title: How to pass a list by reference?
body: <p>I'm trying to implement a function 'add' that combines the list <code>L1</code> with <code>L2</code> into <code>L3</code>:</p>
  
  <pre><code>def add(L1,L2,L3):
      L3 = L1 + L2
  
  L3 = []
  add([1],[0],L3)
  print L3
  </code></pre>
  
  <p>The code above produces an empty list as a result instead of <code>[1,0]</code> - This means that <code>L3</code> wasn't passed by reference.</p>
  
  <p>How to pass <code>L3</code> by reference?</p>