Added __iadd__ test, fixed whatsnew

Author: benthayer

doc/source/whatsnew/v0.20.0.txt

@@ -536,7 +536,8 @@ Deprecations
 - ``Series.sortlevel`` and ``DataFrame.sortlevel`` have been deprecated in favor of ``Series.sort_index`` and ``DataFrame.sort_index`` (:issue:`15099`)
 - importing ``concat`` from ``pandas.tools.merge`` has been deprecated in favor of imports from the ``pandas`` namespace. This should only affect explict imports (:issue:`15358`)
 - ``Series/DataFrame/Panel.consolidate()`` been deprecated as a public method. (:issue:`15483`)
-- ``FrozenList.__add__`` has been depricated in favor of ``FrozenList.union`` (:issue: `15475`)
+- ``FrozenList.__add__`` has been depricated. (:issue: `15475`)
+- ``FrozenList.__iadd__`` has been depricated. (:issue: `15475`)
 
 .. _whatsnew_0200.prior_deprecations:
 

pandas/indexes/frozen.py

@@ -32,7 +32,9 @@ def __add__(self, other):
 
     def __iadd__(self, other):
         warnings.warn("__iadd__ is deprecated, use union(...)", FutureWarning)
-        return self.union(other)
+        if isinstance(other, tuple):
+            other = list(other)
+        return super(FrozenList, self).__iadd__(other)
 
     # Python 2 compat
     def __getslice__(self, i, j):

pandas/tests/indexes/test_frozen.py

@@ -17,6 +17,13 @@ def setUp(self):
     def test_add(self):
         self.assert_produces_warning(FutureWarning)
 
+    def test_iadd(self):
+        q = q1 = [1, 2, 3, 4, 5]
+        q = FrozenList(q)
+        q += [1, 2, 3]
+        expected = FrozenList(q1 + [1, 2, 3])
+        self.check_result(q, expected)
+
     def test_union(self):
         result = self.container.union((1, 2, 3))
         expected = FrozenList(self.lst + [1, 2, 3])