尺寸为3 x 3 x 3的三维阵列，在其三个轴中的每个轴上求和

>>>>>> x array([[[ 0,  1,  2],         [ 3,  4,  5],         [ 6,  7,  8]],        [[ 9, 10, 11],         [12, 13, 14],         [15, 16, 17]],        [[18, 19, 20],         [21, 22, 23],         [24, 25, 26]]]) 
>>> x.sum(axis=0) array([[27, 30, 33],        [36, 39, 42],        [45, 48, 51]]) 
>>> # for sum, axis is the first keyword, so we may omit it, 
>>> # specifying only its value 
>>> x.sum(0), x.sum(1), x.sum(2) (array([[27, 30, 33],         [36, 39, 42],         [45, 48, 51]]),  array([[ 9, 12, 15],         [36, 39, 42],         [63, 66, 69]]),  array([[ 3, 12, 21],         [30, 39, 48],         [57, 66, 75]]))