from tkinter import *

root = Tk()
frame = Frame(root)
frame.pack(padx = 10 ,pady = 10)
v1 = StringVar()
v2 = StringVar()
v3 = StringVar()

def test(content):
    if content.isdigit():
        return True
    else:
        return False

testCMD = root.register(test)
Entry(frame,textvariable=v1,width=10,validate='key',\
      validatecommand = (testCMD,'%P')).grid(row = 0,column = 0)
Label(frame,text='+').grid(row=0,column=1)

Entry(frame,textvariable=v2,width=10,validate='key',\
      validatecommand = (testCMD,'%P')).grid(row = 0,column = 2)
Label(frame,text='+=').grid(row=0,column=3)

Entry(frame,textvariable=v3,width=10,validate='key',\
      validatecommand = (testCMD,'%P')).grid(row = 0,column = 4)

def calc():
    result = int(v1.get()) + int(v2.get())
    v3.set(result)

Button(frame,text='计算结果',command = calc).grid(row=1,column = 2,pady=5)

mainloop()
