title: Ask for 0 or 2 positional arguments with argparse?
body: <p>I'm learning argparse module and I want to ask 0 or 2 files.</p>
  <pre><code>parser.add_argument(&quot;infile&quot;, nargs = {0,2}, type=argparse.FileType('r'))
  </code></pre>
  <p>It should work with :</p>
  <pre><code>python prog.py
  python prog.py infile1 infile2
  </code></pre>
  <p>But not with :</p>
  <pre><code>python prog.py infile1
  python prog.py infile1 infile2 infile3
  </code></pre>
  <p>Actually I use 2 different arguments with &quot;nargs='?' and I test</p>
  <pre><code>sys.argv == 1 or sys.argv == 3
  </code></pre>