How to randomly change boolean value in a list

random.randint(a, b) returns a number between a and b inclusive. If the result of the function call equals len(population), then you're trying to do population[len(population)], which will raise an IndexError because indexing starts at 0.

Simple change: Just minus 1 from len(population):

r = random.randint(0, len(population)-1)

Or use randrange(a, b), which is not inclusive:

r = random.randrange(len(population))

Note that if the first argument is 0 we don't need it since it will assume the start is 0.

According to the documentation, random.randint(a, b)

Return a random integer N such that a <= N <= b.

Since arrays are indexed starting at 0 in Python, len(population) is outside the range of the array (hence your error). As @TerryA indicated, you actually want the range to be from 0 to len(population) - 1.

try :

for x in population:
        if x:
            r = random.randint(0, len(population)-1)
            population[r] = True