dummy = object()
def pair(g):
    """
    yields successive adjacent pairs of values.
    the first and last yielded values will be paired with a dummy value, indicating the boundaries of the iterator.
    """
    last_value = dummy
    for item in g:
        yield last_value, item
        last_value = item
    if last_value is not dummy:
        yield (last_value, dummy)

print(list(pair("hello")))