title: How to run a code whenever a Tkinter widget value changes?
body: <p>I'm using Python and <code>Tkinter</code>, and I want the equivalent of <code>onchange</code> event from other toolkits/languages. I want to run code whenever the user updates the state of some widgets.</p>
  
  <p>In my case, I have many <code>Entry</code>, <code>Checkbutton</code>, <code>Spinbox</code> and <code>Radiobutton</code> widgets. Whenever any one of these changes, I want to run my code (in this case, update a text box on the other panel).</p>
  
  <p>(just remember that user may interact with those widgets using either mouse or keyboard, and even using Ctrl+V to paste text)</p>