Tic-tac-toe input error and crash in Python

Question

I am a beginner with Python, and I am creating a two-player tic-tac-toe game in the terminal.

Basically, this game has all its bugs and kinks worked out, however, I have one last problem. Basically, when prompted to enter a move, if one user enters a letter when prompted for a move to go somewhere, if a letter or non-integer is inputted, it crashes. Here is the code, and then I will put the output when a game is ran, and a user inputs a letter after the prompt for a move.

X = "X"
O = "O"
empty = " "
S = [" ", " ", " ", " ", " ", " ", " ", " ", " "]

def Instructions():
    print "Fill in spaces on the board with number corresponding to the board below."
    print ""
    print "",1,"|",2,"|",3
    print "","---------"
    print "",4,"|",5,"|",6
    print "","---------"
    print "",7,"|",8,"|",9
    print ""

def Board():
    print ""
    print "",S[0],"|",S[1],"|",S[2]
    print "","---------"
    print "",S[3],"|",S[4],"|",S[5]
    print "","---------"
    print "",S[6],"|",S[7],"|",S[8], "\n"

def WhoGoesFirst():
    Instructions()
    global order
    letter = raw_input('Who goes first, X or O? ').upper()
    while not (letter == "X" or letter == "O"):
        letter = raw_input('Who goes first, X or O? ').upper()
    if letter == "X":
        order = [X, O, X, O, X, O, X, O, X]
    else:
        order = [O, X, O, X, O, X, O, X, O]

def CheckWin():
    global winner
    winner = ""
    if S[0] == S[1] == S[2] != empty:
        winner = S[0]
    if S[3] == S[4] == S[5] != empty:
        winner = S[3]
    if S[6] == S[7] == S[8] != empty:
        winner = S[6]
    if S[0] == S[3] == S[6] != empty:
        winner = S[0]
    if S[1] == S[4] == S[7] != empty:
        winner = S[1]
    if S[2] == S[5] == S[8] != empty:
        winner = S[2]
    if S[0] == S[4] == S[8] != empty:
        winner = S[0]
    if S[2] == S[4] == S[6] != empty:
        winner = S[2]

def Move(turn):
    move = input('Choose a Space from 1-9 for ' + str(order[turn]) + ' to Go: ')
    while move not in range (1, 10) or S[int(move) - 1] is not empty:
        move = input('Choose a Space from 1-9 for ' + str(order[turn]) + ' to Go: ')
    S[int(move) - 1] = order[turn]
    Board()
    CheckWin()

def MakeMove():
    turn = 0
    while turn <= 8:
        Move(turn)
        turn += 1
        if winner == X or winner == O:
            while turn <= 8:
                turn += 1
            if winner == X:
                print winner + " Is the Winner!"
            if winner == O:
                print winner + " Is the Winner!"
    if winner == "":
        print "The Game Is a Tie"

WhoGoesFirst()
MakeMove()

OUTPUT

Fill in spaces on the board with number corresponding to the board below.

 1 | 2 | 3
 ---------
 4 | 5 | 6
 ---------
 7 | 8 | 9

Who goes first, X or O? x
Choose a Space from 1-9 for X to Go: 1

 X |   |
 ---------
   |   |
 ---------
   |   |

Choose a Space from 1-9 for O to Go: k
Traceback (most recent call last):
  File "move.py", line 79, in <module>
    MakeMove()
  File "move.py", line 66, in MakeMove
    Move(turn)
  File "move.py", line 56, in Move
    move = input('Choose a Space from 1-9 for ' + str(order[turn]) + ' to Go: ')
  File "<string>", line 1, in <module>
NameError: name 'k' is not defined

Is it possible to fix this problem, so that if a non-integer is inputted, it will give the prompt for a move again, (until a proper move is put in). If it is possible, then how would it be done?

Answer 1

To fix your immediate problem (under Python 2), use raw_input instead of input.

Under Python 2, input will get a value from you then try to evaluate it. Evaluating 1 is okay but evaluating k when there's no such variable is not.

Once you have the string, you can check it however you like before attempting to convert it to an integer, such as with:

def Move(turn):
    move = -1
    while move not in range (1, 10) or S[int(move) - 1] is not empty:
        smove = raw_input('Choose a Space from 1-9 for ' + str(order[turn]) + ' to Go: ')
        try: 
            move = int(smove)
        except ValueError:
            move = -1
    S[int(move) - 1] = order[turn]
    Board()
    CheckWin()

Python 3 is also a fix to this problem since its input function is equivalent to the raw_input of Python 2. In any case, development of Python2 has now stopped and people should be moving their stuff to Python 3 unless they have a very good reason.