How can I sum a column of a list?

Try this:

a = [[1,2,3],
     [1,2,3]]

print [sum(x) for x in zip(*a)]

zip function description

Using a for loop (in a generator expression):

data = [[1,2,3],
        [1,2,3]]

column = 1
print(sum(row[column] for row in data))  # -> 4