list index()

[Alex Martelli]

<zzbbaadd at aol.com> 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)...).


Alex