BUG: loc raising KeyError for string slices in list-like indexer and DatetimeIndex

doc/source/whatsnew/v1.2.0.rst

@@ -639,6 +639,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc` raising ``TypeError`` when non-integer slice was given to select values from :class:`MultiIndex` (:issue:`25165`, :issue:`24263`)
 - Bug in :meth:`Series.at` returning :class:`Series` with one element instead of scalar when index is a :class:`MultiIndex` with one level (:issue:`38053`)
 - Bug in :meth:`DataFrame.loc` returning and assigning elements in wrong order when indexer is differently ordered than the :class:`MultiIndex` to filter (:issue:`31330`, :issue:`34603`)
+- Bug in :meth:`DataFrame.loc` raising ``KeyError`` for list-like indexers with string slices for :class:`DatetimeIndex` (:issue:`27180`)
 - Bug in :meth:`DataFrame.loc` and :meth:`DataFrame.__getitem__`  raising ``KeyError`` when columns were :class:`MultiIndex` with only one level (:issue:`29749`)
 - Bug in :meth:`Series.__getitem__` and :meth:`DataFrame.__getitem__` raising blank ``KeyError`` without missing keys for :class:`IntervalIndex` (:issue:`27365`)
 - Bug in setting a new label on a :class:`DataFrame` or :class:`Series` with a :class:`CategoricalIndex` incorrectly raising ``TypeError`` when the new label is not among the index's categories (:issue:`38098`)

pandas/core/indexes/datetimes.py

@@ -819,6 +819,25 @@ def slice_indexer(self, start=None, end=None, step=None, kind=None):
 
     # --------------------------------------------------------------------
 
+    def _convert_listlike_indexer(self, key):
+        if not isinstance(key, list):
+            # There are no slices, so we can dispatch back
+            return super()._convert_listlike_indexer(key)
+
+        new_indexer = []
+        positions = list(range(len(self)))
+        try:
+            for k in key:
+                # Convert slices to list of integers
+                indexer = positions[self.get_loc(k)]
+                if not isinstance(indexer, list):
+                    indexer = [indexer]
+                new_indexer.extend(indexer)
+        except KeyError:
+            # Dispatch to base method for handling of KeyErrors
+            return super()._convert_listlike_indexer(key)
+        return np.array(new_indexer), key
+
     @property
     def inferred_type(self) -> str:
         # b/c datetime is represented as microseconds since the epoch, make

pandas/tests/indexing/test_datetime.py

@@ -231,3 +231,21 @@ def test_loc_setitem_with_existing_dst(self):
             dtype=object,
         )
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "indexer", [["2001-01", "2001-01-30"], ["2001-01", Timestamp("2001-01-30")]]
+    )
+    def test_loc_getitem_partial_strings_in_list(self, indexer):
+        # GH#27180
+        ser = Series(1, index=date_range("2001-01-29", periods=60))
+        result = ser.loc[indexer]
+        expected = Series(
+            1,
+            index=[
+                Timestamp("2001-01-29"),
+                Timestamp("2001-01-30"),
+                Timestamp("2001-01-31"),
+                Timestamp("2001-01-30"),
+            ],
+        )
+        tm.assert_series_equal(result, expected)