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From: Peter Otten <__peter__@web.de>
Subject: Re: Python 3: dict & dict.keys()
Date: Thu, 25 Jul 2013 10:13:47 +0200
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Xref: csiph.com comp.lang.python:51205

Chris Angelico wrote:

> On Thu, Jul 25, 2013 at 5:04 PM, Steven D'Aprano
> <steve+comp.lang.python@pearwood.info> wrote:
>> - Views support efficient (O(1) in the case of keys) membership testing,
>> which neither iterkeys() nor Python2 keys() does.
> 
> To save me the trouble and potential error of digging through the
> source code: What's the complexity of membership testing on
> values/items? Since you're calling it "efficient" it must be better
> than O(n) which the list form would be, yet it isn't O(1) or you
> wouldn't have qualified "in the case of keys". Does this mean
> membership testing of the values and items views is O(log n) in some
> way, eg a binary search?

keys() and items() is O(1); both look up the key in the dictionary and 
items() then proceeds to compare the value. values() is O(n).