Python: Finding average of a nested list

Question

I have a list

a = [[1,2,3],[4,5,6],[7,8,9]]

Now I want to find the average of these inner list so that

a = [(1+4+7)/3,(2+5+8)/         


        

Answer 1

>>> import itertools
>>> [sum(x)/len(x) for x in itertools.izip(*a)]
[4, 5, 6]