From f5afb7f2331efa8f64080576a75517c3a96442b9 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <rhettinger@users.noreply.github.com>
Date: Mon, 31 Oct 2022 23:44:40 -0500
Subject: [PATCH] GH-98766: Modest speed-up from ChainMap.__iter__ (GH-98946)

---
 Lib/collections/__init__.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index 58607874be9..f07ee143a5a 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):