python如何求列表平均值？

>>>  import numpy as np

>>> num1 = np.array([[1,2,3],[2,3,4],[3,4,5],[4,5,6]])

>>> now2 = np.mat(num1)

>>> now2

matrix([[1, 2, 3],

        [2, 3, 4],

        [3, 4, 5],

        [4, 5, 6]])

>>> np.mean(now2) # 对所有元素求均值

3.5

>>> np.mean(now2,0) # 压缩行，对各列求均值

matrix([[ 2.5,  3.5,  4.5]])

>>> np.mean(now2,1) # 压缩列，对各行求均值

matrix([[ 2.],

        [ 3.],

        [ 4.],

        [ 5.]])