Question

I've pasted the code here:

X = "X"
O = "O"
board = []
EMPTY = ""

def instructions_prompt ():
    print "\t\t\tNoughts and Crosses"
    print \
        """Foolish human. Now that you've entered this Python program,
       there is no exit. None! (At any point in the game if you feel
       like you are intimidated by my presence, hit 0 to exit)

       This challenge of wits will be one of many failures in your life.

       The instructions are as follows:

       1. Select a number from the following key:

                   0 | 1 | 2
                   ---------
                   3 | 4 | 5
                   ---------
                   6 | 7 | 8

       to place X or O which is predetermined by whether or not you start
       the game.

       2. Try to save face while failing. And don't talk about Fight Club."""

def start_prompt ():
    choice = raw_input ("Would you like to go first (Y/N)?")
    lower = choice.lower()
    if lower == "y":
        human = X
        computer = O
        print "You're",human
        print "I am",computer
    elif lower == "n":
        computer = X
        human = O
        print "You're",human
        print "I'm in",computer
    return human, computer

def display_board (board):
    print "",board[0],"|",board[1],"|",board[2],""
    print "---------"
    print "",board[3],"|",board[4],"|",board[5],""
    print "---------"
    print "",board[6],"|",board[7],"|",board[8],""

def turn_switcher (turn):
    if turn == X:
        return O
   else:
        return X 

def fresh_board ():
    for i in range (9):
        board.append (EMPTY)
    return board

def request_move ():
    try:
        square = int(raw_input("Where'd you like your square to be?"))
        if square>8 or square<0:
            print "This program can't proceed as that number is not on the board."
        else:
            return square
    except:
        print "That's not a number on the board. This program can't proceed."

def legal_moves (board):
    legal_moves = []
    for i in range (9):
        if board [i] == EMPTY:
            legal_moves.append(i)
    return legal_moves

def winner (board):
    if board[0] == board [1] == board [2] != EMPTY:
        winner = board [0]
        return winner
    elif board [0] == board [3] == board [6] != EMPTY:
        winner = board [0]
        return winner
    elif board [0] == board [4] == board [8] != EMPTY:
        winner = board [0]
        return winner
    elif board [1] == board [4] == board [7] != EMPTY:
        winner = board [1]
        return winner
    elif board [2] == board [5] == board [8] != EMPTY:
        winner = board [2]
        return winner
    elif board [2] == board [4] == board [6] != EMPTY:
        winner = board [2]
        return winner
    elif board [6] == board [7] == board [8] != EMPTY:
        winner = board [8]
        return winner
    elif EMPTY not in board:
        return None

def human_move (board, square):
    legal = legal_moves(board)
    if square not in legal: 
        print "This is not a legal move"
        sys.exit()
    else:
        return square

def computer_move (computer, board, human):
    best = (4,0,8,2,6,1,3,5,7)
    board = board [:]
    legal = legal_moves(board)
    for i in legal:
        board[i] = computer
        if winner(board) == computer:
            return i
        board = EMPTY

#Stopping human from winning
    for i in legal_moves(board):
        board [i] = human
        if winner(board) == human:
             return i

    for i in best:
        if i in legal_moves(board):
            return i

def main ():
    instructions_prompt ()
    human, computer = start_prompt ()
    turn = X
    board = fresh_board()
    request_move ()
    display_board (board)

    while not winner(board):
        if turn == human:
            square = request_move()
            move = human_move (board, square)
            board[move] = human
        else:
            move = computer_move(computer, board, human)
            board[move] = computer
        display_board(board)
        turn = turn_switcher(turn)

main ()
raw_input ("Enter a key to end.")

TRACKBACK:

Traceback (most recent call last):
  File "C:\Users\COMPAQ\Desktop\NoughtsCrosses.py", line 152, in <module>
  main ()
  File "C:\Users\COMPAQ\Desktop\NoughtsCrosses.py", line 147, in main
  move = computer_move(computer, board, human)
  File "C:\Users\COMPAQ\Desktop\NoughtsCrosses.py", line 118, in computer_move
 board[i] = computer
 TypeError: 'str' object does not support item assignment 

Answer 1

You have your arguments mixed up for the function computer_move. On line 113, the arguments are in the order computer, board, human. However, on line 147, where computer_move is called, the order is board, computer, human.

Python gave you the confusing index error because Python strings are really just lists of characters. E.g.

>>> "foo"[2] == "o"
True

Update: You’re getting this new error because of the last line of computer_move. It should be board[i] = EMPTY rather than board = EMPTY.