For shorter lists:

>>> alphanums = ['a', 1, 'b', 2, 'c', 3, 'd', 4]
 >>> alphas = alphanums[:-1:2]
 >>> nums   = alphanums[1::2]
 >>> alphas
 ['a', 'b', 'c', 'd']
 >>> nums
 [1, 2, 3, 4]

For longer sequences, use iterators:

import itertools
 >>> [ i for i in itertools.islice(['a', 1, 'b', 2, 'c', 3], 1, None, 2) ]
 [1, 2, 3]

Where 1, None, 2 correspond to the start, end and steps to take.