Password Guesser Project

[RedSkeleton007]

I'm having trouble getting the guess string indexes to point to the indexes of AlphaNum.

#!/usr/bin/env python3
#PasswordGuesser.py

AlphaNum = "abc"
passwordThatNeedsToBeGuessed = "cab"
guessMatches = False
guess = ""
guessLength = len(guess)

guessLength = 1 #start with a guess length of just one char
for char in AlphaNum:
    guess[0] = char
    print(guess)

guessLength = 2
for char in AlphaNum:
    guess[0] = char
    for char in AlphaNum:
        guess[1] = char
        guess = guess[1] + guess[0]
        print(guess)

guessLength = 3
for char in AlphaNum:
    guess[0] = char
    print(guess)
    for char in AlphaNum:
        guess[1] = char
        guess = guess[1] + guess[0]
        print(guess)
        for char in AlphaNum:
            guess[2] = char
            guess = guess[2] + guess[1] + guess[0]
            print(guess)

The error in the IDLE shell is:

Traceback (most recent call last):
  File "E:\Python\Python36-32\SamsPrograms\PasswordGuesser.py", line 12, in <module>
    guess[0] = char
TypeError: 'str' object does not support item assignment

Once these for loops are functioning properly, the output of this program should be exactly:

a
b
c

aa
ba
ca
ab
bb
cb
ac
bc
cc

aaa
baa
caa
aba
bba
cba
aca
bca
cca
aab
bab
cab
abb
bbb
cbb
acb
bcb
ccb
aac
bac
cac
abc
bbc
cbc
acc
bcc
ccc