Fix negative slicing of Zarr arrays (#8674)

* Fix negative slicing of Zarr arrays

Closes #8252
Closes #3921

* Cleanup

Author: dcherian

doc/whats-new.rst

@@ -59,6 +59,8 @@ Bug fixes
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
 - Ensure :py:meth:`DataArray.unstack` works when wrapping array API-compliant classes. (:issue:`8666`, :pull:`8668`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
+- Fix negative slicing of Zarr arrays without dask installed. (:issue:`8252`)
+  By `Deepak Cherian <https://github.com/dcherian>`_.
 - Preserve chunks when writing time-like variables to zarr by enabling lazy CF
   encoding of time-like variables (:issue:`7132`, :issue:`8230`, :issue:`8432`,
   :pull:`8575`). By `Spencer Clark <https://github.com/spencerkclark>`_ and

xarray/backends/zarr.py

@@ -86,19 +86,23 @@ def get_array(self):
     def _oindex(self, key):
         return self._array.oindex[key]
 
+    def _vindex(self, key):
+        return self._array.vindex[key]
+
+    def _getitem(self, key):
+        return self._array[key]
+
     def __getitem__(self, key):
         array = self._array
         if isinstance(key, indexing.BasicIndexer):
-            return array[key.tuple]
+            method = self._getitem
         elif isinstance(key, indexing.VectorizedIndexer):
-            return array.vindex[
-                indexing._arrayize_vectorized_indexer(key, self.shape).tuple
-            ]
-        else:
-            assert isinstance(key, indexing.OuterIndexer)
-            return indexing.explicit_indexing_adapter(
-                key, array.shape, indexing.IndexingSupport.VECTORIZED, self._oindex
-            )
+            method = self._vindex
+        elif isinstance(key, indexing.OuterIndexer):
+            method = self._oindex
+        return indexing.explicit_indexing_adapter(
+            key, array.shape, indexing.IndexingSupport.VECTORIZED, method
+        )
 
         # if self.ndim == 0:
         # could possibly have a work-around for 0d data here