class TicTacToe:

""" Making of a Tic-Tac-Toe game (simple version)"""

def __init__(self):

""" start a new game """

self._board = [[' '] *3 for j in range(3)] # make a 3x3 array

self._player = 'X'

def mark(self, i, j):

""" Put an X or O mark at position (i,j) for next player's turn. """

if not (0 <= i <= 2 and 0 <= j <= 2):

raise ValueError('Invalid position')

if self._board[i][j] != ' ':

raise ValueError('Board position occupied')

if self.winner( ) is not None:

raise ValueError('Game is over!')

self._board[i][j] = self._player

if self._player == 'X':

self._player = 'O'

else:

self._player = 'X'

def _is_win(self, mark):

"""Check whether the board configuration is a win for the given player."""

board = self._board

return(mark == board[0][0] == board[0][1] == board[0][2] or # row 0

mark == board[1][0] == board[1][1] == board[1][2] or # row 1

mark == board[2][0] == board[2][1] == board[2][2] or # row 2

mark == board[0][0] == board[1][0] == board[2][0] or # column 0

mark == board[0][1] == board[1][1] == board[2][1] or # column 1

mark == board[0][2] == board[1][2] == board[2][2] or # column 2

mark == board[0][0] == board[1][1] == board[2][2] or # diagonal

mark == board[0][2] == board[1][1] == board[2][0]) # rev diag

def winner(self):

"""Return mark of winning player, or None to indicate a tie."""

for mark in 'XO':

if self._is_win(mark):

return mark

return None

def __str__(self):

"""Return string representation of current game board."""

rows = [' | '.join(self._board[r]) for r in range(3)]

return '\n-------\n'.join(rows)