return random number distributed by student's t distribution with
    `df` degrees of freedom with the specified mean and standard deviation.

def rand_student_t(df, mu=0, std=1):
    """
    return random number distributed by student's t distribution with
    `df` degrees of freedom with the specified mean and standard deviation.
    """

    x = random.gauss(0, std)
    y = 2.0*random.gammavariate(0.5 * df, 2.0)
    return x / (math.sqrt(y / df)) + mu