BUG: Fix length 1 .loc slicing with datetime string GH16071

Author: Christoph Paulik

doc/source/whatsnew/v0.20.0.txt

@@ -557,6 +557,7 @@ Other Enhancements
 - The ``display.show_dimensions`` option can now also be used to specify
   whether the length of a ``Series`` should be shown in its repr (:issue:`7117`).
 - ``parallel_coordinates()`` has gained a ``sort_labels`` keyword arg that sorts class labels and the colours assigned to them (:issue:`15908`)
+- ``loc`` indexer with datetime strings now works correctly for length 1 DataFrames and is consistent with the behaviour of using a ``datetime.datetime`` object (:issue:`16071')
 
 
 .. _ISO 8601 duration: https://en.wikipedia.org/wiki/ISO_8601#Durations

pandas/core/indexes/datetimes.py

@@ -1470,8 +1470,9 @@ def _maybe_cast_slice_bound(self, label, side, kind):
             #   [parsed, parsed + 1 freq)
             # because label may be passed to searchsorted
             # 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):
+            # length > 1 (is_monotonic_decreasing gives True for empty
+            # and length 1 index)
+            if self.is_monotonic_decreasing and len(self) > 1:
                 return upper if side == 'left' else lower
             return lower if side == 'left' else upper
         else:

pandas/tests/indexing/test_loc.py

@@ -628,3 +628,22 @@ def test_loc_empty_list_indexer_is_ok(self):
         tm.assert_frame_equal(df.loc[[]], df.iloc[:0, :],
                               check_index_type=True,
                               check_column_type=True)
+
+
+def test_loc_datetime_length_one():
+    """
+    Test if DataFrame with length one is properly sliced
+    by .loc.
+    """
+    from datetime import datetime
+    df = pd.DataFrame(columns=['1'],
+                      index=pd.date_range('2016-10-01T00:00:00',
+                                          '2016-10-01T23:59:59'))
+    df_sliced_dt = df.loc[datetime(2016, 10, 1):]
+    tm.assert_frame_equal(df, df_sliced_dt)
+
+    df = pd.DataFrame(columns=['1'],
+                      index=pd.date_range('2016-10-01T00:00:00',
+                                          '2016-10-01T23:59:59'))
+    df_sliced_str = df.loc['2016-10-01T00:00:00':]
+    tm.assert_frame_equal(df, df_sliced_str)