'Returns a `set` of random unique elements for the specified length.'

@classmethod
def random_sample_unique(cls, elements=('a', 'b', 'c'), length=None):

    if (length is None):
        length = random.randint(1, len(elements))
    if (length > len(elements)):
        raise ValueError('Sample    length    cannot    be    longer    than    the    number    of    elements    to    pick    from.')
    sample = set()
    while (len(sample) < length):
        sample.add(cls.random_element(elements))
    return sample
