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gh-98766: Lazier ChainMap methods by zrothberg · Pull Request #98767 · python/cpython · GitHub
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gh-98766: Lazier ChainMap methods #98767

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71b6de5
Lazy slicing and iter
zrothberg Oct 27, 2022
eabdf02
correct key_view spelling to match outer
zrothberg Oct 27, 2022
14b21cd
Fix typos
zrothberg Oct 27, 2022
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Lazy slicing and iter
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@zrothberg
zrothberg committed Oct 27, 2022
commit 71b6de5e626078a1c7c9eef531dac826fbe439c3
10 changes: 5 additions & 5 deletions Lib/collections/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,7 @@
from itertools import chain as _chain
from itertools import repeat as _repeat
from itertools import starmap as _starmap
from itertools import islice as _isclice
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from keyword import iskeyword as _iskeyword
from operator import eq as _eq
from operator import itemgetter as _itemgetter
Expand Down Expand Up @@ -1010,9 +1011,8 @@ def __len__(self):
return len(set().union(*self.maps)) # reuses stored hash values if possible

def __iter__(self):
d = {}
for mapping in reversed(self.maps):
d.update(dict.fromkeys(mapping)) # reuses stored hash values if possible
lazy_views = (keyview.keys() for keyview in reversed(self.maps))
d = dict.fromkeys(_chain.from_iterable(lazy_views))

@rhettinger rhettinger Oct 27, 2022

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This doesn't do what you think it does. The chain extracts the keys from the views and then passes those through to dict.fromkeys(). That defeats the goal of retaining the precomputed hash values stored in the underlying mapping.

The dict.fromkeys() class method can use the precomputed hash values ONLY if the input is an exact dict or exact set. An iterable of keys will not suffice.

Also, there isn't any "laziness" at all in these two lines. You make a generator expression and then consume it immediately. It is not much different from converting sum([10, 20, 30]) to sum(iter([10, 20, 30]))` and calling it "lazy".

return iter(d)

def __contains__(self, key):
Expand All @@ -1032,7 +1032,7 @@ def fromkeys(cls, iterable, *args):

def copy(self):
'New ChainMap or subclass with a new copy of maps[0] and refs to maps[1:]'
return self.__class__(self.maps[0].copy(), *self.maps[1:])
return self.__class__(self.maps[0].copy(), *_isclice(self.maps, 1, None))

__copy__ = copy

Expand All @@ -1050,7 +1050,7 @@ def new_child(self, m=None, **kwargs): # like Django's Context.push()
@property
def parents(self): # like Django's Context.pop()
'New ChainMap from maps[1:].'
return self.__class__(*self.maps[1:])
return self.__class__(*_isclice(self.maps, 1, None))

def __setitem__(self, key, value):
self.maps[0][key] = value
Expand Down