sort by index 2d array python

1a = [[9, 9, 2], [9, 9, 3], [9, 9, 8], [9, 9, 4], [9, 9, 1], [9, 9, 5]]
2
3b = sorted(a, key=lambda a:a[2])
4
5#b contains the following arrays, sorted by their second index:
6[[9, 9, 1], [9, 9, 2], [9, 9, 3], [9, 9, 4], [9, 9, 5], [9, 9, 8]]