ENH: Introduce UnsortedIndexError GH11897

Author: Dr-Irv

doc/source/whatsnew/v0.20.0.txt

@@ -41,6 +41,9 @@ Other enhancements
 
 - ``pd.read_excel`` now preserves sheet order when using ``sheetname=None`` (:issue:`9930`)
 
+- New ``UnsortedIndexError`` (subclass of ``KeyError``) thrown when indexing into an
+ unsorted index (:issue:'11897`)
+
 
 .. _whatsnew_0200.api_breaking:
 

pandas/indexes/multi.py

@@ -44,6 +44,8 @@
                                  _index_shared_docs)
 import pandas.indexes.base as ibase
 
+class UnsortedIndexError(KeyError):
+    pass
 
 class MultiIndex(Index):
     """
@@ -1936,7 +1938,8 @@ def get_locs(self, tup):
 
         # must be lexsorted to at least as many levels
         if not self.is_lexsorted_for_tuple(tup):
-            raise KeyError('MultiIndex Slicing requires the index to be fully '
+            raise UnsortedIndexError(
+                           'MultiIndex Slicing requires the index to be fully '
                            'lexsorted tuple len ({0}), lexsort depth '
                            '({1})'.format(len(tup), self.lexsort_depth))
 

pandas/tests/indexes/test_multi.py

@@ -10,6 +10,7 @@
                     CategoricalIndex, compat)
 from pandas.core.common import PerformanceWarning
 from pandas.indexes.base import InvalidIndexError
+from pandas.indexes.multi import UnsortedIndexError
 from pandas.compat import range, lrange, u, PY3, long, lzip
 
 import numpy as np
@@ -2535,3 +2536,19 @@ def test_dropna(self):
         msg = "invalid how option: xxx"
         with tm.assertRaisesRegexp(ValueError, msg):
             idx.dropna(how='xxx')
+
+    def test_unsortedindex(self):
+        #GH 11897
+        mi = pd.MultiIndex.from_tuples([('z','a'),('x','a'),('y','b'),
+                                        ('x','b'),('y','a'),('z','b')], 
+                                        names=['one','two'])
+        df = pd.DataFrame([[i,10*i] for i in lrange(6)], index=mi, 
+                          columns=['one','two'])
+
+        with assertRaises(UnsortedIndexError):
+            df.loc(axis=0)['z',:]
+        df.sort_index(inplace=True)
+        self.assertEqual(len(df.loc(axis=0)['z',:]), 2)
+        
+        with assertRaises(KeyError):
+            df.loc(axis=0)['q',:]