title: python: dictionary comprehension with list index
body: <p>I'd like to create dictionary with a key containing the indexed position of a value in a list. I'm using python 2.7. Consider my attempt:</p>
  
  <pre><code>LL = ["this","is","a","sample","list"]
  LL_lookup = {LL.index(l):l for (LL.index(l), l) in LL}
  
  # desired output
  print LL_lookup[1]
  &gt;&gt; is
  </code></pre>
  
  <p>I recognize that a dictionary isn't necessary in this example- <code>LL[1]</code> would yield the same result. Nonetheless, we can imagine a circumstance where 1) a dictionary is preferable given a more complex example, and b) a dictionary look up may yield marginal performance gains with a large number of iterations. </p>