[DataFrame] Refactor indexers and implement setitem (ray-project#2020)

* Reset to_pandas change; update current

* Fix pd_df bug

python/ray/dataframe/dataframe.py

@@ -5027,9 +5027,8 @@ def loc(self):
         We currently support: single label, list array, slice object
         We do not support: boolean array, callable
         """
-        raise NotImplementedError(
-            "To contribute to Pandas on Ray, please visit "
-            "github.com/ray-project/ray.")
+        from .indexing import _Loc_Indexer
+        return _Loc_Indexer(self)
 
     @property
     def is_copy(self):
@@ -5054,9 +5053,8 @@ def iloc(self):
         We currently support: single label, list array, slice object
         We do not support: boolean array, callable
         """
-        raise NotImplementedError(
-            "To contribute to Pandas on Ray, please visit "
-            "github.com/ray-project/ray.")
+        from .indexing import _iLoc_Indexer
+        return _iLoc_Indexer(self)
 
     def _copartition(self, other, new_index):
         """Colocates the values of other with this for certain operations.

python/ray/dataframe/index_metadata.py

@@ -382,3 +382,13 @@ def convert_to_index_sliceable(self, key):
             key: slice to convert and check
         """
         return convert_to_index_sliceable(self._coord_df, key)
+
+    def get_partition(self, partition_id):
+        """Return a view of coord_df where partition = partition_id
+        """
+        return self._coord_df[self._coord_df.partition == partition_id]
+
+    def sorted_index(self):
+        return (self._coord_df
+                    .sort_values(['partition', 'index_within_partition'])
+                    .index)