finding items that occur before or after an item in lists

[Simon Forman]

Simon Forman wrote:
> I've got a function that I'd like to improve.
>
> It takes a list of lists and a "target" element, and it returns the set
> of the items in the lists that appear either before or after the target
> item.  (Actually, it's a generator, and I use the set class outside of
> it to collect the unique items, but you get the idea. ;-) )
>
> data = [
>     ['this', 'string', 'is', 'nice'],
>     ['this', 'string', 'sucks'],
>     ['string', 'not', 'good'],
>     ['what', 'a', 'string']
>     ]
>
> def f(target, ListOfLists):
>     for N in ListOfLists:
>         try:
>             i = N.index(target)
>         except ValueError:
>             continue
>
>         # item before target
>         if i: yield N[i - 1]
>
>         # item after target
>         try:
>             yield N[i + 1]
>         except IndexError:
>             pass
>
> print set(n for n in f('string', data))
>
> # Prints set(['this', 'not', 'is', 'sucks', 'a'])
>
>
> Now, this works and normally I'd be happy with this and not try to
> improve it unless I found that it was a bottleneck.  However, in this
> case I know that when a try..except statement fails (i.e. executes the
> except part of the statement) it's "slow", *and* I know that the real
> list of lists will have many lists in it in which the target item does
> not appear.
>
> That means that the first try..except statement will be executing it's
> except clause more than half the time, and probably *much* more than
> half the time.


Nevermind,  it turns out that each list in the list of lists will
*always* contain the target item,  making this task much less
interesting.

~Simon