BUG-24212 add comment

pandas/core/reshape/merge.py

@@ -760,6 +760,8 @@ def _get_join_info(self):
  join_index = self.left.index.take(left_indexer)
  if (self.how == 'right' and -1 in left_indexer
  and not isinstance(self.right.index, MultiIndex)):
+ # if values missing (-1) from left index,
+ # take from right index instead
  join_list = join_index.to_numpy()
  absent = left_indexer == -1
  join_list[absent] = self.right.index.to_numpy()[absent]
@@ -773,6 +775,8 @@ def _get_join_info(self):
  join_index = self.right.index.take(right_indexer)
  if (self.how == 'left' and -1 in right_indexer
  and not isinstance(self.left.index, MultiIndex)):
+ # if values missing (-1) from right index,
+ # take from left index instead
  join_list = join_index.to_numpy()
  absent = right_indexer == -1
  join_list[absent] = self.left.index.to_numpy()[absent]

pandas/tests/reshape/merge/test_merge.py

@@ -1170,7 +1170,8 @@ def test_merge_incompat_dtypes_error(self, df1_vals, df2_vals):
  with pytest.raises(ValueError, match=msg):
  pd.merge(df2, df1, on=['A'])
 
- def test_merge_on_index_with_more_values(self): # GH 24212
+ def test_merge_on_index_with_more_values(self):
+ # GH 24212
  df1 = pd.DataFrame([[1, 2], [2, 4], [3, 6], [4, 8]],
  columns=['a', 'b'])
  df2 = pd.DataFrame([[3, 30], [4, 40]],