BUG: fix DatetimeIndex._maybe_cast_slice_bound for empty index (GH14354) (#14501)

Author: jorisvandenbossche

doc/source/whatsnew/v0.19.1.txt

@@ -48,6 +48,7 @@ Bug Fixes
 
 - Bug in ``RangeIndex.intersection`` when result is a empty set (:issue:`14364`).
 - Bug in union of differences from a ``DatetimeIndex`; this is a regression in 0.19.0 from 0.18.1 (:issue:`14323`)
+- Regression in ``DatetimeIndex._maybe_cast_slice_bound`` when index is empty (:issue:`14354`).
 
 - Bug in groupby-transform broadcasting that could cause incorrect dtype coercion (:issue:`14457`)
 
@@ -78,4 +79,5 @@ Bug Fixes
 
 
 
-- Bug in ``pd.pivot_table`` may raise ``TypeError`` or ``ValueError`` when ``index`` or ``columns`` is not scalar and ``values`` is not specified (:issue:`14380`)
+- Bug in ``pd.pivot_table`` may raise ``TypeError`` or ``ValueError`` when ``index`` or ``columns``
+  is not scalar and ``values`` is not specified (:issue:`14380`)

pandas/tseries/index.py

@@ -1453,8 +1453,9 @@ def _maybe_cast_slice_bound(self, label, side, kind):
             # 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:
+            # the bounds need swapped if index is reverse sorted and has a
+            # length (is_monotonic_decreasing gives True for empty index)
+            if self.is_monotonic_decreasing and len(self):
                 return upper if side == 'left' else lower
             return lower if side == 'left' else upper
         else:

pandas/tseries/tests/test_timeseries.py

@@ -3911,6 +3911,18 @@ def test_slice_with_zero_step_raises(self):
         self.assertRaisesRegexp(ValueError, 'slice step cannot be zero',
                                 lambda: ts.ix[::0])
 
+    def test_slice_bounds_empty(self):
+        # GH 14354
+        empty_idx = DatetimeIndex(freq='1H', periods=0, end='2015')
+
+        right = empty_idx._maybe_cast_slice_bound('2015-01-02', 'right', 'loc')
+        exp = Timestamp('2015-01-02 23:59:59.999999999')
+        self.assertEqual(right, exp)
+
+        left = empty_idx._maybe_cast_slice_bound('2015-01-02', 'left', 'loc')
+        exp = Timestamp('2015-01-02 00:00:00')
+        self.assertEqual(left, exp)
+
 
 class TestDatetime64(tm.TestCase):
     """