数组扩展维度问题

import numpy as np

x_train=np.array([[3,5,6,4],[6,5,4,4]])
print(x_train.shape)
print(x_train)


print("n")
x_train=np.expand_dims(x_train,axis=1)
#x_train=np.expand_dims(x_train,axis=3)
print(x_train.shape)
print(x_train)