Python 3: dict & dict.keys()

[Ian Kelly]

On Thu, Jul 25, 2013 at 2:13 AM, Peter Otten <__peter__ at web.de> wrote:
> Chris Angelico wrote:
>
>> On Thu, Jul 25, 2013 at 5:04 PM, Steven D'Aprano
>> <steve+comp.lang.python at 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).

3.x values() is O(n) but avoids the unnecessary step of copying all the
values in the dict that you get when performing the same operation
using 2.x values().  Hence, although the asymptotic complexity is the
same, it's still more efficient.