title: Sum a list which contains 'None' using Python
body: <p>Basically my question is say you have an list containing 'None' how would you try retrieving the sum of the list. Below is an example I tried which doesn't work and I get the error: <code>TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'</code>. Thanks</p>
  
  <pre><code>def sumImport(self):
      my_list = [[1,2,3,None],[1,2,3],[1,1],[1,1,2,2]]
      k = sum(chain.from_iterable(my_list))
      return k
  </code></pre>