title: Why tuple assignment failed?
body: <p>I have tried to assign new data to existing tuple, and it does not work.and actually I want to add    the a and b to get a sum, however only string can be iterated..</p>
  
  <pre><code>A=[('3', '4'), ('3', '11'), ('8', '10')] 
    print A
  for a,b in A:
    a,b = int(a), int(b)+int(a)
  print A
  </code></pre>
  
  <p>results:</p>
  
  <p>[('3', '4'), ('3', '11'), ('8', '10')]</p>
  
  <p>[('3', '4'), ('3', '11'), ('8', '10')]</p>