# TIC TAC TOE
# Started: 10/19/09
# Ended: still in progress
 
#GLOBAL CONSTANTS
X = "X"
O = "O"
Empty = " "
TIE = "TIE"
NUM_SQUARES = 9
 
def display_instruct():
    """Display game instructions."""
    print \
    """
    Welcome to the intellectual challenge of all time: Tic-Tac-Toe.
    You will make your move know by entering a number, 0 - 8. The number
    will correspond to the board position as illustrated:
 
                            0 | 1 | 2
                            _________
 
                            3 | 4 | 5
                            _________
 
                            6 | 7 | 8
 
    Prepare yourself. The ultimate battle is about to begin. \n
    """
 
def ask_yes_no(question):
    """Ask a yes or no question."""
    response = None
    while response not in ("y", "n"):
        response = raw_input(question).lower()
    return response
 
def ask_number(question, low, high):
    """Ask for a number within a range."""
    response = None
    while response not in range(low, high):
        response = int(raw_input(question))
    return response
 
def pieces():
    """Determine if Player 1 or Computer goes first."""
    go_first = ask_yes_no("Do you require the first move? (y/n):")
    if go_first == "y":
        print "\nThen take the first move."
        player1 = X
        computer = O
    else:
        print "\nComputer will go first."
        player2 = X
        computer = O
    return computer, player1
 
def new_board():
    """Create new game board."""
    board = []
    for square in range(NUM_SQUARES):
        board.append(EMPTY)
    return board
 
def display_board(board):
    """Display game board on screen."""
    print "\n\t", board[0], "|", board[1], "|", board[2]
    print "\t", "------"
    print "\t", board[3], "|", board[4], "|", board[5]
    print "\t", "------"
    print "\t", board[6], "|", board[7], "|", board[8], "\n"
 
def legal_moves(board):
    """Create list of legal moves."""
    moves =[]
    for squares in range(NUM_SQUARES):
        if board[square] == EMPTY:
 
            moves.append(square)
    return moves
 
def winner(board):
    """Determine the game winner."""
    WAYS_TO_WIN = ((0, 1, 2),
                   (3, 4, 5),
                   (6, 7, 8),
                   (0, 3, 6),
                   (1, 4, 7),
                   (2, 5, 8),
                   (0, 4, 8),
                   (2, 4, 6))
 
    for row in WAYS_TO_WIN:
        if board[row[0]] == board[row[1]] == board[row[2]] != EMPTY:
            winner = board[row[0]]
            return winner
 
    if EMPTY not in board:
        return TIE
 
    return None
 
def player1_move(board, player1):
    """Get Player 1 move."""
    legal = legal_moves(board)
    move = None
    while move not in legal:
        move = ask_number("Where will you move? (0 - 8): ", 0, NUM_SQUARES)
        if move not in legal:
            print "\nThat square is already occupied. Choose another.\n"
        print "Fine..."
        return move
 
def computer_move(board, computer, player1):
    """ Make computer move."""
    # make a copy to work with since function will be changing list
    board = board[:]
 
    # the best positions to have, in order
    BEST_MOVES = (4, 0, 2, 6, 8, 1, 3, 5, 7)
    print "I shall take square number",
 
 
# if computer can win, block that move
for move in legal_moves(board):
    board[move] = computer
    if winner(board) == computer:
        print move
        return move
    # done checking this move, undo it
    board[move] = EMPTY
 
# if player1 can win, block that move
for move in legal_moves(board):
    board[move] = player1
    if winner(board) == player1:
        print move
        return move
    # done checking this move, undo it
    board[move] = EMPTY
 
    # since no one can win on next move, pick open square
    for move in BEST_MOVES:
        if move in legal_moves(board):
            print move
            return move
        #done checking this move, undo it
        board[move] = EMPTY
 
def next_turn(turn):
    """Switch turns."""
    if turn == X:
        return O
    else:
        return X
 
def congrat_winner(The_winner, computer, player1):
    """Congratulate the winner."""
    if the_winner != TIE:
        print the_winner, "won\n"
    else:
        print "It's a tie!\n"
    if the_winner == computer:
        print " Computer is great.\n"
    elif the_winner == player1:
        print "Player 1 is great.\n"
    elif the_winner == TIE:
        print "You two try again.\n"
 
def main():
    display_instruct()
    computer, player1 = pieses()
    turn = X
    board = new_board()
    display_board(board)
 
    while not winner(board):
        if turn == player1:
            move = player1_move(board, player1)
            board[move] = player1
 
        else:
            turn == computer
            move = computer_move(board, player2)
            board[move] = computer
 
            display_board(board)
            turn = next_turn(turn)
            the_winner = winner(board)
            congrat_winner(the_winner , computer, player1)
 
# start the program
main()
raw_input("\n\nPress the enter key to quit.")

IDLE 2.6.2      
>>> def a(x, y):
	return x + y
 
>>> a(5, 8)
13
>>> x = 5
>>> y = 8
>>> return x + y #this you can not do because outside a function
SyntaxError: 'return' outside function
>>>