title: Check if argparse optional argument is set or not
body: <p>I would like to check whether an optional argparse argument has been set by the user or not.</p>
  
  <p>Can I safely check using isset?</p>
  
  <p>Something like this:</p>
  
  <pre><code>if(isset(args.myArg)):
      #do something
  else:
      #do something else
  </code></pre>
  
  <p>Does this work the same for float / int / string type arguments?</p>
  
  <p>I could set a default parameter and check it (e.g., set myArg = -1, or "" for a string, or "NOT_SET").  However, the value I ultimately want to use is only calculated later in the script.  So I would be setting it to -1 as a default, and then updating it to something else later. This seems a little clumsy in comparison with simply checking if the value was set by the user.</p>