title: Python list of tuples: organize by unique elements to a dictionary
body: <p>I have a list of tuples, say:</p>
  
  <pre><code>list_of_tuples = [('a', 1),('b',2),('c',1),('a',2),('d',3)]
  </code></pre>
  
  <p>I need to get correspoding values for any (unique) second element in a tuple. For example as a dictionary. 
  Output:</p>
  
  <pre><code>dict = {1:['a','c'],2:['b','a'],3:['d']}
  </code></pre>
  
  <p>Whats the most pythonic way of doing it? Help much appriciated!</p>