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

rhettinger · web-flow · commit f5afb7f2331e · 2022-10-31T23:44:40.000-05:00
diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
@@ -1011,8 +1011,8 @@ def __len__(self):
 
     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):
