Why can I I break out of the while loop?

Question

I am new to python and trying to write a python function that generates random n-mers (each character can be one of (ACGT) in the end. But the while loop seems to go on forever. Any suggestions? Here is my code:

def add_base(x):

    random_seqs = []
    for char in "ACGT":
        y = x + char
        random_seqs.append(y)
    return random_seqs


def random_n_mer(n):

    print("Random " + str(n) + " mers")
    i = 1

    random_mers_next = []
    random_mers = add_base("")

    while i < n:

        for base in random_mers:
            print(base)
            random_mers_next.extend(add_base(base))
            print(random_mers)
            print(random_mers_next)
        random_mers = random_mers_next
        i = i+1

random_n_mer(3)

Answer 1

The reason why your while loop goes on forever is that you set your random_mers variable to random_mers_next . Inside your for loop for the random_n_mer(n) function, you extend random_mers_next making the list longer. After, you say that random_mers is equal to that list. Since the for loop is iterating through each value in the random_mers list, and you keep adding to that list, it will never end.

Answer 2

I will go on a limb here and guess:

you're missing

random_mers_next = []

So try this:

while i < n:

    for base in random_mers:
        print(base)
        random_mers_next.extend(add_base(base))
        print(random_mers)
        print(random_mers_next)
    random_mers = random_mers_next
    random_mers_next = []
    i = i+1

Without it you're rapidly extending random_mers - on every i iteration you process previously produced elements twice, which adds exponential growth on top of 4 times growth by the algorithm itself.