Backport PR pandas-dev#35547: Bug fix one element series truncate

Author: gabicca

doc/source/whatsnew/v1.1.1.rst

@@ -48,7 +48,7 @@ Categorical
 
 **Indexing**
 
--
+- Bug in :meth:`Series.truncate` when trying to truncate a single-element series (:issue:`35544`)
 
 **DataFrame**
 - Bug in :class:`DataFrame` constructor failing to raise ``ValueError`` in some cases when data and index have mismatched lengths (:issue:`33437`)

pandas/core/generic.py

@@ -9397,7 +9397,7 @@ def truncate(
             if before > after:
                 raise ValueError(f"Truncate: {after} must be after {before}")
 
-        if ax.is_monotonic_decreasing:
+        if len(ax) > 1 and ax.is_monotonic_decreasing:
             before, after = after, before
 
         slicer = [slice(None, None)] * self._AXIS_LEN

pandas/tests/series/methods/test_truncate.py

@@ -141,3 +141,14 @@ def test_truncate_multiindex(self):
         expected = df.col
 
         tm.assert_series_equal(result, expected)
+
+    def test_truncate_one_element_series(self):
+        # GH 35544
+        series = pd.Series([0.1], index=pd.DatetimeIndex(["2020-08-04"]))
+        before = pd.Timestamp("2020-08-02")
+        after = pd.Timestamp("2020-08-04")
+
+        result = series.truncate(before=before, after=after)
+
+        # the input Series and the expected Series are the same
+        tm.assert_series_equal(result, series)