Tic-Tac-Toe

#Tic-Tac-Toe
#Plays the game of tic-tac-toe, human vs computer

#global constants
X = "X"
O = "O"
EMPTY = " "
TIE = "TIE"
NUM_SQUARES = 9

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

You will make your move by entering a number, 1-9. 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():
    """Detirmine if the player or the 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'll 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])

def legal_moves(board):
    """Create a 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, human, computer):
    """make computer move"""
    #make a copy to work with
    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 checking this move, undo it
        board[move] = EMPTY

    #pick the 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 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 with me. \n"
              "Celebrate today... for this is the best you will ever achieve.")
def main():
    instructions()
    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 enter to exit")