Return independent empty covariance feature outputs

src/pyrecest/utils/pairwise_covariance_features.py

@@ -158,8 +158,11 @@ def pairwise_covariance_shape_components(
     n_a = covariances_a.shape[2]
     n_b = covariances_b.shape[2]
     if n_a == 0 or n_b == 0:
-        empty = zeros((n_a, n_b), dtype=float64)
-        return empty, empty, empty
+        return (
+            zeros((n_a, n_b), dtype=float64),
+            zeros((n_a, n_b), dtype=float64),
+            zeros((n_a, n_b), dtype=float64),
+        )
 
     moved_covariances_a = _symmetrized_covariance_batch(covariances_a)
     moved_covariances_b = _symmetrized_covariance_batch(covariances_b)

tests/test_covariance_empty_outputs.py

@@ -0,0 +1,13 @@
+from pyrecest.backend import zeros
+from pyrecest.utils import pairwise_covariance_shape_components
+
+
+def test_pairwise_covariance_empty_outputs_are_independent_objects():
+    shape_cost, logdet_cost, shape_similarity = pairwise_covariance_shape_components(
+        zeros((2, 2, 0)), zeros((2, 2, 4))
+    )
+
+    assert shape_cost.shape == (0, 4)
+    assert logdet_cost.shape == (0, 4)
+    assert shape_similarity.shape == (0, 4)
+    assert len({id(shape_cost), id(logdet_cost), id(shape_similarity)}) == 3