BUG: Bug in Categorical.__getitem__/__setitem__ with listlike input getting incorrect result from indexer coercion (GH9469)

doc/source/whatsnew/v0.16.0.txt

@@ -185,7 +185,7 @@ Bug Fixes
 - Fixed compatibility issue in ``DatetimeIndex`` affecting architectures where ``numpy.int_`` defaults to ``numpy.int32`` (:issue:`8943`)
 - Bug in Panel indexing with an object-like (:issue:`9140`)
 - Bug in the returned ``Series.dt.components`` index was reset to the default index (:issue:`9247`)
-
+- Bug in ``Categorical.__getitem__/__setitem__`` with listlike input getting incorrect results from indexer coercion (:issue:`9469`)
 
 - Fixed bug in ``to_sql`` when mapping a ``Timestamp`` object column (datetime
   column with timezone info) to the according sqlalchemy type (:issue:`9085`).

pandas/core/categorical.py

@@ -1229,7 +1229,6 @@ def __getitem__(self, key):
             else:
                 return self.categories[i]
         else:
-            key = self._maybe_coerce_indexer(key)
             return Categorical(values=self._codes[key], categories=self.categories,
                                ordered=self.ordered, fastpath=True)
 
@@ -1253,6 +1252,7 @@ def __setitem__(self, key, value):
 
         rvalue = value if is_list_like(value) else [value]
         to_add = Index(rvalue).difference(self.categories)
+
         # no assignments of values not in categories, but it's always ok to set something to np.nan
         if len(to_add) and not isnull(to_add).all():
             raise ValueError("cannot setitem on a Categorical with a new category,"
@@ -1297,7 +1297,6 @@ def __setitem__(self, key, value):
             nan_pos = np.where(isnull(self.categories))[0]
             lindexer[lindexer == -1] = nan_pos
 
-        key = self._maybe_coerce_indexer(key)
         lindexer = self._maybe_coerce_indexer(lindexer)
         self._codes[key] = lindexer
 

pandas/tests/test_categorical.py

@@ -34,6 +34,50 @@ def test_getitem(self):
         subf = self.factor[np.asarray(self.factor) == 'c']
         tm.assert_almost_equal(subf._codes, [2, 2, 2])
 
+    def test_getitem_listlike(self):
+
+        # GH 9469
+        # properly coerce the input indexers
+        np.random.seed(1)
+        c = Categorical(np.random.randint(0, 5, size=150000).astype(np.int8))
+        result = c.codes[np.array([100000]).astype(np.int64)]
+        expected = c[np.array([100000]).astype(np.int64)].codes
+        self.assert_numpy_array_equal(result, expected)
+
+    def test_setitem(self):
+
+        # int/positional
+        c = self.factor.copy()
+        c[0] = 'b'
+        self.assertEqual(c[0], 'b')
+        c[-1] = 'a'
+        self.assertEqual(c[-1], 'a')
+
+        # boolean
+        c = self.factor.copy()
+        indexer = np.zeros(len(c),dtype='bool')
+        indexer[0] = True
+        indexer[-1] = True
+        c[indexer] = 'c'
+        expected = Categorical.from_array(['c', 'b', 'b', 'a',
+                                           'a', 'c', 'c', 'c'])
+
+        self.assert_categorical_equal(c, expected)
+
+    def test_setitem_listlike(self):
+
+        # GH 9469
+        # properly coerce the input indexers
+        np.random.seed(1)
+        c = Categorical(np.random.randint(0, 5, size=150000).astype(np.int8)).add_categories([-1000])
+        indexer = np.array([100000]).astype(np.int64)
+        c[indexer] = -1000
+
+        # we are asserting the code result here
+        # which maps to the -1000 category
+        result = c.codes[np.array([100000]).astype(np.int64)]
+        self.assertEqual(result, np.array([5], dtype='int8'))
+
     def test_constructor_unsortable(self):
 
         # it works!