title: argumentparser close file argument
body: <p>argumentparser can take file type argument and leave the file open directly, for example:</p>
  
  <pre><code>parser.add_argument('infile', nargs='?', type=argparse.FileType('r'))
  args = parser.parse_args().__dict__
  input = args['infile'].readlines()
  </code></pre>
  
  <p>do I need to close <code>args['infile']</code> in my program? Would argumentparser close it for me? I didn't find anywhere mention this in the documentations.</p>