title: python optparse how to set a args of list?
body: <p>I want to pass data to script, like this  </p>
  
  <pre><code>if __name__ == '__main__':
      usage = 'python pull.py [-h &lt;host&gt;][-p &lt;port&gt;][-r &lt;risk&gt;]arg1[,arg2..]'
      parser = OptionParser(usage)
      parser.add_option('-o', '--host', dest='host', default='127.0.0.1',
      ┊   help='mongodb host')
      parser.add_option('-p', '--port', dest='port', default=27017,
      ┊   help="mongodb port")
      parser.add_option('-r', "--risk", dest='risk', default="high",
      ┊   help="the risk of site, choice are 'high', 'middle', 'low', 'all'")
      options, args = parser.parse_args()
  </code></pre>
  
  <p>in this script, if I want to set ./test.py -r high and middle, how can I set <code>['high', 'middle']</code> in <code>optparse</code>?</p>