3-letter Scratch usernames finder (Python)

import time, os, requests, math, concurrent.futures as cf, sys

start_time = time.time()

f = open(os.path.join("/storage/emulated/0/Download", "usernames.txt"), "a")
#f = open("usernames.txt", "a")

CONNECTIONS = 20

def get_usernames():
    my_list = list()

    alphabet = "abcdefghijklmnopqrstuvwxyz1234567890-_"

    a = alphabet[0:26] # letters
    b = alphabet[36:38] # dash and underscore
    c = alphabet[26:36] # numbers

    for l1 in alphabet[14:38]: # opqrstuvwxyz1234567890-_
        for l2 in alphabet:
            for l3 in alphabet:
                user = l1 + l2 + l3

                cond1 = l1 in a and l2 in a and l3 in a
                cond2 = l1 in b and l2 in b and l3 in b
                cond3 = l1 in c and l2 in c and l3 in c

                if not cond1 and not cond2 and not cond3:
                    my_list.append(user)
    return my_list

def check_username(username):
    r = requests.get(f'https://api.scratch.mit.edu/accounts/checkusername/{username}/')
    return r.text != '{"username":"' + username + '","msg":"valid username"}'

def write_to_file(list):
    for item in list:
        f.write(item.upper() + "\n")

usernames = get_usernames()

available_usernames = list()
combinations = int(math.ceil(len(usernames) / 10.0)) * 10 # it's 25536 but we round it up to 25540 because we can divide 25540 by 10
counter = 0

with cf.ThreadPoolExecutor(max_workers = CONNECTIONS) as executor:
    future_to_url = {executor.submit(check_username, username): username for username in usernames}
    for future in cf.as_completed(future_to_url):

        counter += 1
        if counter % (combinations / 10) == 0.0:
            print(str(int((counter / combinations) * 100)) + " % Done")

        username = future_to_url[future]
        if not future.result():
            available_usernames.append(username)

print("100 % Done")

available_usernames = sorted(available_usernames, key = lambda src: ["abcdefghijklmnopqrstuvwxyz1234567890-_".index(char) for char in src])

f.truncate(0) # clear file contents
write_to_file(available_usernames)
f.close()

print("\n" + "Execution Time: " + str(round(((time.time() - start_time)/60), 3)) + " Minutes")
sys.exit(0)