DEPR: Enforce diallowing indexing with single item slice (#49343)

doc/source/whatsnew/v2.0.0.rst

@@ -248,6 +248,7 @@ Removal of prior version deprecations/changes
 - Removed setting Categorical._codes directly (:issue:`41429`)
 - Enforced :meth:`Rolling.count` with ``min_periods=None`` to default to the size of the window (:issue:`31302`)
 - Renamed ``fname`` to ``path`` in :meth:`DataFrame.to_parquet`, :meth:`DataFrame.to_stata` and :meth:`DataFrame.to_feather` (:issue:`30338`)
+- Enforced disallowing indexing a :class:`Series` with a single item list with a slice (e.g. ``ser[[slice(0, 2)]]``). Either convert the list to tuple, or pass the slice directly instead (:issue:`31333`)
 - Enforced the ``display.max_colwidth`` option to not accept negative integers (:issue:`31569`)
 - Removed the ``display.column_space`` option in favor of ``df.to_string(col_space=...)`` (:issue:`47280`)
 - Removed the deprecated method ``mad`` from pandas classes (:issue:`11787`)

pandas/core/indexers/utils.py

@@ -367,12 +367,9 @@ def unpack_1tuple(tup):
 
         if isinstance(tup, list):
             # GH#31299
-            warnings.warn(
+            raise ValueError(
                 "Indexing with a single-item list containing a "
-                "slice is deprecated and will raise in a future "
-                "version.  Pass a tuple instead.",
-                FutureWarning,
-                stacklevel=find_stack_level(),
+                "slice is not allowed. Pass a tuple instead.",
             )
 
         return tup[0]

pandas/tests/series/indexing/test_getitem.py

@@ -284,14 +284,13 @@ def test_getitem_slice_2d(self, datetime_series):
     @pytest.mark.filterwarnings("ignore:Using a non-tuple:FutureWarning")
     def test_getitem_median_slice_bug(self):
         index = date_range("20090415", "20090519", freq="2B")
-        s = Series(np.random.randn(13), index=index)
+        ser = Series(np.random.randn(13), index=index)
 
         indexer = [slice(6, 7, None)]
-        with tm.assert_produces_warning(FutureWarning):
+        msg = "Indexing with a single-item list"
+        with pytest.raises(ValueError, match=msg):
             # GH#31299
-            result = s[indexer]
-        expected = s[indexer[0]]
-        tm.assert_series_equal(result, expected)
+            ser[indexer]
 
     @pytest.mark.parametrize(
         "slc, positions",

pandas/tests/series/indexing/test_indexing.py

@@ -194,12 +194,11 @@ def test_basic_getitem_setitem_corner(datetime_series):
     with pytest.raises(KeyError, match=msg):
         datetime_series[:, 2] = 2
 
-    # weird lists. [slice(0, 5)] will work but not two slices
-    with tm.assert_produces_warning(FutureWarning):
+    # weird lists. [slice(0, 5)] raises but not two slices
+    msg = "Indexing with a single-item list"
+    with pytest.raises(ValueError, match=msg):
         # GH#31299
-        result = datetime_series[[slice(None, 5)]]
-    expected = datetime_series[:5]
-    tm.assert_series_equal(result, expected)
+        datetime_series[[slice(None, 5)]]
 
     # OK
     msg = r"unhashable type(: 'slice')?"