def standard_deviation_of_a_sample(*variables, mean=None):
    """Calculate the standard deviation of a sample.
       Its arithmetic mean can be given
       but is not required.
       Input: *float (all data points as arguments), float
       Output: float
    """

    sigma, sample_size = 0, 0
    for x in variables:
        sigma += (x - mean)**2
        sample_size += 1

    if sample_size < 2:
        return 0

    sigma = (sigma / (sample_size-1))**0.5

    return sigma