fix: expose array interface for CUDA (#2327)

* fix: expose array interface for CUDA

* test: cover other cases

src/awkward/index.py

@@ -149,8 +149,13 @@ def raw(self, nplike):
     def __len__(self):
         return self.length
 
-    def __array__(self, dtype=None):
-        return self._nplike.asarray(self._data, dtype=dtype)
+    @property
+    def __cuda_array_interface__(self):
+        return self._data.__cuda_array_interface__
+
+    @property
+    def __array_interface__(self):
+        return self._data.__array_interface__
 
     def __repr__(self):
         return self._repr("", "", "")

tests/test_2327_array_interface.py

@@ -0,0 +1,31 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
+
+import numpy as np
+import pytest
+
+import awkward as ak
+
+
+def test_wrap_index_cupy():
+    cp = pytest.importorskip("cupy")
+    data = cp.arange(10, dtype=cp.int64)
+    index = ak.index.Index64(data)
+    other_index = ak.index.Index64(index)
+    other_data = cp.asarray(other_index)
+    assert cp.shares_memory(data, other_data)
+
+
+def test_wrap_index_numpy():
+    data = np.arange(10, dtype=np.int64)
+    index = ak.index.Index64(data)
+    other_index = ak.index.Index64(index)
+    other_data = np.asarray(other_index)
+    assert np.shares_memory(data, other_data)
+
+
+def test_wrap_bare_list():
+    data = [1, 2, 3, 4, 5]
+    index = ak.index.Index64(data)
+    other_index = ak.index.Index64(index)
+    other_data = np.asarray(other_index)
+    assert other_data.tolist() == data