Get 0d slices of ndarrays directly from indexing (#2625)

* Add test to ensure that 0d slices are views

* Get 0d slices of ndarrays directly from indexing

* Add 0d slice documentation

Author: danielwe

doc/whats-new.rst

@@ -56,6 +56,9 @@ Enhancements
 - :py:meth:`DataArray.resample` and :py:meth:`Dataset.resample` now supports the
   ``loffset`` kwarg just like Pandas.
   By `Deepak Cherian <https://github.com/dcherian>`_
+- 0d slices of ndarrays are now obtained directly through indexing, rather than
+  extracting and wrapping a scalar, avoiding unnecessary copying. By `Daniel
+  Wennberg <https://github.com/danielwe>`_.
 
 Bug fixes
 ~~~~~~~~~

xarray/core/indexing.py

@@ -1142,15 +1142,6 @@ def __init__(self, array):
                             'Trying to wrap {}'.format(type(array)))
         self.array = array
 
-    def _ensure_ndarray(self, value):
-        # We always want the result of indexing to be a NumPy array. If it's
-        # not, then it really should be a 0d array. Doing the coercion here
-        # instead of inside variable.as_compatible_data makes it less error
-        # prone.
-        if not isinstance(value, np.ndarray):
-            value = utils.to_0d_array(value)
-        return value
-
     def _indexing_array_and_key(self, key):
         if isinstance(key, OuterIndexer):
             array = self.array
@@ -1160,7 +1151,10 @@ def _indexing_array_and_key(self, key):
             key = key.tuple
         elif isinstance(key, BasicIndexer):
             array = self.array
-            key = key.tuple
+            # We want 0d slices rather than scalars. This is achieved by
+            # appending an ellipsis (see
+            # https://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#detailed-notes).  # noqa
+            key = key.tuple + (Ellipsis,)
         else:
             raise TypeError('unexpected key type: {}'.format(type(key)))
 
@@ -1171,7 +1165,7 @@ def transpose(self, order):
 
     def __getitem__(self, key):
         array, key = self._indexing_array_and_key(key)
-        return self._ensure_ndarray(array[key])
+        return array[key]
 
     def __setitem__(self, key, value):
         array, key = self._indexing_array_and_key(key)

xarray/tests/test_variable.py

@@ -1147,6 +1147,11 @@ def test_getitem_basic(self):
         assert v_new.dims == ('x', )
         assert_array_equal(v_new, v._data[:, 1])
 
+        # test that we obtain a modifiable view when taking a 0d slice
+        v_new = v[0, 0]
+        v_new[...] += 99
+        assert_array_equal(v_new, v._data[0, 0])
+
     def test_getitem_with_mask_2d_input(self):
         v = Variable(('x', 'y'), [[0, 1, 2], [3, 4, 5]])
         assert_identical(v._getitem_with_mask(([-1, 0], [1, -1])),