WoLfhOUnD

import random

def choice_equel(seq, p):
        p = list(p)
    for i in range(len(p) - 1):
        p[i+1] = p[i+1] + p[i]
 
    rnd = random.random()
    for index, value in enumerate(p):
        if rnd <= value:
            return index 
 
if __name__ == '__main__':
    system = [
        'SMALL',
        'MEDIUM',
        'LARGE'
    ]
    stats = [0,0,0]
    for i in xrange(1000000):
        index = choice_equel(system, (0.3, 0.5, 0.2))
        stats[index] = stats[index] + 1
    print stats