From: Raymond Hettinger <rhettinger@users.noreply.github.com>
Date: Tue, 1 Nov 2022 04:44:40 +0000 (-0500)
Subject: GH-98766: Modest speed-up from ChainMap.__iter__ (GH-98946)
X-Git-Tag: v3.12.0a2~179
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=f5afb7f2331efa8f64080576a75517c3a96442b9;p=thirdparty%2FPython%2Fcpython.git

GH-98766: Modest speed-up from ChainMap.__iter__ (GH-98946)
---

diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index 58607874be93..f07ee143a5af 100644
--- a/Lib/collections/__init__.py
+++ b/Lib/collections/__init__.py
@@ -1011,8 +1011,8 @@ class ChainMap(_collections_abc.MutableMapping):
 
     def __iter__(self):
         d = {}
-        for mapping in reversed(self.maps):
-            d.update(dict.fromkeys(mapping))    # reuses stored hash values if possible
+        for mapping in map(dict.fromkeys, reversed(self.maps)):
+            d |= mapping                        # reuses stored hash values if possible
         return iter(d)
 
     def __contains__(self, key):