BinGrouper holds the sorted binners, give the indexer to reorder axis if needed, eg. build grouping labels

Author: ruiann

pandas/core/groupby.py

@@ -1730,13 +1730,14 @@ class BaseGrouper(object):
     """
 
     def __init__(self, axis, groupings, sort=True, group_keys=True,
-                 mutated=False):
+                 mutated=False, indexer=None):
         self._filter_empty_groups = self.compressed = len(groupings) != 1
         self.axis = axis
         self.groupings = groupings
         self.sort = sort
         self.group_keys = group_keys
         self.mutated = mutated
+        self.indexer = indexer
 
     @property
     def shape(self):
@@ -2282,11 +2283,12 @@ def generate_bins_generic(values, binner, closed):
 
 class BinGrouper(BaseGrouper):
 
-    def __init__(self, bins, binlabels, filter_empty=False, mutated=False):
+    def __init__(self, bins, binlabels, filter_empty=False, mutated=False, indexer=None):
         self.bins = _ensure_int64(bins)
         self.binlabels = _ensure_index(binlabels)
         self._filter_empty_groups = filter_empty
         self.mutated = mutated
+        self.indexer = indexer
 
     @cache_readonly
     def groups(self):
@@ -2554,6 +2556,9 @@ def _make_labels(self):
             if isinstance(self.grouper, BaseGrouper):
                 labels, _, _ = self.grouper.group_info
                 uniques = self.grouper.result_index
+                if self.grouper.indexer is not None:
+                    sorter = np.lexsort((labels, self.grouper.indexer))
+                    labels = labels[sorter]
             else:
                 labels, uniques = algorithms.factorize(
                     self.grouper, sort=self.sort)

pandas/core/resample.py

@@ -250,7 +250,7 @@ def _get_binner(self):
         """
 
         binner, bins, binlabels = self._get_binner_for_time()
-        bin_grouper = BinGrouper(bins, binlabels)
+        bin_grouper = BinGrouper(bins, binlabels, indexer=self.groupby.indexer)
         return binner, bin_grouper
 
     def _assure_grouper(self):
@@ -879,7 +879,14 @@ def _downsample(self, how, **kwargs):
 
         if is_subperiod(ax.freq, self.freq):
             # Downsampling
-            return self._groupby_and_aggregate(how, grouper=self.grouper)
+            if len(new_index) == 0:
+                bins = []
+            else:
+                i8 = memb.asi8
+                rng = np.arange(i8[0], i8[-1] + 1)
+                bins = memb.searchsorted(rng, side='right')
+            grouper = BinGrouper(bins, new_index, indexer=self.groupby.indexer)
+            return self._groupby_and_aggregate(how, grouper=grouper)
         elif is_superperiod(ax.freq, self.freq):
             if how == 'ohlc':
                 # GH #13083