title: Merging two lists of dictionaries to create a third one in python
body: <p>Let say I have the following:</p>
  
  <pre><code>list_a = [
      {u'username': u'firstuser', u'userid': u'1'},
      {u'username': u'seconduser', u'userid': u'2'},
      {u'username': u'thirduser', u'userid': u'3'},
  ]
  </code></pre>
  
  <p>and this one:</p>
  
  <pre><code>list_b = [
      {u'username': u'firstuser', u'userid': u'101'},
      {u'username': u'seconduser', u'userid': u'102'},
  ]
  </code></pre>
  
  <p>How can I obtain a third one like this:</p>
  
  <pre><code>list_result = [
      {u'username': u'firstuser', u'userid_one': u'1', u'userid_two': u'101'},
      {u'username': u'seconduser', u'userid_one': u'2', u'userid_two': u'102'},
      {u'username': u'thirduser', u'userid_one': u'3', u'userid_two': u''},
  ]
  </code></pre>