Use different seeds for subspace sampling (#885)

Co-authored-by: Uri Granta <[email protected]>

tests/unit/test_space.py

@@ -973,6 +973,18 @@ def test_collection_space_sampling_raises_for_invalid_sample_size(
         collection_space.sample(num_samples)
 
 
+@pytest.mark.parametrize("search_space_type", [TaggedMultiSearchSpace, TaggedProductSearchSpace])
+def test_collection_space_sampling_uses_different_seeds_for_subspaces(
+    search_space_type: Type[CollectionSearchSpace],
+) -> None:
+    box = Box([0], [1])
+    collection_space = search_space_type(spaces=[box, box])
+    samples = collection_space.sample(5, seed=42)
+    # check that all the points are unique despite the seed
+    flattened = samples.numpy().flatten()
+    assert len(flattened) == len(set(flattened))
+
+
 @pytest.mark.parametrize(
     "search_space_type, space_A",
     [

trieste/space.py

@@ -1229,7 +1229,11 @@ def subspace_sample(self, num_samples: int, seed: Optional[int] = None) -> Seque
         tf.debugging.assert_non_negative(num_samples)
         if seed is not None:  # ensure reproducibility
             tf.random.set_seed(seed)
-        return [self._spaces[tag].sample(num_samples, seed=seed) for tag in self._tags]
+        return [
+            # ensure subspaces (which may be identical) don't all use the same seed
+            self._spaces[tag].sample(num_samples, seed=None if seed is None else seed + i)
+            for i, tag in enumerate(self._tags)
+        ]
 
     def __eq__(self, other: object) -> bool:
         """