这段代码为什么输入框不显示任何输入，只有去掉 validate = 'key' 才可以，但是这样又失去了验证功能:

from tkinter import *

master = Tk()

v1 = StringVar()
v2 = StringVar()
v3 = StringVar()

def test(content) :
    return content.isdigit()

testCMD = master.register(test)

e1 = Entry(master,textvariable = v1, validate = 'key',\
      validatecommand = (testCMD,'%p')).grid(row = 0,column =0)
      
Label(master,text = '+').grid(row = 0,column =1)

e2 = Entry(master, textvariable=v2, validate = 'key',\
           validatecommand=(testCMD, '%p')).grid(row = 0,column =2)
           
Label(master,text = '=').grid(row = 0,column =3)

e3 = Entry(master,textvariable = v3,state = 'readonly').grid(row = 0,column =4)

def calc() :
    result = int(v1.get()) + int(v2.get())
    v3.set(str(result))
    
Button(master,text = '计算结果',command = calc).grid(row = 1,column =2)

mainloop()