title: Keeping track of progress using Process.Pool
body: <p>In a <code>for</code> loop, it is easy to track progress by doing:</p>
  
  <pre><code>total_num = 1000
  for num, url in enumerate(urls):
      print '%s / %s' (num+1, total_num)
      # do something
  </code></pre>
  
  <p>How would I keep track of progress with using <code>Pool</code>? </p>
  
  <pre><code># input data has 4M items
  pool = Pool(parallel_workers)
  pool.map(run_item, input_data)
  </code></pre>