From a07d5fc6d813d2ce7e00c039667328014e4bc7a0 Mon Sep 17 00:00:00 2001
From: tp <terji78@yaho.com>
Date: Wed, 6 Jun 2018 17:21:29 +0100
Subject: [PATCH] Add __contains__ to CategoricalIndex

---
 asv_bench/benchmarks/categoricals.py | 13 +++++++++++++
 doc/source/whatsnew/v0.23.1.txt      |  2 ++
 pandas/core/indexes/category.py      | 16 +++++++++-------
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/asv_bench/benchmarks/categoricals.py b/asv_bench/benchmarks/categoricals.py
index 5464e7cba22c3..48f42621d183d 100644
--- a/asv_bench/benchmarks/categoricals.py
+++ b/asv_bench/benchmarks/categoricals.py
@@ -193,3 +193,16 @@ def time_categorical_series_is_monotonic_increasing(self):
 
     def time_categorical_series_is_monotonic_decreasing(self):
         self.s.is_monotonic_decreasing
+
+
+class Contains(object):
+
+    goal_time = 0.2
+
+    def setup(self):
+        N = 10**5
+        self.ci = tm.makeCategoricalIndex(N)
+        self.cat = self.ci.categories[0]
+
+    def time_contains(self):
+        self.cat in self.ci
diff --git a/doc/source/whatsnew/v0.23.1.txt b/doc/source/whatsnew/v0.23.1.txt
index 9c29c34adb7dd..e23db591181fa 100644
--- a/doc/source/whatsnew/v0.23.1.txt
+++ b/doc/source/whatsnew/v0.23.1.txt
@@ -33,6 +33,8 @@ Performance Improvements
 
 - Improved performance of :meth:`CategoricalIndex.is_monotonic_increasing`, :meth:`CategoricalIndex.is_monotonic_decreasing` and :meth:`CategoricalIndex.is_monotonic` (:issue:`21025`)
 - Improved performance of :meth:`CategoricalIndex.is_unique` (:issue:`21107`)
+- 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:`21107`)
 -
 -
 
diff --git a/pandas/core/indexes/category.py b/pandas/core/indexes/category.py
index 150eca32e229d..9643d8c3ddc26 100644
--- a/pandas/core/indexes/category.py
+++ b/pandas/core/indexes/category.py
@@ -328,16 +328,18 @@ def __contains__(self, key):
         if self.categories._defer_to_indexing:
             return key in self.categories
 
-        return key in self.values
+        try:
+            code_value = self.categories.get_loc(key)
+        except KeyError:
+            if isna(key):
+                code_value = -1
+            else:
+                return False
+        return code_value in self._engine
 
     @Appender(_index_shared_docs['contains'] % _index_doc_kwargs)
     def contains(self, key):
-        hash(key)
-
-        if self.categories._defer_to_indexing:
-            return self.categories.contains(key)
-
-        return key in self.values
+        return key in self
 
     def __array__(self, dtype=None):
         """ the array interface, return my values """