title: difference between adding lists in python with + and +=
body: <p>I've noticed when experimenting with lists that  <code>p= p+i</code> is different then <code>p +=  i</code> 
  For example:</p>
  
  <pre><code>test = [0, 1, 2, 3,]
  p = test
  test1 = [8]
  p = p + test1
  print test
  </code></pre>
  
  <p>In the above code <code>test</code> prints out to the original value of <code>[0, 1, 2, 3,]</code></p>
  
  <p>But if I switch <code>p = p + test1</code> with <code>p += test1</code> As  in the following</p>
  
  <pre><code>test = [0, 1, 2, 3,]
  p = test
  test1 = [8]
  
  p += test1
  
  print test
  </code></pre>
  
  <p><code>test</code> now equals <code>[0, 1, 2, 3, 8]</code></p>
  
  <p>What is the reason for the different value?</p>