ENH: allows Index - timedelta to make it symmetric to Index + timedelta

Author: Chang She

pandas/core/index.py

@@ -436,7 +436,10 @@ def __add__(self, other):
     __ge__ = _indexOp('__ge__')
 
     def __sub__(self, other):
-        return self.diff(other)
+        if isinstance(other, Index):
+            return self.diff(other)
+        else:
+            return Index(self.view(np.ndarray) - other)
 
     def __and__(self, other):
         return self.intersection(other)

pandas/tests/test_index.py

@@ -221,8 +221,20 @@ def test_add(self):
         tm.assert_contains_all(self.strIndex, secondCat)
         tm.assert_contains_all(self.dateIndex, firstCat)
 
+    def test_timedelta(self):
         # this is valid too
         shifted = self.dateIndex + timedelta(1)
+        back = shifted + timedelta(-1)
+        self.assert_(tm.equalContents(self.dateIndex, back))
+        self.assertEqual(shifted.freq, self.dateIndex.freq)
+        self.assertEqual(shifted.freq, back.freq)
+
+        lead = self.dateIndex - timedelta(1)
+        back = lead - timedelta(-1)
+        self.assert_(tm.equalContents(self.dateIndex, back))
+        self.assertEqual(self.dateIndex.freq, back.freq)
+        self.assertEqual(lead.freq, back.freq)
+
 
     def test_append_multiple(self):
         index = Index(['a', 'b', 'c', 'd', 'e', 'f'])