To generate random sequence of letters and digits

random_choice.py

import string
import random

print string.ascii_letters
print string.digits

allow_letters = string.ascii_letters + string.digits

print allow_letters

random_string = []
duplicated = 0
for j in range(50000):
    s = ''.join(random.choice(allow_letters) for i in range(3))
    if s not in random_string:
        random_string.append(s)    
    else:
        print 'duplicated'
        duplicated = duplicated + 1
print 'end'    
print duplicated