title: Colon (:) in Python list index
body: <p>I'm new to Python. I see <code>:</code> used in list indices especially when it's associated with function calls. </p>
  
  <p>Python 2.7 documentation suggests that <code>lists.append</code> translates to <code>a[len(a):] = [x]</code>. Why does one need to suffix <code>len(a)</code> with a colon?</p>
  
  <p>I understand that <code>:</code> is used to identify keys in dictionary.</p>