title: to create a button when pressed print the number on that button in the entry box in python 3.3
body: <p>i want to make a button on which '2' is written .... now when anyone click it , it will show the number '2' in the entry box... 
  the error  is:
  before clicking , it is already showing '2' in  the entry box </p>
  
  <p>So please help me removing this error
  here is my program</p>
  
  <pre><code>from tkinter import * 
  
  root = Tk()  
  def add(x):  
      e1=Entry(root)  
      e1.insert(INSERT, x)  
      e1.pack()
  
  a=Button(root, text='2', command=add(2))  
  a.pack()
  
  root.mainloop()
  </code></pre>