title: Using operator = in list
body: <p>I would like to understand why when I'm using the a=(with append)
  the result is none.
  why the var list is none.</p>
  
  <pre><code>a = ['a','b','b']
  b = 'y'
  a = a.append(b)
  print (a) ---result none
  </code></pre>