a =np.array([1,2,3,4,5,6,7,8,9])
> print a
[1 2 3 4 5 6 7 8 9]

b =a[:3]
> print b
[1 2 3]

b =a[3:]
> print b
[4 5 6 7 8 9]

b =a[-3:]
> print b
[7 8 9]

b =a[:-3]
> print b
[1 2 3 4 5 6]

a=np.eye(5)
> b=a[:,:３]
> print a
[[1. 0. 0. 0. 0.]
 [0. 1. 0. 0. 0.]
 [0. 0. 1. 0. 0.]
 [0. 0. 0. 1. 0.]
 [0. 0. 0. 0. 1.]]
> print b
[[1. 0. 0.]
 [0. 1. 0.]
 [0. 0. 1.]
 [0. 0. 0.]
 [0. 0. 0.]]

b=a[:3,:3]
> print b
[[1. 0. 0.]
 [0. 1. 0.]
 [0. 0. 1.]]