tkinter:为什么在"tk.call“方法中，"self._w”用来指代小部件，而不是"self“？

import tkinter as tk

class MyWidget(tk.Frame):
    def __init__(self, master=None):
        super().__init__(master)
        self.pack()
        self.create_widgets()

    def create_widgets(self):
        self.button = tk.Button(self, text="Click Me", command=self.on_button_click)
        self.button.pack()

    def on_button_click(self):
        # 使用self._w调用Tk的低级接口
        tk.call("tk", "messageBox", "-title", "Info", "-message", "Button Clicked!")

root = tk.Tk()
app = MyWidget(master=root)
app.mainloop()