add tests and comments

Author: reidy-p

doc/source/whatsnew/v0.22.0.txt

@@ -40,7 +40,7 @@ Backwards incompatible API changes
 Other API Changes
 ^^^^^^^^^^^^^^^^^
 
--
+- :func:`Series.truncate` and :func:`DataFrame.truncate` will raise a ``ValueError`` if the index is not sorted (:issue:`17935`)
 -
 -
 

pandas/core/generic.py

@@ -6356,6 +6356,8 @@ def truncate(self, before=None, after=None, axis=None, copy=True):
         axis = self._get_axis_number(axis)
         ax = self._get_axis(axis)
 
+        # GH 17935
+        # Check that index is sorted
         if (not ax.is_monotonic_increasing and
            not ax.is_monotonic_decreasing):
             raise ValueError("truncate requires a sorted index")

pandas/tests/frame/test_timeseries.py

@@ -384,7 +384,9 @@ def test_truncate_nonsortedindex(self):
             df.truncate(before=3, after=9)
 
         rng = pd.date_range('2011-01-01', '2012-01-01', freq='W')
-        ts = pd.Series(np.random.randn(len(rng)), index=rng)
+        ts = pd.DataFrame({'A': np.random.randn(len(rng)),
+                           'B': np.random.randn(len(rng))}, 
+                           index=rng)
         with tm.assert_raises_regex(ValueError,
                                     'truncate requires a sorted index'):
             ts.sort_values(ascending=False).truncate(before='2011-11',

pandas/tests/series/test_timeseries.py

@@ -236,6 +236,22 @@ def test_truncate(self):
                       before=self.ts.index[-1] + offset,
                       after=self.ts.index[0] - offset)
 
+    def test_truncate_nonsortedindex(self):
+        # GH 17935
+
+        s = pd.Series(['a', 'b', 'c', 'd', 'e'],
+                       index=[5, 3, 2, 9, 0])
+        with tm.assert_raises_regex(ValueError,
+                                    'truncate requires a sorted index'):
+            s.truncate(before=3, after=9)
+
+        rng = pd.date_range('2011-01-01', '2012-01-01', freq='W')
+        ts = pd.Series(np.random.randn(len(rng)), index=rng)
+        with tm.assert_raises_regex(ValueError,
+                                    'truncate requires a sorted index'):
+            ts.sort_values(ascending=False).truncate(before='2011-11',
+                                                     after='2011-12')
+
     def test_asfreq(self):
         ts = Series([0., 1., 2.], index=[datetime(2009, 10, 30), datetime(
             2009, 11, 30), datetime(2009, 12, 31)])