list index()

Ricardo Aráoz ricaraoz at
Fri Aug 31 04:38:22 CEST 2007

Alex Martelli wrote:
> <zzbbaadd at> wrote:
>    ...
>> In my case of have done os.listdir() on two directories. I want to see
>> what files are in directory A that are not in directory B.
> So why would you care about WHERE, in the listdir of B, are to be found
> the files that are in A but not B?!  You should call .index only if you
> CARE about the position.
> def inAnotB(A, B):
>     inA = os.listdir(A)
>     inBs = set(os.listdir(B))
>     return [f for f in inA if f not in inBs]
> is the "one obvious way to do it" (the set(...) is just a simple and
> powerful optimization -- checking membership in a set is roughly O(1),
> while checking membership in a list of N items is O(N)...).

And what is the order of passing a list into a set? O(N)+?

> Alex

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