class IterN (object):
    def __init__ (self, iterable, n, end_marker=None):
        if n == 1:
            raise TypeError ("n = 1? woot?")

        self.iterator = iter (iterable)
        self.n = n
        self.end_marker = end_marker
        self.finished = False

    def __iter__ (self):
        return self

    def next (self):
        if self.finished:
            raise StopIteration ()

        res = []
        i = 0
        try:
            # pull n items from the iterator
            for i in range (self.n):
                res.append (self.iterator.next ())
            # set the first-invalid-index indicator to one past the length
            i += 1
        except StopIteration, x:
            if i == 0:
                # we couldn't get anything from the iterator
                raise StopIteration ()

            # we got less than expected, fix the remaining with end markers and
            # make sure to stop the next time
            res.extend ([self.end_marker] * (self.n - i))
            self.finished = True
        return res, i

def main ():
    for seq, n in IterN ([1, 2, 3, 4, 5], 2):
        if n < 2:
            break

        a, b = seq
        # do something useful with a and b
        print a, b

if __name__ == '__main__':
    main ()