title: python argparse: unrecognized arguments
body: <p>When I run <code>parsePlotSens.py -s bw hehe</code>, it says that <code>hehe</code> is an unrecognized argument. However, if I run <code>parsePlotSens.py hehe -s bw</code>, it's OK. Ideally, I would like it work for both cases. </p>
  
  <p>Any tips? The following is my code:</p>
  
  <pre><code>if __name__ == '__main__' :
  
      parser = argparse.ArgumentParser(prog='parsePlotSens');
      parser.add_argument('-s', '--sort', nargs =1, action = 'store', choices = ['mcs', 'bw'], default='mcs', help=sorthelp)
      parser.add_argument('filename', nargs ='+', action = 'store')
      option = parser.parse_args(sys.argv)
  </code></pre>