def seconds_str(num, prefix=None):
    r"""
    Returns:
        str

    Example:
        >>> # ENABLE_DOCTEST
        >>> from utool.util_str import *  # NOQA
        >>> import utool as ut
        >>> num_list = sorted([4.2 / (10.0 ** exp_)
        >>>                    for exp_ in range(-13, 13, 4)])
        >>> secstr_list = [seconds_str(num, prefix=None) for num in num_list]
        >>> result = (', '.join(secstr_list))
        >>> print(result)
        0.04 ns, 0.42 us, 4.20 ms, 0.04 ks, 0.42 Ms, 4.20 Gs, 42.00 Ts
    """
    exponent_list = [-12, -9, -6, -3, 0, 3, 6, 9, 12]
    small_prefix_list = ['p', 'n', 'u', 'm', '', 'k', 'M', 'G', 'T']
    #large_prefix_list = ['pico', 'nano', 'micro', 'mili', '', 'kilo', 'mega',
    # 'giga', 'tera']
    #large_suffix = 'second'
    small_suffix = 's'
    suffix = small_suffix
    prefix_list = small_prefix_list
    base = 10.0
    secstr = order_of_magnitude_str(num, base, prefix_list, exponent_list,
                                    suffix, prefix=prefix)
    return secstr