'Sketch for a C code implementation of pairwise()'

null = object()

class Pairwise:

    __slots__ = ('it', 'old')

    def __init__(self, iterable):
        self.it = iter(iterable)
        self.old = null

    def __iter__(self):
        return self

    def __next__(self):
        if self.it is null:
            raise StopIteration
        if self.old is null:
            self.old = next(self.it, null)
            if self.old is null:
                self.it = null
                raise StopIteration
        old = self.old
        new = next(self.it, null)
        if new is null:
            self.it = null
            self.old = null
            raise StopIteration
        result = old, new
        self.old = new
        return result

if __name__ == '__main__':

    from itertools import tee

    def pairwise(iterable):
        "s -> (s0,s1), (s1,s2), (s2, s3), ..."
        a, b = tee(iterable)
        next(b, None)
        return zip(a, b)

    assert list(pairwise('')) == list(Pairwise(''))
    assert list(pairwise('a')) == list(Pairwise('a'))
    assert list(pairwise('ab')) == list(Pairwise('ab'))
    assert list(pairwise('abc')) == list(Pairwise('abc'))
    assert list(pairwise('abcd')) == list(Pairwise('abcd'))
    assert list(pairwise('abcde')) == list(Pairwise('abcde'))
    for pair in Pairwise('abcdefgh'):
        print(pair)