Understanding square bracket use

[wahsape]

I'm currently reading through "Python Programming for the Absolute Beginner, Third Edition" and I need some help to understand a section of a program in the book.

# Tic-Tac-Toe
# Plays the game of tic-tac-toe against a human opponent
   
# global constants
X = "X"
O = "O"
EMPTY = " "
TIE = "TIE"
NUM_SQUARES = 9


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

    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
    """
    )


def ask_yes_no(question):
    """Ask a yes or no question."""
    response = None
    while response not in ("y", "n"):
        response = 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(input(question))
    return response


def pieces():
    """Determine if player 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.  You will need it.")
        human = X
        computer = O
    else:
        print("\nYour bravery will be your undoing... I will go first.")
        computer = X
        human = O
    return computer, human


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 square 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 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, foolish human.  Choose another.\n")
    print("Fine...")
    return move


def computer_move(board, computer, human):
    """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", end=" ")
    
    # 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 checkin 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


def next_turn(turn):
    """Switch turns."""
    if turn == X:
        return O
    else:
        return X

    
def congrat_winner(the_winner, computer, human):
    """Congratulate the winner."""
    if the_winner != TIE:
        print(the_winner, "won!\n")
    else:
        print("It's 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("No, no!  It cannot be!  Somehow you tricked me, human. \n" \
              "But never again!  I, the computer, so swear it!")

    elif the_winner == TIE:
        print("You were most lucky, human, and somehow managed to tie me.  \n" \
              "Celebrate today... for this is the best you will ever achieve.")


def main():
    display_instruct()
    computer, human = pieces()
    turn = X
    board = new_board()
    display_board(board)

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

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


# start the program
main()
input("\n\nPress the enter key to quit.")

The section I'm having problems understanding is

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

How is this determining a winner in the game, the way I'm reading it it looks like it is looking at the top row of the board only.

I know it is doing more than that since when I run the program it works fine.

[Windspar]

def winner(board):
    """Determine the game winner."""
    # create a tuple list
    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))
    
    # loops through tuple list 
    for row in WAYS_TO_WIN: #(0,1,2) next loop (3,4,5) next loop (6,7,8) and so on.
        # check out board position. 
        # first loop (0, 1, 2), row[0] = 0, row[1] = 1, row[2] = 2
        # if board[0] == board[1] == board[2] != EMPTY:
        # second loop (3, 4, 5), row[0] = 3, row[1] = 4, row[2] = 5
        # if board[3] == board[4] == board[5] != EMPTY:
        # so on
        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

[league55]

WAYS-TO-WIN is a collection of different rows. the line

for row in WAYS-TO-WIN

...iterates over that collection and tests every item in the collection until a win is found, the board is full with no win, or there's no win item in that collection.

[wahsape]

Thank you both for your answer, I see that I was reading it correctly just failing to understand the loop process.

[kismat77]

Thank you both for your answer, I see that I was reading it correctly just failing to understand the loop process.

Good