title: Create a python dictionary with unique keys that have list as their values
body: <p>I want to create a dictionary as following - </p>
  
  <pre><code>{'a':[1, 2, 3, 4, 5], 'b':[1, 3, 5], 'c':[2, 3, 5]}
  </code></pre>
  
  <p>The way I have implemented it is</p>
  
  <pre><code>mydict = dict()
  letters = ['a', 'b', 'a', 'c', 'a'] 
  
  #please mark the list has multiple occurence of a, 
  #hence I would want to check if a key with 'a' exists. Please do not advise to make the list unique. 
  for l in letters:
      if not mydict.get(l):
          mydict[l] =  &lt;values from another place via some filter&gt;
      else:
          mydict[l].append(&lt;values from another dict&gt;)
  </code></pre>
  
  <p>Is there a better approach to do this?</p>