np.newaxis

在原有得到矩阵中增加一个axis，

实际上上np.newaxis的值为None

print(np.newaxis)
#None

np.newaxis的位置决定要在哪个维度增加

a = np.arange(1, 10)
a.shape
#(9,)
print(a[np.newaxis].shape)
#(1, 9)
print(a[:, np.newaxis].shape)
# (9, 1)
print(a[:, np.newaxis, np.newaxis].shape)
# (9, 1, 1)
print(a[:, np.newaxis, np.newaxis, np.newaxis].shape)
# (9, 1, 1, 1)

a = np.arange(1, 10).reshape(3, 3)
print(a.shape)
b = a[:, np.newaxis, np.newaxis]
print(b.shape)