BUG: address GH #185, prevent use of step in label-based slicing

pandas/core/indexing.py

@@ -198,6 +198,10 @@ def _convert_to_indexer(self, obj, axis=0):
         if isinstance(obj, slice):
             if _is_label_slice(index, obj):
                 i, j = index.slice_locs(obj.start, obj.stop)
+
+                if obj.step is not None:
+                    raise Exception('Non-zero step not supported with '
+                                    'label-based slicing')
                 return slice(i, j)
             else:
                 return obj
@@ -238,6 +242,10 @@ def _get_slice_axis(self, slice_obj, axis=0):
         if _is_label_slice(labels, slice_obj):
             i, j = labels.slice_locs(slice_obj.start, slice_obj.stop)
             slicer = slice(i, j)
+
+            if slice_obj.step is not None:
+                raise Exception('Non-zero step not supported with label-based '
+                                'slicing')
         else:
             slicer = slice_obj
 

pandas/tests/test_frame.py

@@ -252,6 +252,10 @@ def test_getitem_fancy_2d(self):
         exp.values[5:10] = 5
         assert_frame_equal(f, exp)
 
+    def test_getitem_fancy_slice_integers_step(self):
+        df = DataFrame(np.random.randn(10, 5))
+        self.assertRaises(Exception, df.ix.__getitem__, slice(0, 8, 2))
+
     def test_setitem_fancy_2d(self):
         f = self.frame
         ix = f.ix