How to randomly select a character everytime it's needed

How do I randomly select a character from a string of characters every time I want it to be changed, for example:

import random

def user_input():
    chars = 'abcdefghijklmnopqrstuvwxyz'
    present = random.choice(chars)
    while True:
        print present
        to_eval = raw_input('Enter key: ')
        if to_eval == present:
            print 'Correct!'
            break
        else:
            # change the key and ask again

user_input()