Attempt to fix indexing for Dask

This is a naive attempt to make `isel` work with Dask

Known limitation: it triggers the computation.

xarray/core/indexing.py

@@ -18,7 +18,7 @@
     is_duck_dask_array,
     sparse_array_type,
 )
-from .utils import maybe_cast_to_coords_dtype
+from .utils import is_duck_array, maybe_cast_to_coords_dtype
 
 
 def expanded_indexer(key, ndim):
@@ -307,7 +307,7 @@ def __init__(self, key):
         for k in key:
             if isinstance(k, slice):
                 k = as_integer_slice(k)
-            elif isinstance(k, np.ndarray):
+            elif is_duck_array(k):
                 if not np.issubdtype(k.dtype, np.integer):
                     raise TypeError(
                         f"invalid indexer array, does not have integer dtype: {k!r}"
@@ -320,7 +320,7 @@ def __init__(self, key):
                         "invalid indexer key: ndarray arguments "
                         f"have different numbers of dimensions: {ndims}"
                     )
-                k = np.asarray(k, dtype=np.int64)
+                k = k.astype(np.int64)
             else:
                 raise TypeError(
                     f"unexpected indexer type for {type(self).__name__}: {k!r}"

xarray/tests/test_indexing.py

@@ -9,6 +9,8 @@
 
 from . import IndexerMaker, ReturnItem, assert_array_equal
 
+da = pytest.importorskip("dask.array")
+
 B = IndexerMaker(indexing.BasicIndexer)
 
 
@@ -729,3 +731,16 @@ def test_indexing_1d_object_array() -> None:
     expected = DataArray(expected_data)
 
     assert [actual.data.item()] == [expected.data.item()]
+
+
+def test_indexing_dask_array():
+    da = DataArray(
+        np.ones(10 * 3 * 3).reshape((10, 3, 3)),
+        dims=("time", "x", "y"),
+    ).chunk(dict(time=-1, x=1, y=1))
+    da[{"time": 9}] = 42
+
+    idx = da.argmax("time")
+    actual = da.isel(time=idx)
+
+    assert np.all(actual == 42)