def random_permutation(iterable, r=None):
    """Return a random *r* length permutation of the elements in *iterable*.

    If *r* is not specified or is ``None``, then *r* defaults to the length of
    *iterable*.

        >>> random_permutation(range(5))  # doctest:+SKIP
        (3, 4, 0, 1, 2)

    This equivalent to taking a random selection from
    ``itertools.permutations(iterable, r)``.

    """
    pool = tuple(iterable)
    r = len(pool) if r is None else r
    return tuple(sample(pool, r))