Choose an elements from sequence with weight at random.

choice_on.py

#!/usr/bin/env python
import random

def choice_by(seq, criteria):
    """
    Choose an elements from sequence with weight at random.
    """
    #Initial value
    itr = iter(seq)
    chosen_so_far = itr.next()
    cumsum = criteria( chosen_so_far )
    #Choose
    for next_node in itr:
        threshold = criteria( next_node )
        cumsum += threshold
        r = random.random() * cumsum
        if r < threshold:
            chosen_so_far = next_node
        #else: chosen_so_far is not changed

    return chosen_so_far

if __name__ == '__main__':
    k_weight = { 'A': 1, 'B': 1.5, 'C': 0 }
    k_count = { 'A': 0, 'B': 0, 'C': 0 }
    cri = lambda k: k_weight[k]
    for i in range(50000):
        k_count[ choice_by( k_weight, cri ) ] += 1
    print k_count # e.g. => {'A': 19922, 'C': 0, 'B': 30078}