If some of the elements in the array are NaNs, you can use the nansum() function of Numpy. It returns the sum of array elements over a given axis treating NaNs as zeroes.
Here are examples:
>>> import numpy as np
>>> aa = np.array([[1,2,3],[4,np.nan,6],[7,8,9],[10,11,np.nan]])
>>> np.nansum(aa, axis=1)
array([ 6., 10., 24., 21.])
>>> np.nansum(aa, axis=0)
array([22., 21., 18.])