TST: Do not allow inplace operations on indexes that require type promotion.

Author: josteinbf

pandas/tests/test_index.py

@@ -2078,6 +2078,12 @@ def test_coerce_list(self):
     def test_dtype(self):
         self.assertEqual(self.index.dtype, np.int64)
 
+    def test_promote_type_inplace(self):
+        # Related to GH-9966
+        index = Int64Index(np.arange(5))
+        with self.assertRaises(TypeError):
+            index *= 0.333
+
     def test_is_monotonic(self):
         self.assertTrue(self.index.is_monotonic)
         self.assertTrue(self.index.is_monotonic_increasing)