Retro "Tic Tac Toe" in python, wrote for fun

tictactoe.py

#! /usr/bin/env python
# All right reserved. Distributed Under BSD Lisence

__author__="Arun.K.R <the1.arun@gmail.com>"
__date__ ="$May 11, 2011 6:23:17 PM$"

import os, time
from random import choice

WIDTH = 3
BOARD = [str(i+1) for i in range(WIDTH*WIDTH)]
USERS = {True:'X', False:'O'}

def check_for_win(player, position, marked):
    row=(position-1)/WIDTH
    #horizontal equality test
    if len(filter(marked, BOARD[row*WIDTH: row*WIDTH+WIDTH])) == WIDTH:
        game_over(player)
    #vertical equality test
    if len(filter(marked, [BOARD[itm]
            for itm in range((position-1) % WIDTH, WIDTH*WIDTH, WIDTH)])) == WIDTH:
        game_over(player)
    #diagonal (forward slash) test
    if len(filter(marked, [BOARD[itm]
            for itm in range(0, WIDTH*WIDTH, WIDTH+1)])) == WIDTH:
        game_over(player)
    #diagonal (backward slash) test
    if len(filter(marked, [BOARD[itm]
            for itm in range(WIDTH-1, WIDTH*WIDTH, WIDTH-1)[:-1]])) == WIDTH:
        game_over(player)

def check_for_tie():
    if len(filter((lambda x: x not in ['X', 'O']), BOARD)) == 0:
        draw(ask=False)
        print "\n", "Game is in tie!!!"
        exit()

def ask_input(player):
    try:
        print "Player " + USERS[player] + "'s turn ->",
        pos = int(raw_input("Enter a number from board: "))
        assert pos>0 and pos<=WIDTH*WIDTH
    except Exception:
        print "Enter a valied number in the board!"
        time.sleep(1)
        draw(player)
    else:
        play(pos, player)

def draw(player=choice([True, False]), ask=True):
    os.system('clear')
    for i in range(WIDTH):
        print ' | '.join(BOARD[i*WIDTH:i*WIDTH+WIDTH])
        print '-'*(WIDTH * WIDTH)
    if ask:
        ask_input(player)

def play(position, player):
    if BOARD[position-1] == str(position):
        BOARD[position-1] = USERS[player]
    else:
        draw(player)
    #check for win
    check_for_win(player, position, marked=(lambda x: x==USERS[player]))
    #check for draw
    check_for_tie()
    #switch player (simple LOGIC ANDing)
    draw(not player)

def game_over(player):
    draw(ask=False)
    print "\n", "Player " + USERS[player] + " WON!!!"
    exit()

if __name__ == "__main__":
    draw()