如何在Tkinter开发的SOS游戏中默认选中单选按钮'S'？

我正在用tkinter开发SOS棋盘游戏，通过createBoard(tk.Tk)类处理游戏的GUI界面。其中playerChoice(self,game_window)函数为红、蓝两名玩家分别创建了一组可选'S'或'O'的单选按钮。我希望游戏界面创建完成后，每组单选按钮默认处于选中'S'的状态，但尝试调用.invoke()方法以及对StringVar执行.set('S')操作均未生效。playerChoice函数是在createOptionsDisplay函数中被调用的，相关代码如下：

import tkinter as tk
from tkinter import ttk
from tkinter.messagebox import showinfo
from tkinter import font
from typing import NamedTuple
from itertools import cycle

#game logic class

class createBoard(tk.Tk):
    def __init__(self,game):
        super().__init__()
        
        self.board_win = ''
        self.cells = {}
        self.title("SOS Game")
        self._game = game
        self.createOptionsDisplay()


    def playerChoice(self,game_window):
            #Blue player label and 'S' or 'O' radio buttons.
            blue_player_frame = tk.Frame(game_window)
            blue_player_label = ttk.Label(blue_player_frame, text='Blue player')
            blue_player_label.grid(row = 0, column = 0)

            blue_var = tk.StringVar()

            blue_player_s_btn = ttk.Radiobutton(blue_player_frame, text = 'S', variable = blue_var, value = 'S',command=self.setS)
            blue_player_o_btn = ttk.Radiobutton(blue_player_frame, text = 'O', variable = blue_var, value = 'O',command=self.setO)
            #does not work:
            #blue_player_s_btn.invoke()
            #blue_var.set('S')

            blue_player_s_btn.grid(row = 1, column = 0)
            blue_player_o_btn.grid(row = 2, column = 0)

            blue_player_frame.grid(row=0,column=0)

            #Red player label and 'S' or 'O' radio buttons.
            red_player_frame = tk.Frame(game_window)
            red_player_label = ttk.Label(red_player_frame, text='Red player')
            red_player_label.grid(row = 0, column = 0)

            red_var = tk.StringVar()

            red_player_s_btn = ttk.Radiobutton(red_player_frame, text = 'S', variable = red_var, value = 'S',command=self.setS)
            red_player_o_btn = ttk.Radiobutton(red_player_frame, text = 'O', variable = red_var, value = 'O', command=self.setO)
            #does not work:
            #red_player_s_btn.invoke()
            #red_var.set('S')

            red_player_s_btn.grid(row = 1, column = 0)
            red_player_o_btn.grid(row = 2, column = 0)

            red_player_frame.grid(row = 0, column = 2)

问题出在blue_var和red_var是playerChoice函数的局部变量，函数执行完毕后这两个变量会被Python垃圾回收机制销毁，导致单选按钮与StringVar的绑定关系失效，所以设置选中状态的操作无法生效。

解决方法是将这两个StringVar改为类的实例属性，让它们的生命周期与createBoard实例保持一致，具体修改如下：

1. 将blue_var = tk.StringVar()改为self.blue_var = tk.StringVar()
2. 将red_var = tk.StringVar()改为self.red_var = tk.StringVar()
3. 在创建完单选按钮后，调用self.blue_var.set('S')和self.red_var.set('S')设置默认选中，或者调用按钮的.invoke()方法

修改后的playerChoice函数代码：

def playerChoice(self,game_window):
        #Blue player label and 'S' or 'O' radio buttons.
        blue_player_frame = tk.Frame(game_window)
        blue_player_label = ttk.Label(blue_player_frame, text='Blue player')
        blue_player_label.grid(row = 0, column = 0)

        self.blue_var = tk.StringVar()

        blue_player_s_btn = ttk.Radiobutton(blue_player_frame, text = 'S', variable = self.blue_var, value = 'S',command=self.setS)
        blue_player_o_btn = ttk.Radiobutton(blue_player_frame, text = 'O', variable = self.blue_var, value = 'O',command=self.setO)
        # 现在可正常生效
        self.blue_var.set('S')
        # 也可使用 blue_player_s_btn.invoke()

        blue_player_s_btn.grid(row = 1, column = 0)
        blue_player_o_btn.grid(row = 2, column = 0)

        blue_player_frame.grid(row=0,column=0)

        #Red player label and 'S' or 'O' radio buttons.
        red_player_frame = tk.Frame(game_window)
        red_player_label = ttk.Label(red_player_frame, text='Red player')
        red_player_label.grid(row = 0, column = 0)

        self.red_var = tk.StringVar()

        red_player_s_btn = ttk.Radiobutton(red_player_frame, text = 'S', variable = self.red_var, value = 'S',command=self.setS)
        red_player_o_btn = ttk.Radiobutton(red_player_frame, text = 'O', variable = self.red_var, value = 'O', command=self.setO)
        # 现在可正常生效
        self.red_var.set('S')
        # 也可使用 red_player_s_btn.invoke()

        red_player_s_btn.grid(row = 1, column = 0)
        red_player_o_btn.grid(row = 2, column = 0)

        red_player_frame.grid(row = 0, column = 2)

修改后，StringVar会与createBoard实例绑定，不会被提前回收，设置默认选中的操作就能正常生效。

内容的提问来源于stack exchange，提问作者mikedoorhandle