[Tutor] Todays Learning Python Question From a Newbie ;)

Jon Moore jonathan.r.moore at gmail.com
Thu Feb 2 11:31:05 CET 2006

On 02/02/06, Alan Gauld <alan.gauld at freenet.co.uk> wrote:
> Bob,
> >> Write a new computer_move() function for the tic-tac-toe game to plug
> >> the hole in the computers stratergy. See if you can create an opponent
> >> that is unbeatable!
> >>
> >> My main problem is that I can not see how the computers stratergy can
> >> be improved as at best I can only manage a tie with the computer!
> Does the computer ever go first?

Yes if you let it!

Does the computer start with a random location?

No, if you look at the code below, it has a  predefined set of 'best moves'.

If yes to the above then the computer can be beaten since the entire
> course of a Tic-Tac-Toe game (or OXO as we call it in the UK!)
> depends upon the first move location.

Thanks to  André, there is a way to win every time if you take the first
move (see below), so there MUST be a whole in the computers stratergy! Based
on what we all know about the game, I would say that you can not make it so
that the computer can win every time, but it should be possable to make it

x: 0
o: 4
x: 7
o: 2
x: 6

If no to the above then, provided the gameplay is OK, I don't know
> what the author means either.
> Alan G.

The code is as follows:

# set global constants
X = "X"
O = "O"
EMPTY = " "

# set game instructions
def display_instruct():
    """Display game instructions."""
    print \
    Welcome to the greatest intellectual challenge of all time:
    This will be a showdown between your human brain and my silicon

    You will make your move known 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, human.  The ultimate battle is about to begin. \n

# set question
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

# set ask number
def ask_number(question, low, high):
    """Ask for a number within the range"""
    response = None
    while response not in range(low, high):
        response =  int(raw_input(question))
    return response

# set pieces
def pieces():
    """Determine if player or computer goes first."""
    go_first =  ask_yes_no("Do you wish to go first? (y/n): ")
    if go_first == "y":
        print "\nThen take the first move. You will need it ;)"
        human = X
        computer = O
        print "\nYour bravery will be your undoing....I will go first."
        computer = X
        human = O
    return computer, human

# create new board
def new_board():
    """Create a new game board."""
    board = []
    for square in range(NUM_SQUARES):
    return board

# display the board
def display_board(board):
    """Display the board on the 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"

# set legal moves
def legal_moves(board):
    """Create list of legal moves."""
    moves = []
    for square in range(NUM_SQUARES):
        if board[square] == EMPTY:
    return moves

# set winner
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

# set human move
def human_move(board, human):
    """Get human 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. Please choose
    print "Fine..."
    return move

# set computer move
def computer_move(board, computer, human):
    """Make computer move."""
    # Make a copy of the board to work with since the function will be
changing the 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 a square number",
    # if computer can win, take 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 human can win, block that move
    for move in legal_moves(board):
        board[move] = human
        if winner(board) ==  human:
            print move
            return move
        # done checking this move, undo it
        board[move] = EMPTY

    # since no one can win on next move, pick best open square
    for move in BEST_MOVES:
        if move in legal_moves(board):
            print move
            return move

# set next turn
def next_turn(turn):
    """Switch turns."""
    if turn == X:
        return O
        return X

# congratulate winner
def congrat_winner(the_winner, computer, human):
    """Congratulate the winner"""
    if the_winner != TIE:
        print the_winner, "won!\n"
        print "Its a tie!\n"

    if the_winner == computer:
        print "As I predicted human, I am triumphant once more. \n" \
              "Proof that computers are superior to humans in all regards."

    elif the_winner == human:
        print "Nooooo! It cannot be! Somehow you tricked me human! \n" \
              "But never again.......I will win next time!"

    elif the_winner == TIE:
        print "You were most lucky human, and somehow managed to tie me! \n"
              "Celebrate today....for this will not happen again!"

# set main function
def main():
    computer, human = pieces()
    turn = X
    board = new_board()

    while not winner(board):
        if turn == human:
            move = human_move(board, human)
            board[move] = human
            move = computer_move(board, computer, human)
            board[move] = computer
        turn = next_turn(turn)

    the_winner =  winner(board)
    congrat_winner(the_winner, computer, human)

# start the program

raw_input("\n\nPress the enter key to exit.")
Best Regards

Jon Moore
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