为什么在代码中找不到“example_size”？

Question

import  numpy as np

def data_iter_random(data_indices, num_steps, batch_size):

    example_size = len(data_indices)/num_steps

    epoch_size = example_size/batch_size

    example = [data_indices[i*num_steps:i*num_steps + num_steps] 

              for i in range(int(example_size))]

    shuffle_example = np.random.shuffle(example)

    print(shuffle_example)

data_iter_random(list(range(30)), 5, 2)

输出是 None

谁能告诉我出了什么问题？