title: TypeError: unsupported operand type(s) for +: 'instance' and 'instance' Python 2.7
body: <p>I am trying to use Tkinterface to make a simple calculator, that adds, subtracts, multiplies and divides, but I have a issue, when I try to execute my program, i get this error:</p>
  
  <p>TypeError: unsupported operand type(s) for +: 'instance' and 'instance'</p>
  
  <p>I have personally no idea why this is happening, I have been looking around, and have found issues with similar errors, but not with "instance" and "instance".
  Here is my code:</p>
  
  <pre><code>#!/usr/bin/python2.7
  
  #Imports necessary modules
  
  from Tkinter import *
  #import Tkinter.messagebox
  import time as t
  import os
  
  #End of importing
  
  #Defines interface
  
  calc = Tk()
  calc.title("GUI Calculator")
  calc.geometry("500x350+200+200")
  
  #End of interface definition
  
  #Defines methods
  
  def add(val1, val2):
      global ans
      ans = int(val1 + val2)
      return ans
  
  def sub(val1, val2):
      global ans
      ans = int(val1 - val2)
      return ans
  
  def mul(val1, val2):
      global ans
      ans = int(val1 * val2)
      return ans
  
  def div(val1, val2):
      global ans
      ans = float(val1 % val2)
      return ans
  
  #End of method defining
  
  #Heading
  but1Text = StringVar()
  but1Text.set("Welcome to the GUI Calculator")
  label1 = Label(calc, textvariable=but1Text, height=2)
  label1.pack()
  
  #Instruction 1
  but2Text = StringVar()
  but2Text.set("Enter value number one!")
  label2 = Label(calc, textvariable=but2Text, height=2)
  label2.pack()
  
  #Val 1 input
  num1 = IntVar()
  val1 = Entry(calc, textvariable=num1)
  val1.pack()
  
  #Instruction 2
  but3Text = StringVar()
  but3Text.set("Enter value number one!")
  label3 = Label(calc, textvariable=but2Text, height=2)
  label3.pack()
  
  #Val 2 input
  num2 = IntVar()
  val2 = Entry(calc, textvariable=num2)
  val2.pack()
  
  #Operation selector
  
  but4Text = StringVar()
  but4Text.set(None)
  operator = Radiobutton(calc, text = "Addition", value = "Add", variable = but4Text,command = add(val1,val2)).pack()
  operator = Radiobutton(calc, text = "Subtract", value = "Sub", variable = but4Text,command = sub(val1,val2)).pack()
  operator = Radiobutton(calc, text = "Multiply", value = "Mul", variable = but4Text,command = mul(val1,val2)).pack()
  operator = Radiobutton(calc, text = "Divide",   value = "Div", variable = but4Text,command = div(val1,val2)).pack()
  
  #Answer
  but5Text = StringVar()
  but5Text.set("Answer=")
  label4 = Label(calc, textvariable=but5Text, height=2)
  label4.pack()
  
  #Answer
  but6Text = StringVar()
  but6Text.set(ans)
  label5 = Label(calc, textvariable=but6Text, height=2)
  label5.pack()
  
  #Mainloop
  calc.mainloop()
  </code></pre>
  
  <p>Here is the full error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "./tkCalc.py", line 76, in &lt;module&gt;
      operator = Radiobutton(calc, text = "Addition", value = "Add", variable = but4Text,command = add(val1,val2)).pack()
    File "./tkCalc.py", line 24, in add
      ans = int(val1 + val2)
  TypeError: unsupported operand type(s) for +: 'instance' and 'instance'
  </code></pre>