title: How can I cannot index into the values list of reduce?
body: <p>I am using in-mapper combining in a Map Reduce job via the Python mrjob module. Because I wrote a mapper_final function that emits a single pair, I am sure that only a single key-value pair is emitted to my reducers.</p>
  
  <p>However, my reduce function is erring:</p>
  
  <pre><code>  def reducer(self, key, occurrences):
      '''
      Calculates the final value.
      '''
      yield 'Final Value: ', occurrences[0] / 2
  </code></pre>
  
  <p>The error reads</p>
  
  <pre><code>File "calculateFinalValue.py", line 354, in reducer
      yield 'Final Value: ', occurrences[0] / 2
  TypeError: 'generator' object has no attribute '__getitem__'
  </code></pre>
  
  <p>Why can I not index into <code>occurrences</code>? There should only be a single pair in that list, right?</p>