Another tip, there are some great built in functions using the string library to get those letter sets. Try these out for example:
import string
print(string.ascii_uppercase)
print(string.ascii_lowercase)
print(string.ascii_letters)
print(string.digits)
print(string.printable)
They will output what you need automatically:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789
0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~