Stricter testing of 'monotocity' when reindexing with ffill or bfill.

Author: prossahl

pandas/core/index.py

@@ -811,12 +811,12 @@ def get_indexer(self, target, method=None, limit=None):
                             'objects')
 
         if method == 'pad':
-            if not self.is_monotonic:
-                raise AssertionError('Must be monotonic for forward fill')
+            if not self.is_monotonic or not target.is_monotonic:
+                raise ValueError('Must be monotonic for forward fill')
             indexer = self._engine.get_pad_indexer(target.values, limit)
         elif method == 'backfill':
-            if not self.is_monotonic:
-                raise AssertionError('Must be monotonic for backward fill')
+            if not self.is_monotonic or not target.is_monotonic:
+                raise ValueError('Must be monotonic for backward fill')
             indexer = self._engine.get_backfill_indexer(target.values, limit)
         elif method is None:
             indexer = self._engine.get_indexer(target.values)

pandas/tests/test_frame.py

@@ -1629,6 +1629,29 @@ def test_nested_exception(self):
             repr(df)
         except Exception as e:
             self.assertNotEqual(type(e), UnboundLocalError)
+            
+    def test_reverse_reindex_ffill_raises(self):
+        dr = pd.date_range('2013-08-01', periods=6, freq='B')
+        data = np.random.randn(6,1)
+        df = pd.DataFrame(data, index=dr, columns=list('A'))
+        df['A'][3] = np.nan
+        df_rev = pd.DataFrame(data, index=dr[::-1], columns=list('A'))
+        # Reverse index is not 'monotonic'
+        self.assertRaises(ValueError, df_rev.reindex, df.index, method='pad')
+        self.assertRaises(ValueError, df_rev.reindex, df.index, method='ffill')
+        self.assertRaises(ValueError, df_rev.reindex, df.index, method='bfill')
+        
+    def test_reversed_reindex_ffill_raises(self):
+        dr = pd.date_range('2013-08-01', periods=6, freq='B')
+        data = np.random.randn(6,1)
+        df = pd.DataFrame(data, index=dr, columns=list('A'))
+        df['A'][3] = np.nan
+        df = pd.DataFrame(data, index=dr, columns=list('A'))
+        # Reversed reindex is not 'monotonic'
+        self.assertRaises(ValueError, df.reindex, dr[::-1], method='pad')
+        self.assertRaises(ValueError, df.reindex, dr[::-1], method='ffill')
+        self.assertRaises(ValueError, df.reindex, dr[::-1], method='bfill')
+        
 
 _seriesd = tm.getSeriesData()
 _tsd = tm.getTimeSeriesData()

pandas/tseries/tests/test_timeseries.py

@@ -546,7 +546,7 @@ def test_pad_require_monotonicity(self):
 
         rng2 = rng[::2][::-1]
 
-        self.assertRaises(AssertionError, rng2.get_indexer, rng,
+        self.assertRaises(ValueError, rng2.get_indexer, rng,
                           method='pad')
 
     def test_frame_ctor_datetime64_column(self):