REGR: fix bug in DatetimeIndex slicing with irregular or unsorted indices

doc/source/whatsnew/v1.1.4.rst

@@ -20,6 +20,7 @@ Fixed regressions
 - Fixed regression in :class:`RollingGroupby` with ``sort=False`` not being respected (:issue:`36889`)
 - Fixed regression in :meth:`Series.astype` converting ``None`` to ``"nan"`` when casting to string (:issue:`36904`)
 - Fixed regression in :class:`RollingGroupby` causing a segmentation fault with Index of dtype object (:issue:`36727`)
+- Fixed regression where slicing :class:`DatetimeIndex` failed on irregular time series with ``pd.NaT`` or on unsorted indices (:issue:`36953` and :issue:`35509`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/generic.py

@@ -3739,7 +3739,6 @@ def _slice(self: FrameOrSeries, slobj: slice, axis=0) -> FrameOrSeries:
 
         Slicing with this method is *always* positional.
         """
-        assert isinstance(slobj, slice), type(slobj)
         axis = self._get_block_manager_axis(axis)
         result = self._constructor(self._mgr.get_slice(slobj, axis=axis))
         result = result.__finalize__(self)

pandas/tests/indexing/test_partial.py

@@ -681,3 +681,22 @@ def test_index_name_empty(self):
             {"series": [1.23] * 4}, index=pd.RangeIndex(4, name="series_index")
         )
         tm.assert_frame_equal(df, expected)
+
+    def test_slice_irregular_datetime_index_with_nan(self):
+        # GH36953
+        index = pd.to_datetime(["2012-01-01", "2012-01-02", "2012-01-03", None])
+        df = pd.DataFrame(range(len(index)), index=index)
+        expected = pd.DataFrame(range(len(index[:3])), index=index[:3])
+        tm.assert_frame_equal(df["2012-01-01":"2012-01-04"], expected)
+
+    def test_slice_datetime_index(self):
+        # GH35509
+        df = pd.DataFrame(
+            {"col1": ["a", "b", "c"], "col2": [1, 2, 3]},
+            index=pd.to_datetime(["2020-08-01", "2020-07-02", "2020-08-05"]),
+        )
+        expected = pd.DataFrame(
+            {"col1": ["a", "c"], "col2": [1, 3]},
+            index=pd.to_datetime(["2020-08-01", "2020-08-05"]),
+        )
+        tm.assert_frame_equal(df.loc["2020-08"], expected)