[Python-checkins] Speed-up building enums by value, e.g. http.HTTPStatus(200) (#11318)

[Ethan Furman]

https://github.com/python/cpython/commit/34ae04f74dcf4ac97d07c3e82eaf8f619d80cedb
commit: 34ae04f74dcf4ac97d07c3e82eaf8f619d80cedb
branch: master
author: Andrew Svetlov <andrew.svetlov at gmail.com>
committer: Ethan Furman <ethan at stoneleaf.us>
date: 2018-12-26T10:45:33-08:00
summary:

Speed-up building enums by value, e.g. http.HTTPStatus(200) (#11318)

bpo-35585: Speed up enum by-value lookup

files:
A Misc/NEWS.d/next/Library/2018-12-26-02-28-00.bpo-35585.Lkzd3Z.rst
M Lib/enum.py

diff --git a/Lib/enum.py b/Lib/enum.py
index fec1aed9b25c..f7452f0cc0aa 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -563,8 +563,10 @@ def __new__(cls, value):
         # by-value search for a matching enum member
         # see if it's in the reverse mapping (for hashable values)
         try:
-            if value in cls._value2member_map_:
-                return cls._value2member_map_[value]
+            return cls._value2member_map_[value]
+        except KeyError:
+            # Not found, no need to do long O(n) search
+            pass
         except TypeError:
             # not there, now do long search -- O(n) behavior
             for member in cls._member_map_.values():
diff --git a/Misc/NEWS.d/next/Library/2018-12-26-02-28-00.bpo-35585.Lkzd3Z.rst b/Misc/NEWS.d/next/Library/2018-12-26-02-28-00.bpo-35585.Lkzd3Z.rst
new file mode 100644
index 000000000000..247a4ae6800f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-12-26-02-28-00.bpo-35585.Lkzd3Z.rst
@@ -0,0 +1 @@
+Speed-up building enums by value, e.g. http.HTTPStatus(200).