BUG: scalar getitem with a CI

Author: jreback

doc/source/whatsnew/v0.20.0.txt

@@ -1628,6 +1628,7 @@ Indexing
 - Bug in the display of ``.info()`` where a qualifier (+) would always be displayed with a ``MultiIndex`` that contains only non-strings (:issue:`15245`)
 - Bug in ``pd.concat()`` where the names of ``MultiIndex`` of resulting ``DataFrame`` are not handled correctly when ``None`` is presented in the names of ``MultiIndex`` of input ``DataFrame`` (:issue:`15787`)
 - Bug in ``DataFrame.sort_index()`` and ``Series.sort_index()`` where ``na_position`` doesn't work with a ``MultiIndex`` (:issue:`14784`, :issue:`16604`)
+- Bug in indexing with a scalar and a ``CategoricalIndex`` (:issue:`16123`)
 
 I/O
 ^^^

pandas/core/indexes/category.py

@@ -10,7 +10,8 @@
     is_list_like,
     is_interval_dtype,
     is_scalar)
-from pandas.core.common import _asarray_tuplesafe
+from pandas.core.common import (_asarray_tuplesafe,
+                                _values_from_object)
 from pandas.core.dtypes.missing import array_equivalent
 
 
@@ -352,6 +353,22 @@ def get_loc(self, key, method=None):
             raise KeyError(key)
         return self._engine.get_loc(codes)
 
+    def get_value(self, series, key):
+        """
+        Fast lookup of value from 1-dimensional ndarray. Only use this if you
+        know what you're doing
+        """
+        try:
+            k = _values_from_object(key)
+            k = self._convert_scalar_indexer(k, kind='getitem')
+            indexer = self.get_loc(k)
+            return series.iloc[indexer]
+        except KeyError:
+            pass
+
+        # we might be a positional inexer
+        return super(CategoricalIndex, self).get_value(series, key)
+
     def _can_reindex(self, indexer):
         """ always allow reindexing """
         pass