import random


def random_saturation(im, lower=0.5, upper=1.5):
    """Random change saturation
    
    Args:
        im(numpy.array): one H,W,3 image
        lower(float): change lower limit 
        upper(float): change upper limit
    """
    if random.random() < 0.5:
        im[:, :, 1] *= random.uniform(lower, upper)
    return im