title: Convert all dictionary values into proportions
body: <p>I have defined a dictionary as</p>
  
  <pre><code>coocurences = {('a', 'b'): 74, ('a', 'c'): 33, ('b', 'c'): 26}
  </code></pre>
  
  <p>and I would like to change the values of this dictionary into proportions of the sum of its values:</p>
  
  <pre><code>{('a', 'b'): 0,556390977443609‬, ('a', 'c'): 0,2481203007518797, ('b', 'c'): 0,1954887218045113‬‬}
  </code></pre>
  
  <p>I have tried with:</p>
  
  <pre><code>for x in coocurences.values():
      x = (x/sum(coocurences.values()))
  </code></pre>
  
  <p>but it tells me: TypeError: 'builtin_function_or_method' object is not iterable.
  Any suggestions how to accomplish this?</p>