井字游戏 - 比较值时出错

我写了一个井字游戏程序，一切正常，但是当我输入数字 7 时，它说：选择不可用！也就是说，好像 7 无法与 available(location, board) 函数中的 7 进行比较。我不明白错误在哪里。

from IPython.display import clear_output #to clear the output (specific to Jupyter notebooks and ipython)
from random import randint

def draw(board):

for row in board:
    print ("\t      {} | {} | {}".format(*row))
    print ("\t    ","-"*10)




def available(location, board):

for row in board:
    for col in row:
        if location == col:
            return False
        else:
            return True


def mark(player, location, board):
for row in board:
    if location in row:

        ind = row.index(location)
        row.pop(ind)
        row.insert(ind, player)

def check_win(board):


if board[0][0] == board[1][0] and board[1][0] == board[2][0]:
    return True
elif board[0][1] == board[1][1] and board[1][1] == board[2][1]:
    return True
elif board[0][2] == board[1][2] and board[1][2] == board[2][2]:
    return True
elif board[0][0] == board[0][1] and board[0][1] == board[0][2]:
    return True
elif board[1][0] == board[1][1] and board[1][1] == board[1][2]:
    return True
elif board[2][0] == board[2][1] and board[2][1] == board[2][2]:
    return True
elif board[0][0] == board[1][1] and board[1][1] == board[2][2]:
    return True
elif board[0][2] == board[1][1] and board[1][1] == board[2][0]:
    return True
else:
    return False

def check_tie(board):

    for row in board:
        for col in row:
            if col.isdigit():
                return False
                break
            else:
                return True


def dashes():

print("o" + 35 *'-' + "o")

def display(message):

print("|{:^35s}|".format(message))

def main():
# initializing game
board = [['7', '8', '9'], ['4', '5', '6'], ['1', '2', '3']]
# select the first player randomly
player = ['X', 'O']
turn = randint(0, 1)

win = False
tie = False
while(not win and not tie):
    # switch players
    turn = (turn + 1) % 2
    current_player = player[turn] # contains 'X' or 'O'

    clear_output()

    # display header
    dashes()
    display("TIC TAC TOE")
    dashes()

    # display game board
    print()
    draw(board)
    print()

    # display footer
    dashes()
    # player select a location to mark
    while True:
        location = input("|{:s} Turn, select a number (1, 9): ".format(current_player))
        if available(location, board):
            break # Only the user input loop, main loop does NOT break
        else:
            print("Selection not available!")
    dashes()

    # mark selected location with player symbol ('X' or 'O')
    mark(current_player, location, board)

    # check for win
    win = check_win(board)

    # check for tie
    tie = check_tie(board)


# Display game over message after a win or a tie
clear_output()

# display header
dashes()
display("TIC TAC TOE")
dashes()

# display game board (Necessary to draw the latest selection)
print()
draw(board)
print()

# display footer
dashes()
display("Game Over!")
if(tie):
    display("Tie!")
elif(win):
    display("Winner:")
    display(current_player)
dashes()



main()