This simple piece of code gives an error when it gets looped for the second time

import random
import string



for x in range(0,15):
    print "something"
    string= str(random.choice(string.letters)+str(random.randint(100,10000))+random.choice(string.letters)+str(random.randint(0,100)))
    print  string

Why does this code throw an error when it runs for the second time inside the for loop? I have no idea how it works perfectly for the first time and throws this error:

something J6554r15 something

Traceback (most recent call last): File "C:\Users\test\Desktop\soooo.py", line 8, in string= str(random.choice(string.letters)+str(random.randint(100,10000))+ran dom.choice(string.letters)+str(random.randint(0,100))) AttributeError: 'str' object has no attribute 'letters'

What am I missing here?