BUG: partial string indexing with scalar

Closes pandas-dev#27516

Author: TomAugspurger

doc/source/whatsnew/v0.25.1.rst

@@ -82,7 +82,7 @@ Interval
 Indexing
 ^^^^^^^^
 
--
+- Bug in partial-string indexing returning a NumPy array rather than a ``Series`` when indexing with a scalar like ``.loc['2015']`` (:issue:`27516`)
 -
 -
 

pandas/core/indexing.py

@@ -1704,6 +1704,11 @@ def _is_scalar_access(self, key: Tuple):
             if isinstance(ax, MultiIndex):
                 return False
 
+            if isinstance(k, str) and ax.is_all_dates:
+                # partial string indexing, df.loc['2000', 'A']
+                # should not be considered scalar
+                return False
+
             if not ax.is_unique:
                 return False
 

pandas/tests/indexes/datetimes/test_partial_slicing.py

@@ -468,3 +468,14 @@ def test_getitem_with_datestring_with_UTC_offset(self, start, end):
         with pytest.raises(ValueError, match="The index must be timezone"):
             df = df.tz_localize(None)
             df[start:end]
+
+    def test_slice_reduce_to_series(self):
+        # GH 27516
+        df = pd.DataFrame(
+            {"A": range(24)}, index=pd.date_range("2000", periods=24, freq="M")
+        )
+        expected = pd.Series(
+            range(12), index=pd.date_range("2000", periods=12, freq="M"), name="A"
+        )
+        result = df.loc["2000", "A"]
+        tm.assert_series_equal(result, expected)