recognizing empty iterators

[John Roth]

"Bengt Richter" <bokr at oz.net> wrote in message
news:bfhvnk$hvd$0 at 216.39.172.122...
> On 21 Jul 2003 07:26:15 -0700, mis6 at pitt.edu (Michele Simionato) wrote:
>
> >After a recent thread on .endswith, I have been thinking about iterators.
> >I realized that I don't know a satisfactory way to check if an
> >iterator is empty. In other words I am looking for an
> >"isempty" function to use in "if" statements such as
> >
> >if isempty(iterator):
> >   do_something()
> >
> >without side effects. Here are some unsatisfactory ways of implementing
> >"isempty":
> >
> >#1: converting to a list or tuple
> >def isempty(iterator):
> >    return not list(iterator)
> >
> >Easy, but one has to create the entire list, thus defecting the basic
> >purpose of the iterator, i.e. lazy evaluation.
> >
> >#2: checking for StopIteration
> >def isempty(iterator):
> >    try:
> >        iterator.next()
> >    except StopIteration:
> >        return True
> >    else:
> >        return False
> >
> >This works, for instance
> >
> >print isempty(iter([]))
> >
> >gives True and
> >
> >it=iter([1,2,3])
> >print isempty(it)
> >
> >gives False. However, there is a side effect: after the check, the
> >iterator has advanced of one step and now "it.next()" gives 2, not 1.
> >In order this to work without side effects, I should be able to restart
> >the iterator from the beginning, but I don't know how to do that.
> >Is it possible?
> >
> >#3: comparing with the empty iterator
> >
> >emptyiterator=iter([])
> >
> >it=iter([])
> >
> >if it == emptyiterator: print 'Ok!'
> >
> >This simply doesn't work.
> >
> >I wonder if the itertools module should contain a function to check for
> >empty iterators, thus simplifying my life ;) Of course, I may well be
> >missing something obvious, if so, please enlighten me.
> >
> If iterators had a .peek() method for 1-item lookahead, maybe that would
> be easy to implement. If empty, it could raise StopIteration, and
otherwise
> return what next() would return, without disturbing the state w.r.t.
next().
>
> Alternatively, it could return [nextitem] vs [] if at the end
> (without raising StopIteration).

I think that's two separate functions. isEmpty() is not the same
thing as willBeEmptyOnNextCall().

That said, there are places where a 1 item lookahead can be quite
valuable, and lack of a sliding window is the last major thing I'd
like to see addressed in the for loop.

John Roth

>
> Regards,
> Bengt Richter