How do I sum the columns in 2D list?

Use zip

col_totals = [ sum(x) for x in zip(*my_list) ]

>>> map(sum,zip(*my_list))
[3, 6, 8, 10]

Or the itertools equivalent

>>> from itertools import imap, izip
>>> imap(sum,izip(*my_list))
<itertools.imap object at 0x00D20370>
>>> list(_)
[3, 6, 8, 10]