title: Changing one list unexpectedly changes another, too
body: <p>I have a list of the form</p>
  
  <pre><code>v = [0,0,0,0,0,0,0,0,0]
  </code></pre>
  
  <p>Somewhere in the code I do</p>
  
  <pre><code>vec=v
  vec[5]=5
  </code></pre>
  
  <p>and this changes both <code>v</code> and <code>vec</code>: </p>
  
  <pre><code>&gt;&gt;&gt; print vec
  [0, 0, 0, 0, 0, 5, 0, 0, 0]
  &gt;&gt;&gt; print v
  [0, 0, 0, 0, 0, 5, 0, 0, 0]
  </code></pre>
  
  <p>Why does <code>v</code> change at all?</p>