make CategoricalIndex.__contains__ compatible with np<1.13

doc/source/whatsnew/v0.23.2.txt

@@ -24,7 +24,9 @@ Fixed Regressions
 Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
--
+- Improved performance of membership checks in :class:`CategoricalIndex`
+  (i.e. ``x in ci``-style checks are much faster). :meth:`CategoricalIndex.contains`
+  is likewise much faster (:issue:`21369`)
 -
 
 Documentation Changes

doc/source/whatsnew/v0.24.0.txt

@@ -65,9 +65,6 @@ Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 - Improved performance of :func:`Series.describe` in case of numeric dtpyes (:issue:`21274`)
-- Improved performance of membership checks in :class:`CategoricalIndex`
-  (i.e. ``x in ci``-style checks are much faster). :meth:`CategoricalIndex.contains`
-  is likewise much faster (:issue:`21369`)
 
 .. _whatsnew_0240.docs:
 

pandas/core/indexes/category.py

@@ -324,18 +324,31 @@ def _reverse_indexer(self):
     @Appender(_index_shared_docs['__contains__'] % _index_doc_kwargs)
     def __contains__(self, key):
         hash(key)
-        if isna(key):
+
+        if isna(key):  # is key NaN?
             return self.isna().any()
-        elif self.categories._defer_to_indexing:  # e.g. Interval values
+
+        # is key in self.categories? Then get its location.
+        # If not (i.e. KeyError), it logically can't be in self either
+        try:
             loc = self.categories.get_loc(key)
-            return np.isin(self.codes, loc).any()
-        elif key in self.categories:
-            return self.categories.get_loc(key) in self._engine
-        else:
+        except KeyError:
             return False
 
+        # loc is the location of key in self.categories, but also the value
+        # for key in self.codes and in self._engine. key may be in categories,
+        # but still not in self, check this. Example:
+        # 'b' in CategoricalIndex(['a'], categories=['a', 'b']) #  False
+        if is_scalar(loc):
+            return loc in self._engine
+        else:
+            # if self.categories is IntervalIndex, loc is an array
+            # check if any scalar of the array is in self._engine
+            return any(loc_ in self._engine for loc_ in loc)
+
     @Appender(_index_shared_docs['contains'] % _index_doc_kwargs)
     def contains(self, key):
+        hash(key)
         return key in self
 
     def __array__(self, dtype=None):