title: assigning to list[:] using list comprehension
body: <p>from this answer <a href="https://stackoverflow.com/a/1207461/8074522">https://stackoverflow.com/a/1207461/8074522</a>
  I'm trying to find the difference between using </p>
  
  <pre><code>somelist = [1,2,3,4,5,6,7,8,9]
  somelist = [x for x in somelist if x&gt;4]
  and 
  somelist[:] = [x for x in somelist if x&gt;4]
  </code></pre>
  
  <p>in his answer, he is saying </p>
  
  <blockquote>
    <p>Or, by assigning to the slice somelist[:], you can mutate the existing
    list to contain only the items you want:</p>
  </blockquote>
  
  <p>"to contain only" &lt; I can use assign to the somelist without slicing.</p>
  
  <p>So what's the need of somelist[:] <a href="https://stackoverflow.com/a/1207461/8074522">here?</a></p>