Given a TLD will search for all single letter domans that are _not_ registered

domain_lookup.py

from string import ascii_lowercase
import subprocess
import sys
from time import sleep

tld = sys.argv[1] # ie python main.py capital
for c in ascii_lowercase:
    output = subprocess.check_output("whois {}.{}".format(c, tld), shell=True)
    if "Domain not found" in str(output):
        print("The domain {}.{} is available".format(c, tld))
    sleep(0.25)