import itertools


def functional(a):
    return itertools.chain(
        itertools.chain.from_iterable(zip(a[1::2], a[::2])),
        [a[-1]] if len(a) % 2 else []
    )


def gener(it):
    while True:
        try:
            a = next(it)
        except StopIteration:
            break
        try:
            b = next(it)
        except StopIteration:
            yield a
            break
        yield b
        yield a


for upper in (10, 11):
    a = list(range(1, upper))

    print(list(functional(a)))
    print(list(gener(iter(a))))