Exercise 2 solution

import random

def run(rng, p, size, max_gens=2000):
    for gen in range(1, max_gens + 1):
        k = sum(rng.random() < p for _ in range(2 * size))
        p = k / (2 * size)
        if p in (0, 1):
            return p, gen
    return p, max_gens

rng = random.Random(2)
reps = 400
results = [run(rng, 0.2, 25) for _ in range(reps)]
fixed = sum(1 for p, g in results if p == 1)
lost = sum(1 for p, g in results if p == 0)
mean_gens = sum(g for p, g in results) / reps
print(fixed, lost, fixed / reps, round(mean_gens, 1))

Output:

75 325 0.1875 50.5

Reading it: the allele was fixed in 75 of 400 runs (0.1875) and lost in 325. The fixed fraction is close to the starting frequency of 0.2: in this simple model, the chance that an allele eventually takes over equals its starting frequency (check). The average time to reach either end was about 50 generations.