$ python --version
Python 3.6.0
$ python -m timeit -s 'import random; l = list(range(18))' 'random.choice(l)'
1000000 loops, best of 3: 0.591 usec per loop
$ python -m timeit -s 'd = {str(n): list(range(15)) for n in range(18)}' 'd["12"]'
10000000 loops, best of 3: 0.0224 usec per loop
Context: a Stack Overflow question.