ENH: support __radd__ operation on Index (GH10083)

Author: mortada

pandas/core/index.py

@@ -1181,12 +1181,18 @@ def argsort(self, *args, **kwargs):
     def __add__(self, other):
         if com.is_list_like(other):
             warnings.warn("using '+' to provide set union with Indexes is deprecated, "
-                          "use '|' or .union()",FutureWarning)
+                          "use '|' or .union()", FutureWarning)
         if isinstance(other, Index):
             return self.union(other)
         return Index(np.array(self) + other)
+
+    def __radd__(self, other):
+        if com.is_list_like(other):
+            warnings.warn("using '+' to provide set union with Indexes is deprecated, "
+                          "use '|' or .union()", FutureWarning)
+        return Index(other + np.array(self))
+
     __iadd__ = __add__
-    __radd__ = __add__
 
     def __sub__(self, other):
         warnings.warn("using '-' to provide set differences with Indexes is deprecated, "

pandas/tests/test_index.py

@@ -705,6 +705,13 @@ def test_add(self):
         tm.assert_contains_all(self.strIndex, secondCat)
         tm.assert_contains_all(self.dateIndex, firstCat)
 
+        # test add and radd
+        idx = Index(list('abc'))
+        expected = Index(['a1', 'b1', 'c1'])
+        self.assert_index_equal(idx + '1', expected)
+        expected = Index(['1a', '1b', '1c'])
+        self.assert_index_equal('1' + idx, expected)
+
     def test_append_multiple(self):
         index = Index(['a', 'b', 'c', 'd', 'e', 'f'])