programs = [pid for pid in island_programs if pid != parent.id]
-
-        if len(other_programs) < num_inspirations:
-            # Not enough programs in island, use what we have
-            inspiration_ids = other_programs
-        else:
-            # Sample inspirations
-            inspiration_ids = random.sample(other_programs, num_inspirations)
-
-        inspirations = [self.programs[pid] for pid in inspiration_ids if pid in self.programs]
+        inspirations = self._sample_inspirations(parent, n=num_inspirations, island_id=island_id)
 
         logger.debug(
             f"Sampled parent {parent.id} and {len(inspirations)} inspirations from island {island_id} "
@@ -1569,7 +1561,9 @@ def _sample_from_archive_for_island(self, island_id: int) -> Program:
             parent_id = random.choice(valid_archive)
             return self.programs[parent_id]
 
-    def _sample_inspirations(self, parent: Program, n: int = 5) -> List[Program]:
+    def _sample_inspirations(
+        self, parent: Program, n: int = 5, island_id: Optional[int] = None
+    ) -> List[Program]:
         """
         Sample inspiration programs for the next evolution step.
 
@@ -1579,14 +1573,23 @@ def _sample_inspirations(self, parent: Program, n: int = 5) -> List[Program]:
         Args:
             parent: Parent program
             n: Number of inspirations to sample
+            island_id: Explicit island to sample from. If omitted, use the
+                parent program's island metadata.
 
         Returns:
             List of inspiration programs from the current island
         """
         inspirations = []
 
-        # Get the parent's island (should be current_island)
-        parent_island = parent.metadata.get("island", self.current_island)
+        # Prefer an explicitly requested island. This matters for
+        # sample_from_island(), where archive fallback may return a parent whose
+        # metadata belongs to a different island.
+        if island_id is None:
+            parent_island = parent.metadata.get("island", self.current_island)
+        else:
+            parent_island = island_id
+
+        parent_island %= len(self.islands)
 
         # Get all programs from the current island
         island_program_ids = list(self.islands[parent_island])

tests/test_sample_from_island_inspirations.py

@@ -0,0 +1,135 @@
+"""Regression tests for issue #437: island-aware inspiration sampling."""
+
+import unittest
+from unittest.mock import patch
+
+from openevolve.config import Config
+from openevolve.database import Program, ProgramDatabase
+
+
+class TestSampleFromIslandInspirations(unittest.TestCase):
+    def setUp(self):
+        config = Config()
+        config.database.in_memory = True
+        config.database.num_islands = 2
+        config.database.exploration_ratio = 1.0
+        config.database.exploitation_ratio = 0.0
+        self.db = ProgramDatabase(config.database)
+
+        self.parent = Program(
+            id="parent",
+            code="def parent(): pass",
+            metrics={"score": 0.5},
+            metadata={"island": 1},
+        )
+        self.elite = Program(
+            id="elite",
+            code="def elite(): pass",
+            metrics={"score": 1.0},
+            metadata={"island": 1},
+        )
+
+        self.db.programs = {
+            self.parent.id: self.parent,
+            self.elite.id: self.elite,
+        }
+        self.db.islands = [set(), {self.parent.id, self.elite.id}]
+        self.db.island_best_programs = [None, self.elite.id]
+
+    def test_sample_from_island_reuses_strategy_aware_inspiration_sampler(self):
+        with (
+            patch.object(
+                self.db,
+                "_sample_from_island_random",
+                return_value=self.parent,
+            ),
+            patch.object(
+                self.db,
+                "_sample_inspirations",
+                return_value=[self.elite],
+            ) as sample_inspirations,
+        ):
+            parent, inspirations = self.db.sample_from_island(
+                island_id=1,
+                num_inspirations=2,
+            )
+
+        self.assertIs(parent, self.parent)
+        self.assertEqual(inspirations, [self.elite])
+        sample_inspirations.assert_called_once_with(
+            self.parent,
+            n=2,
+            island_id=1,
+        )
+
+    def test_explicit_island_overrides_parent_metadata_for_inspirations(self):
+        island_zero_best = Program(
+            id="island-zero-best",
+            code="def island_zero(): pass",
+            metrics={"score": 2.0},
+            metadata={"island": 0},
+        )
+        parent_from_zero = Program(
+            id="parent-from-zero",
+            code="def parent_zero(): pass",
+            metrics={"score": 0.1},
+            metadata={"island": 0},
+        )
+        island_one_best = Program(
+            id="island-one-best",
+            code="def island_one(): pass",
+            metrics={"score": 1.0},
+            metadata={"island": 1},
+        )
+
+        self.db.programs = {
+            parent_from_zero.id: parent_from_zero,
+            island_zero_best.id: island_zero_best,
+            island_one_best.id: island_one_best,
+        }
+        self.db.islands = [
+            {parent_from_zero.id, island_zero_best.id},
+            {island_one_best.id},
+        ]
+        self.db.island_best_programs = [
+            island_zero_best.id,
+            island_one_best.id,
+        ]
+
+        inspirations = self.db._sample_inspirations(
+            parent_from_zero,
+            n=1,
+            island_id=1,
+        )
+
+        self.assertEqual([program.id for program in inspirations], [island_one_best.id])
+        self.assertTrue(all(program.metadata.get("island") == 1 for program in inspirations))
+
+    def test_zero_inspirations_is_forwarded_without_random_sampling(self):
+        with (
+            patch.object(
+                self.db,
+                "_sample_from_island_random",
+                return_value=self.parent,
+            ),
+            patch.object(
+                self.db,
+                "_sample_inspirations",
+                return_value=[],
+            ) as sample_inspirations,
+        ):
+            _, inspirations = self.db.sample_from_island(
+                island_id=1,
+                num_inspirations=0,
+            )
+
+        self.assertEqual(inspirations, [])
+        sample_inspirations.assert_called_once_with(
+            self.parent,
+            n=0,
+            island_id=1,
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()