bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ (GH-23534)
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@ -1001,7 +1001,7 @@ class ChainMap(_collections_abc.MutableMapping):
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def __iter__(self):
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d = {}
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for mapping in reversed(self.maps):
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d.update(mapping) # reuses stored hash values if possible
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d.update(dict.fromkeys(mapping)) # reuses stored hash values if possible
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return iter(d)
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def __contains__(self, key):
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@ -196,6 +196,22 @@ class TestChainMap(unittest.TestCase):
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('e', 55), ('f', 666), ('g', 777), ('h', 88888),
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('i', 9999), ('j', 0)])
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def test_iter_not_calling_getitem_on_maps(self):
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class DictWithGetItem(UserDict):
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def __init__(self, *args, **kwds):
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self.called = False
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UserDict.__init__(self, *args, **kwds)
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def __getitem__(self, item):
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self.called = True
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UserDict.__getitem__(self, item)
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d = DictWithGetItem(a=1)
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c = ChainMap(d)
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d.called = False
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set(c) # iterate over chain map
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self.assertFalse(d.called, '__getitem__ was called')
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def test_dict_coercion(self):
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d = ChainMap(dict(a=1, b=2), dict(b=20, c=30))
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self.assertEqual(dict(d), dict(a=1, b=2, c=30))
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@ -0,0 +1 @@
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ChainMap.__iter__ no longer calls __getitem__ on underlying maps
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