BUG: date slicing with reverse sorted index

closes #14316

Author: Chris <cbartak@gmail.com>

Closes #14317 from chris-b1/rev-date-slice and squashes the following commits:

04feea1 [Chris] add comment / clarity
dbace2b [Chris] BUG: date slicing with rev index

Author: chris-b1

doc/source/whatsnew/v0.19.0.txt

@@ -1526,7 +1526,7 @@ Bug Fixes
 - Bug in ``.shift`` raises ``AmbiguousTimeError`` if data contains datetime near DST boundary (:issue:`13926`)
 - Bug in ``pd.read_hdf()`` returns incorrect result when a ``DataFrame`` with a ``categorical`` column and a query which doesn't match any values (:issue:`13792`)
 - Bug in ``.iloc`` when indexing with a non lex-sorted MultiIndex (:issue:`13797`)
-
+- Bug in ``.loc`` when indexing with date strings in a reverse sorted ``DatetimeIndex`` (:issue:`14316`)
 
 - Bug in ``Series`` comparison operators when dealing with zero dim NumPy arrays (:issue:`13006`)
 - Bug in ``.combine_first`` may return incorrect ``dtype`` (:issue:`7630`, :issue:`10567`)

pandas/tests/indexing/test_indexing.py

@@ -100,7 +100,7 @@ class TestIndexing(tm.TestCase):
     _multiprocess_can_split_ = True
 
     _objs = set(['series', 'frame', 'panel'])
-    _typs = set(['ints', 'labels', 'mixed', 'ts', 'floats', 'empty'])
+    _typs = set(['ints', 'labels', 'mixed', 'ts', 'floats', 'empty', 'ts_rev'])
 
     def setUp(self):
 
@@ -137,6 +137,15 @@ def setUp(self):
         self.panel_ts = Panel(np.random.randn(4, 4, 4),
                               items=date_range('20130101', periods=4))
 
+        dates_rev = (date_range('20130101', periods=4)
+                     .sort_values(ascending=False))
+        self.series_ts_rev = Series(np.random.randn(4),
+                                    index=dates_rev)
+        self.frame_ts_rev = DataFrame(np.random.randn(4, 4),
+                                      index=dates_rev)
+        self.panel_ts_rev = Panel(np.random.randn(4, 4, 4),
+                                  items=dates_rev)
+
         self.frame_empty = DataFrame({})
         self.series_empty = Series({})
         self.panel_empty = Panel({})
@@ -1358,6 +1367,10 @@ def test_loc_getitem_label_slice(self):
                           'ix', slice('20130102', '20130104'),
                           typs=['ts'], axes=2, fails=TypeError)
 
+        # GH 14316
+        self.check_result('ts slice rev', 'loc', slice('20130104', '20130102'),
+                          'indexer', [0, 1, 2], typs=['ts_rev'], axes=0)
+
         self.check_result('mixed slice', 'loc', slice(2, 8), 'ix', slice(2, 8),
                           typs=['mixed'], axes=0, fails=TypeError)
         self.check_result('mixed slice', 'loc', slice(2, 8), 'ix', slice(2, 8),

pandas/tseries/index.py

@@ -1446,8 +1446,14 @@ def _maybe_cast_slice_bound(self, label, side, kind):
             freq = getattr(self, 'freqstr',
                            getattr(self, 'inferred_freq', None))
             _, parsed, reso = parse_time_string(label, freq)
-            bounds = self._parsed_string_to_bounds(reso, parsed)
-            return bounds[0 if side == 'left' else 1]
+            lower, upper = self._parsed_string_to_bounds(reso, parsed)
+            # lower, upper form the half-open interval:
+            #   [parsed, parsed + 1 freq)
+            # because label may be passed to searchsorted
+            # the bounds need swapped if index is reverse sorted
+            if self.is_monotonic_decreasing:
+                return upper if side == 'left' else lower
+            return lower if side == 'left' else upper
         else:
             return label