2x~5x speed up for isel() in most cases

doc/whats-new.rst

@@ -37,7 +37,6 @@ Bug fixes
 - Fix plotting with transposed 2D non-dimensional coordinates. (:issue:`3138`, :pull:`3441`)
   By `Deepak Cherian <https://github.com/dcherian>`_.
 
-
 Documentation
 ~~~~~~~~~~~~~
 - Switch doc examples to use nbsphinx and replace sphinx_gallery with
@@ -58,8 +57,10 @@ Documentation
 
 Internal Changes
 ~~~~~~~~~~~~~~~~
-
-
+- 2x to 5x speed boost (on small arrays) for :py:meth:`Dataset.isel`,
+  :py:meth:`DataArray.isel`, and :py:meth:`DataArray.__getitem__` when indexing by int,
+  slice, list of int, scalar ndarray, or 1-dimensional ndarray.
+  (:pull:`3533`) by `Guido Imperiale <https://github.com/crusaderky>`_.
 - Removed internal method ``Dataset._from_vars_and_coord_names``, 
   which was dominated by ``Dataset._construct_direct``. (:pull:`3565`)
   By `Maximilian Roos <https://github.com/max-sixty>`_
@@ -190,6 +191,7 @@ Documentation
 
 Internal Changes
 ~~~~~~~~~~~~~~~~
+
 - Added integration tests against `pint <https://pint.readthedocs.io/>`_.
   (:pull:`3238`, :pull:`3447`, :pull:`3493`, :pull:`3508`)
   by `Justus Magin <https://github.com/keewis>`_.

xarray/coding/cftime_offsets.py

@@ -42,6 +42,7 @@
 
 import re
 from datetime import timedelta
+from distutils.version import LooseVersion
 from functools import partial
 from typing import ClassVar, Optional
 
@@ -50,7 +51,6 @@
 from ..core.pdcompat import count_not_none
 from .cftimeindex import CFTimeIndex, _parse_iso8601_with_reso
 from .times import format_cftime_datetime
-from distutils.version import LooseVersion
 
 
 def get_date_type(calendar):

xarray/core/dataarray.py

@@ -50,7 +50,8 @@
 )
 from .dataset import Dataset, split_indexes
 from .formatting import format_item
-from .indexes import Indexes, propagate_indexes, default_indexes
+from .indexes import Indexes, default_indexes, propagate_indexes
+from .indexing import is_fancy_indexer
 from .merge import PANDAS_TYPES, _extract_indexes_from_coords
 from .options import OPTIONS
 from .utils import Default, ReprObject, _check_inplace, _default, either_dict_or_kwargs
@@ -1027,8 +1028,27 @@ def isel(
         DataArray.sel
         """
         indexers = either_dict_or_kwargs(indexers, indexers_kwargs, "isel")
-        ds = self._to_temp_dataset().isel(drop=drop, indexers=indexers)
-        return self._from_temp_dataset(ds)
+        if any(is_fancy_indexer(idx) for idx in indexers.values()):
+            ds = self._to_temp_dataset()._isel_fancy(indexers, drop=drop)
+            return self._from_temp_dataset(ds)
+
+        # Much faster algorithm for when all indexers are ints, slices, one-dimensional
+        # lists, or zero or one-dimensional np.ndarray's
+
+        variable = self._variable.isel(indexers)
+
+        coords = {}
+        for coord_name, coord_value in self._coords.items():
+            coord_indexers = {
+                k: v for k, v in indexers.items() if k in coord_value.dims
+            }
+            if coord_indexers:
+                coord_value = coord_value.isel(coord_indexers)
+                if drop and coord_value.ndim == 0:
+                    continue
+            coords[coord_name] = coord_value
+
+        return self._replace(variable=variable, coords=coords)
 
     def sel(
         self,

xarray/core/dataset.py

@@ -66,6 +66,7 @@
     propagate_indexes,
     roll_index,
 )
+from .indexing import is_fancy_indexer
 from .merge import (
     dataset_merge_method,
     dataset_update_method,
@@ -78,8 +79,8 @@
     Default,
     Frozen,
     SortedKeysDict,
-    _default,
     _check_inplace,
+    _default,
     decode_numpy_dict_values,
     either_dict_or_kwargs,
     hashable,
@@ -1907,6 +1908,48 @@ def isel(
         DataArray.isel
         """
         indexers = either_dict_or_kwargs(indexers, indexers_kwargs, "isel")
+        if any(is_fancy_indexer(idx) for idx in indexers.values()):
+            return self._isel_fancy(indexers, drop=drop)
+
+        # Much faster algorithm for when all indexers are ints, slices, one-dimensional
+        # lists, or zero or one-dimensional np.ndarray's
+        invalid = indexers.keys() - self.dims.keys()
+        if invalid:
+            raise ValueError("dimensions %r do not exist" % invalid)
+
+        variables = {}
+        dims: Dict[Hashable, Tuple[int, ...]] = {}
+        coord_names = self._coord_names.copy()
+        indexes = self._indexes.copy() if self._indexes is not None else None
+
+        for var_name, var_value in self._variables.items():
+            var_indexers = {k: v for k, v in indexers.items() if k in var_value.dims}
+            if var_indexers:
+                var_value = var_value.isel(var_indexers)
+                if drop and var_value.ndim == 0 and var_name in coord_names:
+                    coord_names.remove(var_name)
+                    if indexes:
