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bpo-32792: Preserve mapping order in ChainMap() (GH-5586) (#GH-5617)

(cherry picked from commit 3793f95)

Co-authored-by: Raymond Hettinger <>
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miss-islington and rhettinger committed Feb 11, 2018
1 parent e603320 commit 170b3f79506480f78275a801822c9ff1283e16f2
@@ -914,7 +914,10 @@ def __len__(self):
return len(set().union(*self.maps)) # reuses stored hash values if possible

def __iter__(self):
return iter(set().union(*self.maps))
d = {}
for mapping in reversed(self.maps):
d.update(mapping) # reuses stored hash values if possible
return iter(d)

def __contains__(self, key):
return any(key in m for m in self.maps)
@@ -141,6 +141,23 @@ def __missing__(self, key):
with self.assertRaises(KeyError):

def test_order_preservation(self):
d = ChainMap(
OrderedDict(j=0, h=88888),
OrderedDict(i=9999, d=4444, c=3333),
OrderedDict(f=666, b=222, g=777, c=333, h=888),
OrderedDict(e=55, b=22),
OrderedDict(a=1, b=2, c=3, d=4, e=5),
self.assertEqual(''.join(d), 'abcdefghij')
[('a', 1), ('b', 222), ('c', 3333), ('d', 4444),
('e', 55), ('f', 666), ('g', 777), ('h', 88888),
('i', 9999), ('j', 0)])

def test_dict_coercion(self):
d = ChainMap(dict(a=1, b=2), dict(b=20, c=30))
self.assertEqual(dict(d), dict(a=1, b=2, c=30))
@@ -0,0 +1 @@
collections.ChainMap() preserves the order of the underlying mappings.

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