title: Dictionary Comprehension for list values
body: <p>I want to know if there's a more Pythonic way of doing the following, perhaps using dictionary comprehensions:</p>
  
  <pre><code>A = some list
  D = {}
  for i,v in enumerate(A):
      if v in D:
          D[v].append(i)
      else:
          D[v] = [i]
  </code></pre>