title: Setting default value for optional argument that depends on another optional argument in python argparse
body: <p>I am using python <code>argparse()</code> for command line parsing. I want something like this,</p>
  
  <p><code>-a</code> -> optional argument</p>
  
  <p><code>-b</code> -> optional argument that depends on <code>a</code> with <code>default value x</code></p>
  
  <p><strong>Condition</strong>: <code>-b</code> should set(either to <code>default</code> or custom value) if and only if <code>a</code> is set, otherwise <code>b</code> should be <code>None</code>. </p>
  
  <p>Could someone help me to achieve this?</p>