Simply change your list comp to be:
b = [el[0] for el in a]
Or:
from operator import itemgetter
b = map(itemgetter(0), a)
Or, if you’re dealing with “proper arrays”:
import numpy as np
a = [ [1,2], [2,9], [3,7] ]
na = np.array(a)
print na[:,0]
# array([1, 2, 3])
And zip
:
print zip(*a)[0]