添加，减去和存储条目小部件的输入值（Tkinter，Python2.7）

在这个程序中，我创建了三个条目小部件，我可以在其中放置任何值并通过等式 1st_entrybox - 2nd_entrybox + 3rd_entrybox 将它们连接起来。单击“总计”按钮后，我应该会在名为“能量跨度”的框中获得全部答案（该名称与我的项目有关）。但我没有得到答案。能否请你帮忙？

提前致谢。

这是我的代码：

from __future__ import division
from Tkinter import *
root = Tk()


btn1=Frame(root, width=600, height=600, bd=16, relief="raise")
btn1.pack(side=LEFT)

btn2=Frame(root, width=600, height=600, bd=16, relief="raise")
btn2.pack(side=LEFT)


def Sum():
    if var.get == 1:
        Qty1 = float(Trn.get())
        Qty2 = float(Intr.get())
        Qty3 = float(delG.get())
        Sumup = Qty1 - Qty2 + Qty3 
        Total.set(Sumup)

def Reset():
    Trn.set("0")
    Intr.set("0")
    delG.set("0")
    Total.set("0")


var = IntVar()
Trn = IntVar()
Intr = IntVar()
delG = IntVar()
Total = IntVar()


rb1 = Radiobutton(btn1, variable=var, value=1,
             font=('Helvetica',22,'bold')).grid(row=0, column=0, sticky=W)

lbl = Label(btn1, font=('Helvetica',22,'bold'), text="TS", 
  fg="black").grid(row=1, column=0)

ent = Entry(btn1, font=('Helvetica', 22, 'bold'), bd=4, width=13, bg= 
  "white",textvariable=Trn).grid(row=1, column=1)

lbl1 = Label(btn1, font=('Helvetica',22,'bold'), text="INT", fg="black", 
   bd=16).grid(row=2, column=0)

ent1 = Entry(btn1, font=('Helvetica', 22, 'bold'), bd=4, width=13, bg= 
  "white",textvariable=Intr).grid(row=2, column=1)

lbl2 = Label(btn1, font=('Helvetica',22,'bold'), text="Delta G", fg="black", 
   bd=16).grid(row=3, column=0, sticky=W)

ent2 = Entry(btn1, font=('Helvetica', 22, 'bold'), bd=4, width=13, bg= "white",
          textvariable=delG).grid(row=3, column=1)

lblTotal = Label(btn1, font=('Helvetica', 22, 'bold'),text = "Energy Span", 
       fg="black", bd=16, justify="left")
lblTotal.grid(row=4, column=0)

lblans = Entry(btn1, font=('Helvetica',22,'bold'), bd=4, width=12, 
  bg="white", textvariable=Total, relief= "sunken").grid(row=5,column=1)

btnTotal = Button(btn2,pady=8, bd=8, fg="black", font=('Helvetica', 25, 
'bold'), width=16,height=2,text = "Total", bg="white", 
       command=Sum).grid(row=1, column=0) 

btnReset = Button(btn2,pady=8, bd=8, fg="black", font=('Helvetica', 25, 
 'bold'), width=16,height=2,text = "Reset", bg="white", 
       command=Reset).grid(row=2, column=0) 

root.mainloop()

输出图像：简单计算