538 Number Sequence

seq.py

from __future__ import division, print_function

def ratio(length):

    number_concurrent_3s = 0
    the_two_we_are_before = 1

    threes_that_should_be_before_this_two = {1: 3}

    number_3s = 0
    number_2s = 0

    for index in xrange(1, length + 1):
        if number_concurrent_3s == threes_that_should_be_before_this_two[the_two_we_are_before]:
            number_2s += 1
            the_two_we_are_before += 1
            threes_that_should_be_before_this_two[index] = 2
            number_concurrent_3s = 0
        else:
            number_3s += 1
            threes_that_should_be_before_this_two[index] = 3
            number_concurrent_3s += 1

    return number_3s / number_2s

if __name__ == '__main__':
    print(ratio(1000000))