对于我的tic-tac-toe程序，我决定创建一个函数来检查玩家的输入是否可用。我定义了一个名为 checkOverride() 检查玩家的输入。如果已经选择了输入并且 "X" 或者一个 "O" 函数将返回 True . 否则它会回来的 False . 但是，在测试程序时，我试图使函数返回 True ，但函数返回的值为 None . 为什么会这样？

def main():
    # Welcome the players
    print("Welcome players to Tic-Tac-Toe!")

    # Variables for creating game board
    top     = "1|2|3"
    middle  = "4|5|6"
    bottom  = "7|8|9"
    divider = "-+-+-"

    # Display game board
    Board = createGameBoard(top, middle, bottom, divider)
    print(Board)

    playerOneWin = False
    playerTwoWin = False

    while playerOneWin == False and playerTwoWin == False:
        # Get input from player One
        playerOneInput = input("x's turn to choose a square (1-9): ")

        # Check for override
        override = checkOverride(Board, playerOneInput)

        if override == True:
            print(f"{playerOneInput} has already been filled. Please choose another square.")
            continue

        elif override == False:
            Board = playerOneMove(Board, playerOneInput)
            print(Board)

        # Get input from player Two
        playerTwoInput = input("o's' turn to choose a square(1-9): ")

        # Check for override
        override = checkOverride(Board, playerTwoInput)

        if override == True:
            print(f"{playerTwoInput} has already been filled. Please choose another square.")
            continue

        elif override == False:
            Board = playerTwoMove(Board, playerTwoInput)
            print(Board)

def createGameBoard(top, middle, bottom, divider):
    ''' 
    Create a Tic-Tac-Toe game board.
    '''
    Board = f"{top}\n{divider}\n{middle}\n{divider}\n{bottom}"
    return Board

def playerOneMove(Board, playerOneInput):
    '''
    Take player one's input and display it onto the board.
    '''
    # Identify where player one's move and replace number with 'X'
    if playerOneInput in Board:
        Board = Board.replace(playerOneInput, 'X')

    return Board

def playerTwoMove(Board, playerTwoInput):
    '''
    Take player two's input and display it onto the board.
    '''
    # Identify where player one's move and replace number with 'X'
    if playerTwoInput in Board:
        Board = Board.replace(playerTwoInput, "O")

    return Board

def checkOverride(Board, playerOneInput="", playerTwoInput=""):
    '''
    Look at the input of player one or player two and see if their input is already taken.
    '''

    if playerOneInput:
        if playerOneInput in Board:
            square = Board.index(playerOneInput)
            if Board[square] == "X" or Board[square] == "O":    
                return True
            else:
                return False

    elif playerTwoInput:
        if playerTwoInput in Board:
            square = Board.index(playerTwoInput)
            if Board[square] == "X" or Board[square] == "O":
                return True
            else:
                return False