Add slice functionality (ray-project#1832)

python/ray/dataframe/dataframe.py

@@ -2945,9 +2945,7 @@ def __getitem__(self, key):
         # see if we can slice the rows
         indexer = self._row_metadata.convert_to_index_sliceable(key)
         if indexer is not None:
-            raise NotImplementedError("To contribute to Pandas on Ray, please"
-                                      "visit github.com/ray-project/ray.")
-            # return self._getitem_slice(indexer)
+            return self._getitem_slice(indexer)
 
         if isinstance(key, (pd.Series, np.ndarray, pd.Index, list)):
             return self._getitem_array(key)
@@ -3015,6 +3013,15 @@ def _getitem_indiv_col(self, key, part):
             lambda df: df.__getitem__(index),
             self._col_partitions[part])
 
+    def _getitem_slice(self, key):
+        new_cols = _map_partitions(lambda df: df[key],
+                                   self._col_partitions)
+
+        index = self.index[key]
+        return DataFrame(col_partitions=new_cols,
+                         index=index,
+                         columns=self.columns)
+
     def __getattr__(self, key):
         """After regular attribute access, looks up the name in the columns