list index()

[MRAB]

On Aug 30, 7:00 pm, Steve Holden <st... at holdenweb.com> wrote:
> zzbba... at aol.com wrote:
> >> How could it not be an exception, in the plain English sense of the
> >> word? Most certainly you're asking for the index because you want to do
> >> something with the index. If the item is not found, you have no index,
> >> so that's a special case that must be handled separately. There is no
> >> logical difference between handling that special case in an except
> >> clause versus handling it with an if-branch.
>
> > 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.
> > I have used exceptions in other languages and only do so on logic that
> > should never happen. In this case it is known that some of the files
> > will not be in both lists. I just want to know which ones.
>
> And, as is so often the case, once the *real* problem is stated a more
> elegant solution become available - in this case, using sets.
>
> afiles = set(os.listdir(dira))
> bfiles = set(os.listdir(dirb))
>
> for f in (afiles - bfiles):
>    print "Not common:", f
>
> You can also generate the files that are in one directory but ot the
> other with
>
> (afiles | bfiles) - (afiles & bfiles)
>
... which can be written more concisely as:

    afiles ^ bfiles

:-)