title: Getting byte offset with MRJob
body: <p>According to "The Definitive Guide to Hadoop", the input format <code>TextInputFormat</code> gives key value pairs <code>(k, v) = (byte offset, line)</code>. However, in MRJob, the key in the mapper input is always <code>None</code>. It should be easy to get the byte offset as key, since that's what TextInputFormat does. How do I get this?</p>
  
  <p>I know that you can use the environment variable 'map_input_start' and calculate byte offsets yourself, but this has caused problems and I would like to do it the much simpler way of just getting the offset as key.</p>