A[::-1]的含义

>>> import numpy as np
>>> a=np.arange(4)
>>> a
array([0, 1, 2, 3])
>>> a[::-1]
array([3, 2, 1, 0])
>>> b=a.reshape(2,2)
>>> b
array([[0, 1],
       [2, 3]])
>>> b[::-1]
array([[2, 3],
       [0, 1]])