generate a random string in python that is not used before code example

Example 1: generate random string python

import string
import random

length=5
#python2
randomstr = ''.join(random.sample(string.ascii_letters+string.digits,length))


#python3
randomstr = ''.join(random.choices(string.ascii_letters+string.digits,k=length))

Example 2: python generate random string

''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(N))