ENH: Eliminate __setitem__ in pd.Index

`Index` inherits from np.ndarray, yet implements an immutable datatype,
Pythonic duck-typing would suggest that the presence of __setitem__
signals a mutable datatype, while the overriden implementaion
just raises an exception - a bait and switch.
Python does not offer a really clean way to eliminate an attribute
inherited from a superclass, but overriding __getattribute__ gives
us the same end result.
squash with 218fe0a

Author: y-p

pandas/core/index.py

@@ -305,9 +305,11 @@ def __contains__(self, key):
     def __hash__(self):
         return hash(self.view(np.ndarray))
 
-    def __setitem__(self, key, value):
-        """Disable the setting of values."""
-        raise Exception(str(self.__class__) + ' object is immutable')
+    def __getattribute__(self,name):
+        if name=="__setitem__": # emulate an Immutable ndarray
+            raise AttributeError(str(self.__class__) + ' object is immutable')
+        else:
+            return object.__getattribute__(self,name)
 
     def __getitem__(self, key):
         """Override numpy.ndarray's __getitem__ method to work as desired"""

pandas/tests/test_format.py

@@ -765,7 +765,7 @@ def test_to_html_with_classes(self):
             <table border="1" class="dataframe sortable draggable">
               <tbody>
                 <tr>
-                  <td>Index([], dtype=object)</td>
+                  <td>Index((), dtype=object)</td>
                   <td>Empty DataFrame</td>
                 </tr>
               </tbody>

pandas/tests/test_index.py

@@ -57,8 +57,7 @@ def test_sort(self):
         self.assertRaises(Exception, self.strIndex.sort)
 
     def test_mutability(self):
-        self.assertRaises(Exception, self.strIndex.__setitem__, 5, 0)
-        self.assertRaises(Exception, self.strIndex.__setitem__, slice(1,5), 0)
+        self.assertFalse(hasattr(self.strIndex,"__setitem__"))
 
     def test_constructor(self):
         # regular instance creation