title: python argparse default value for optional argument
body: <pre><code>usage:  [-h] [--foo FOO] bar
  </code></pre>
  
  <ol>
  <li>How do I make sure that default value of FOO is <code>abc</code> if I execute my script like below</li>
  </ol>
  
  <p><code>./myscript.py --foo bar</code> --> <code>bar</code> is positional argument here. but <code>args.foo</code> is considering <code>bar</code> as argument of <code>'--foo'</code>. I want <code>args.foo</code> to be <code>abc</code> and <code>bar</code> to be the positional argument</p>