title: Dynamodb update attribute based on another
body: <p>I want to update an item in Dynamodb such that I can set the value of one attribute based on the value of an existing attribute in that item. For example: I have table with the following item <code>{"id": 1, "valueone": 30}</code>. </p>
  
  <p>I want to update this item such that I can add another attribute <code>valuetwo</code> whose value is twice that of <code>valueone</code>: <code>{"id": 1, "valueone": 30, "valuetwo": 60}</code></p>
  
  <p>Something like this, but not sure how to represent <code>valueone</code> in the ExpressionAttributeValues:</p>
  
  <pre><code>table.update_item(Key={'id': 1}, UpdateExpression="set vtwo = :two * :r", ExpressionAttributeValues={':r': valueone, ':two': 2},ReturnValues="ALL_NEW"))
  </code></pre>
  
  <p>I can control whether <code>valuetwo</code> is a new attribute in the item or already exists with a dummy value (say zero).</p>