p350.py
self.master指什么？
main函数里的ex跟root怎么关联在一起的？

from tkinter import *
from tkinter import filedialog

class MenuApp2(Frame):
    def __init__(self):
        super().__init__()
        self.master.title('File dialog')
        self.pack(fill=BOTH,expand=True)

        menubar=Menu(self.master)
        self.master.config(menu=menubar)

        filemenu=Menu(menubar)
        filemenu.add_command(label='Open...',command=self.onOpen)
        menubar.add_cascade(label='File',menu=filemenu)

        self.txt=Text(self)
        self.txt.pack(fill=BOTH,expand=True)

    def onOpen(self):
        ftypes=[('Python files','*.py'),('All files','*')]
        dlg=filedialog.Open(self,filetypes=ftypes)
        f1=dlg.show()

        if f1!='':
            text=self.readFile(f1)
            self.txt.insert(END,text)

    def readFile(self,filename):
        with open(filename,'r',encoding='utf-8') as f:
            text=f.read()
        return text

def main():
    root=Tk()
    ex=MenuApp2()
    root.geometry('300x250+300+300')
    root.mainloop()

if __name__=='__main__':
    main()
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