numpy切片引用一列

import numpy as np
a = np.array([[1,2,3], [4,5,6], [7,8,9]])
b = a[:,[-1]]
c = a[:,-1]
[-1] 加中括号表示引用最后一列，输出二维
[[3]
[6]
[9]]
-1 不加中括号表示引用最后一列，输出一维
[3 6 9]