title: TypeError : takes 0 positional arguments but 3 were given. Can't figure out what arguments
body: <p>I'm writing a tkinter UI.
  I wanna be able to access the Entry at every changes (The User add a character)
  So I figured out I should use a callback function like so</p>
  
  <pre><code>def callback():
       s = entry.get()
       print (s)
       varStr.trace_variable("w", callback)
  </code></pre>
  
  <p>And i then call that function . But I'm not sure how they work and can't find out.</p>
  
  <p>When I run the program i got this error once I put a character in the Entry field:</p>
  
  <pre><code>Exception in Tkinter callback
  Traceback (most recent call last):  
    File "C:\Program Files\Python33\lib\tkinter\__init__.py", line 1475, in __call__
      return self.func(*args)
  TypeError: callback() takes 0 positional arguments but 3 were given
  </code></pre>
  
  <p>I found somewhere I could add 'self' but i still got two more arguments to fill. Could you help me know what they should be ?</p>