pyTicTacToe

# Tic Tac Toe
# By Mike Kerry, December 2020 - Email: acclivity2@gmail.com
# For Python Programmers (Beginners) Facebook Group

def play_turn(letter):          # letter is X or O
    global board
    # Define the 8 possible winning combinations
    wins = [(1, 2, 3), (4, 5, 6), (7, 8, 9), (1, 4, 7), (2, 5, 8), (3, 6, 9), (1, 5, 9), (3, 5, 7)]

    # Keep looping until player selects a valid empty cell, 1 to 9
    while True:
        print(letter + " your turn: ")
        user = input("Select a square 1 to 9: ")
        if user in board:
            x = board.index(user)
            break
        print("Invalid cell, or cell already occupied, try again")

    board[x] = letter

    # Print the updated board
    for row in range(3):
        print(board[row * 3], "|", board[row * 3 + 1], "|", board[row * 3 + 2])

    # Look to see if this last player has won, by getting three in a row
    for tup in wins:
        work = [" "] + board
        if work[tup[0]] == letter and work[tup[1]] == letter and work[tup[2]] == letter:
            return letter       # Return a "WIN" indication
    return None                 # Return "No Winner" indication


alt = False     # this is a 1/0 flip flop which determimes who goes first, X or O
while True:
    # Start a new round.  Create 9 empty (numbered) cells
    # Cells are numbered 1 (top left) to 9 (bottom right)
    board = ["1", "2", "3", "4", "5", "6", "7", "8", "9"]
    for row in range(3):    # Print the empty board
        print(board[row * 3], "|", board[row * 3 + 1], "|", board[row * 3 + 2])
    alt = not alt       # Alternate between O and X going first

    # Loop around, allowing X and O to play in turns
    for ch in "XOXOXOXOXO"[alt:]:
        if play_turn(ch) == ch:
            print(ch, "wins!\n=================")
            break
        for cell in board:
            if cell.isdigit():
                break
        else:
            # All cells are occupied, but no one won
            print("It's a draw!\n=================")
            break

    if input("Play again? (Y/N): ").upper() != "Y":
        break