python .npy 存取 dict

import numpy as np

dict = {'a' : {1,2,3}, 'b': {4,5,6}}

np.save('dict.npy', dict)
    
dict_load=np.load('dict.npy', allow_pickle=True)
    
print("dict =", dict_load.item())
print("dict['a'] =", dict_load.item()['a'])

读取的时候如果不用dict_load.item()['a'],而是直接用dict_load['a']，会报如下错误：

IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices