title: Python: Something like `map` that works on threads
body: <p>I was sure there was something like this in the standard library, but it seems I was wrong.</p>
  
  <p>I have a bunch of urls that I want to <code>urlopen</code> in parallel. I want something like the builtin <code>map</code> function, except the work is done in parallel by a bunch of threads.</p>
  
  <p>Is there a good module that does this?</p>