Getting the member of a singleton set

[Gabriel Genellina]

En Sun, 20 May 2007 17:27:20 -0300, Arnaud Delobelle  
<arnodel at googlemail.com> escribió:

> Hi all,
>
> I often find myself needing to get (non-destructively) the value of
> the member of a singleton set.  Is there a good way to do this (as an
> expression?)  None of the ones I can think of satisfy me, eg:
>
> * list(myset)[0]
> * iter(myset).next()
> * set(myset).pop()
>
> What I would like is something like a 'peek()' function or method
> (would return the same as pop() but wouldn't pop anything).  I would
> like to know of a better idiom if it exists.  If not, isn't there a
> need for one?

Yes, something like peek() or any() would be useful. But you're not  
restricted by the builtin methods, you could write your own:

def peek(iterable):
   return iter(iterable).next()

maybe converting the possible StopIteration into another exception like  
EmptyContainer(ValueError).

> Note: it is comparatively easier to do this destructively:
>     myset.pop()
> or to bind a name to the member:
>     element, = myset

If you know that your set contains exactly one element, I like the later  
form.

> PS: this problem is not restricted to sets but could occur with many
> 'container' types.

Yes, and I've seen all your expressions, plus some more, like: for x in  
container: break
All of them are rather ugly...

-- 
Gabriel Genellina