random_composition.py

import random

import numpy as np


composition = [1/3, 1/3, 1/3]

sites = np.zeros(shape=(300,))

val_1 = composition[2] + composition[1]
val_2 = composition[2]
val_3 = 0

for i, site in enumerate(sites):
    val = random.random()
    if val > val_1:
        sites[i] = 1
    elif val > val_2:
        sites[i] = 2
    elif val > val_3:
        sites[i] = 3

for val in range(4):
    print(val, len(np.where(sites == val)[0]))