BUG: Adjust truncate for decreasing index (pandas-dev#33769)

Author: dsaxton

doc/source/whatsnew/v1.1.0.rst

@@ -568,6 +568,7 @@ Indexing
 - Bug in :meth:`DatetimeIndex.insert` and :meth:`TimedeltaIndex.insert` causing index ``freq`` to be lost when setting an element into an empty :class:`Series` (:issue:33573`)
 - Bug in :meth:`Series.__setitem__` with an :class:`IntervalIndex` and a list-like key of integers (:issue:`33473`)
 - Bug in :meth:`Series.__getitem__` allowing missing labels with ``np.ndarray``, :class:`Index`, :class:`Series` indexers but not ``list``, these now all raise ``KeyError`` (:issue:`33646`)
+- Bug in :meth:`DataFrame.truncate` and :meth:`Series.truncate` where index was assumed to be monotone increasing (:issue:`33756`)
 
 Missing
 ^^^^^^^

pandas/core/generic.py

@@ -9196,6 +9196,9 @@ def truncate(
             if before > after:
                 raise ValueError(f"Truncate: {after} must be after {before}")
 
+        if ax.is_monotonic_decreasing:
+            before, after = after, before
+
         slicer = [slice(None, None)] * self._AXIS_LEN
         slicer[axis] = slice(before, after)
         result = self.loc[tuple(slicer)]

pandas/tests/frame/methods/test_truncate.py

@@ -87,3 +87,20 @@ def test_truncate_nonsortedindex(self):
         msg = "truncate requires a sorted index"
         with pytest.raises(ValueError, match=msg):
             df.truncate(before=2, after=20, axis=1)
+
+    @pytest.mark.parametrize(
+        "before, after, indices",
+        [(1, 2, [2, 1]), (None, 2, [2, 1, 0]), (1, None, [3, 2, 1])],
+    )
+    @pytest.mark.parametrize("klass", [pd.Int64Index, pd.DatetimeIndex])
+    def test_truncate_decreasing_index(self, before, after, indices, klass):
+        # https://github.com/pandas-dev/pandas/issues/33756
+        idx = klass([3, 2, 1, 0])
+        if klass is pd.DatetimeIndex:
+            before = pd.Timestamp(before) if before is not None else None
+            after = pd.Timestamp(after) if after is not None else None
+            indices = [pd.Timestamp(i) for i in indices]
+        values = pd.DataFrame(range(len(idx)), index=idx)
+        result = values.truncate(before=before, after=after)
+        expected = values.loc[indices]
+        tm.assert_frame_equal(result, expected)

pandas/tests/series/methods/test_truncate.py

@@ -80,6 +80,23 @@ def test_truncate_nonsortedindex(self):
         with pytest.raises(ValueError, match=msg):
             ts.sort_values(ascending=False).truncate(before="2011-11", after="2011-12")
 
+    @pytest.mark.parametrize(
+        "before, after, indices",
+        [(1, 2, [2, 1]), (None, 2, [2, 1, 0]), (1, None, [3, 2, 1])],
+    )
+    @pytest.mark.parametrize("klass", [pd.Int64Index, pd.DatetimeIndex])
+    def test_truncate_decreasing_index(self, before, after, indices, klass):
+        # https://github.com/pandas-dev/pandas/issues/33756
+        idx = klass([3, 2, 1, 0])
+        if klass is pd.DatetimeIndex:
+            before = pd.Timestamp(before) if before is not None else None
+            after = pd.Timestamp(after) if after is not None else None
+            indices = [pd.Timestamp(i) for i in indices]
+        values = pd.Series(range(len(idx)), index=idx)
+        result = values.truncate(before=before, after=after)
+        expected = values.loc[indices]
+        tm.assert_series_equal(result, expected)
+
     def test_truncate_datetimeindex_tz(self):
         # GH 9243
         idx = date_range("4/1/2005", "4/30/2005", freq="D", tz="US/Pacific")