+                        indexes.pop(var_name, None)
+                    continue
+                if indexes and var_name in indexes:
+                    if var_value.ndim == 1:
+                        indexes[var_name] = var_value.to_index()
+                    else:
+                        del indexes[var_name]
+            variables[var_name] = var_value
+            dims.update(zip(var_value.dims, var_value.shape))
+
+        return self._construct_direct(
+            variables=variables,
+            coord_names=coord_names,
+            dims=dims,
+            attrs=self._attrs,
+            indexes=indexes,
+            encoding=self._encoding,
+            file_obj=self._file_obj,
+        )
+
+    def _isel_fancy(self, indexers: Mapping[Hashable, Any], *, drop: bool) -> "Dataset":
         # Note: we need to preserve the original indexers variable in order to merge the
         # coords below
         indexers_list = list(self._validate_indexers(indexers))

xarray/core/formatting_html.py

@@ -1,11 +1,11 @@
 import uuid
-import pkg_resources
 from collections import OrderedDict
 from functools import partial
 from html import escape
 
-from .formatting import inline_variable_array_repr, short_data_repr
+import pkg_resources
 
+from .formatting import inline_variable_array_repr, short_data_repr
 
 CSS_FILE_PATH = "/".join(("static", "css", "style.css"))
 CSS_STYLE = pkg_resources.resource_string("xarray", CSS_FILE_PATH).decode("utf8")

xarray/core/indexing.py

@@ -1213,6 +1213,19 @@ def posify_mask_indexer(indexer):
     return type(indexer)(key)
 
 
+def is_fancy_indexer(indexer: Any) -> bool:
+    """Return False if indexer is a int, slice, a 1-dimensional list, or a 0 or
+    1-dimensional ndarray; in all other cases return True
+    """
+    if isinstance(indexer, (int, slice)):
+        return False
+    if isinstance(indexer, np.ndarray):
+        return indexer.ndim > 1
+    if isinstance(indexer, list):
+        return bool(indexer) and not isinstance(indexer[0], int)
+    return True
+
+
 class NumpyIndexingAdapter(ExplicitlyIndexedNDArrayMixin):
     """Wrap a NumPy array to use explicit indexing."""
 

xarray/core/variable.py

@@ -617,7 +617,10 @@ def _broadcast_indexes_outer(self, key):
                 k = k.data
             if not isinstance(k, BASIC_INDEXING_TYPES):
                 k = np.asarray(k)
-                if k.dtype.kind == "b":
+                if k.size == 0:
+                    # Slice by empty list; numpy could not infer the dtype
+                    k = k.astype(int)
+                elif k.dtype.kind == "b":
                     (k,) = np.nonzero(k)
             new_key.append(k)
 

xarray/tests/test_dask.py

@@ -16,6 +16,7 @@
 from xarray.testing import assert_chunks_equal
 from xarray.tests import mock
 
+from ..core.duck_array_ops import lazy_array_equiv
 from . import (
     assert_allclose,
     assert_array_equal,
@@ -25,7 +26,6 @@
     raises_regex,
     requires_scipy_or_netCDF4,
 )
-from ..core.duck_array_ops import lazy_array_equiv
 from .test_backends import create_tmp_file
 
 dask = pytest.importorskip("dask")

xarray/tests/test_dataarray.py

@@ -16,7 +16,6 @@
 from xarray.core.common import full_like
 from xarray.core.indexes import propagate_indexes
 from xarray.core.utils import is_scalar
-
 from xarray.tests import (
     LooseVersion,
     ReturnItem,

xarray/tests/test_missing.py

@@ -9,8 +9,8 @@
     NumpyInterpolator,
     ScipyInterpolator,
     SplineInterpolator,
-    get_clean_interp_index,
     _get_nan_block_lengths,
+    get_clean_interp_index,
 )
 from xarray.core.pycompat import dask_array_type
 from xarray.tests import (

xarray/tests/test_variable.py

@@ -1156,6 +1156,26 @@ def test_items(self):
     def test_getitem_basic(self):
         v = self.cls(["x", "y"], [[0, 1, 2], [3, 4, 5]])
 
+        # int argument
+        v_new = v[0]
+        assert v_new.dims == ("y",)
+        assert_array_equal(v_new, v._data[0])
+
+        # slice argument
+        v_new = v[:2]
+        assert v_new.dims == ("x", "y")
+        assert_array_equal(v_new, v._data[:2])
+
+        # list arguments
+        v_new = v[[0]]
+        assert v_new.dims == ("x", "y")
+        assert_array_equal(v_new, v._data[[0]])
+
+        v_new = v[[]]
+        assert v_new.dims == ("x", "y")
+        assert_array_equal(v_new, v._data[[]])
+
+        # dict arguments
         v_new = v[dict(x=0)]
         assert v_new.dims == ("y",)
         assert_array_equal(v_new, v._data[0])
@@ -1196,6 +1216,8 @@ def test_isel(self):
         assert_identical(v.isel(time=0), v[0])
         assert_identical(v.isel(time=slice(0, 3)), v[:3])
         assert_identical(v.isel(x=0), v[:, 0])
+        assert_identical(v.isel(x=[0, 2]), v[:, [0, 2]])
+        assert_identical(v.isel(time=[]), v[[]])
         with raises_regex(ValueError, "do not exist"):
             v.isel(not_a_dim=0)