Tic-Tac-Toe

Tic-tac-toe is a classic pencil-and-paper game played on a 3 × 3 grid. Players take turns placing their X or O marks, trying to get three in a row. Most games of tic-tac-toe end in a tie, but it is possible to outsmart your opponent if they’re not careful.

The Program in Action

When you run tictactoe.py, the output will look like this:

Welcome to Tic-Tac-Toe!

       | |   1 2 3
      -+-+-
       | |   4 5 6
      -+-+-
       | |   7 8 9
What is X's move? (1-9)
> 1

      X| |   1 2 3
      -+-+-
       | |   4 5 6
      -+-+-
       | |   7 8 9
What is O's move? (1-9)
--snip--
      X|O|X  1 2 3
      -+-+-
      X|O|O  4 5 6
      -+-+-
      O|X|X  7 8 9
The game is a tie!
Thanks for playing!

How It Works

To represent tic-tac-toe boards in this program, we use a dictionary with keys '1' through '9' for the spaces on the board. The numbered spaces are arranged in the same way as a phone’s keypad. The values in this dictionary are the string 'X' or 'O' for a player’s mark and ' ' for an empty space.

Download Source Code

  1. """Tic-Tac-Toe, by Al Sweigart [email protected]
  2. The classic board game.
  3. View this code at https://nostarch.com/big-book-small-python-projects
  4. Tags: short, board game, game, two-player"""
  5.
  6. ALL_SPACES = ['1', '2', '3', '4', '5', '6', '7', '8', '9']
  7. X, O, BLANK = 'X', 'O', ' '  # Constants for string values.
  8.
  9.
 10. def main():
 11.     print('Welcome to Tic-Tac-Toe!')
 12.     gameBoard = getBlankBoard()  # Create a TTT board dictionary.
 13.     currentPlayer, nextPlayer = X, O  # X goes first, O goes next.
 14.
 15.     while True:  # Main game loop.
 16.         # Display the board on the screen:
 17.         print(getBoardStr(gameBoard))
 18.
 19.         # Keep asking the player until they enter a number 1-9:
 20.         move = None
 21.         while not isValidSpace(gameBoard, move):
 22.             print('What is {}\'s move? (1-9)'.format(currentPlayer))
 23.             move = input('> ')
 24.         updateBoard(gameBoard, move, currentPlayer)  # Make the move.
 25.
 26.         # Check if the game is over:
 27.         if isWinner(gameBoard, currentPlayer):  # Check for a winner.
 28.             print(getBoardStr(gameBoard))
 29.             print(currentPlayer + ' has won the game!')
 30.             break
 31.         elif isBoardFull(gameBoard):  # Check for a tie.
 32.             print(getBoardStr(gameBoard))
 33.             print('The game is a tie!')
 34.             break
 35.         # Switch turns to the next player:
 36.         currentPlayer, nextPlayer = nextPlayer, currentPlayer
 37.     print('Thanks for playing!')
 38.
 39.
 40. def getBlankBoard():
 41.     """Create a new, blank tic-tac-toe board."""
 42.     # Map of space numbers: 1|2|3
 43.     #                       -+-+-
 44.     #                       4|5|6
 45.     #                       -+-+-
 46.     #                       7|8|9
 47.     # Keys are 1 through 9, the values are X, O, or BLANK:
 48.     board = {}
 49.     for space in ALL_SPACES:
 50.         board[space] = BLANK  # All spaces start as blank.
 51.     return board
 52.
 53.
 54. def getBoardStr(board):
 55.     """Return a text-representation of the board."""
 56.     return '''
 57.       {}|{}|{}  1 2 3
 58.       -+-+-
 59.       {}|{}|{}  4 5 6
 60.       -+-+-
 61.       {}|{}|{}  7 8 9'''.format(board['1'], board['2'], board['3'],
 62.                                 board['4'], board['5'], board['6'],
 63.                                 board['7'], board['8'], board['9'])
 64.
 65. def isValidSpace(board, space):
 66.     """Returns True if the space on the board is a valid space number
 67.     and the space is blank."""
 68.     return space in ALL_SPACES and board[space] == BLANK
 69.
 70.
 71. def isWinner(board, player):
 72.     """Return True if player is a winner on this TTTBoard."""
 73.     # Shorter variable names used here for readability:
 74.     b, p = board, player
 75.     # Check for 3 marks across 3 rows, 3 columns, and 2 diagonals.
 76.     return ((b['1'] == b['2'] == b['3'] == p) or  # Across top
 77.             (b['4'] == b['5'] == b['6'] == p) or  # Across middle
 78.             (b['7'] == b['8'] == b['9'] == p) or  # Across bottom
 79.             (b['1'] == b['4'] == b['7'] == p) or  # Down left
 80.             (b['2'] == b['5'] == b['8'] == p) or  # Down middle
 81.             (b['3'] == b['6'] == b['9'] == p) or  # Down right
 82.             (b['3'] == b['5'] == b['7'] == p) or  # Diagonal
 83.             (b['1'] == b['5'] == b['9'] == p))    # Diagonal
 84.
 85. def isBoardFull(board):
 86.     """Return True if every space on the board has been taken."""
 87.     for space in ALL_SPACES:
 88.         if board[space] == BLANK:
 89.             return False  # If any space is blank, return False.
 90.     return True  # No spaces are blank, so return True.
 91.
 92.
 93. def updateBoard(board, space, mark):
 94.     """Sets the space on the board to mark."""
 95.     board[space] = mark
 96.
 97.
 98. if __name__ == '__main__':
 99.     main()  # Call main() if this module is run, but not when imported.

Exploring the Program

Try to find the answers to the following questions. Experiment with some modifications to the code and rerun the program to see what effect the changes have.

What happens if you change X, O, BLANK = 'X', 'O', ' ' on line 7 to X, O, BLANK = 'X', 'X', ' '?
What happens if you change board[space] = mark on line 95 to board[space] = X?
What happens if you change board[space] = BLANK on line 50 to board[space] = X?