title: how can argparse set default value of optional parameter to null or empty?
body: <p>I am using python argparse to read an optional parameter that defines an "exemplar file" that users sometimes provide via the command line.  I want the default value of the variable to be empty, that is no file found.</p>
  
  <pre><code>parser.add_argument("--exemplar_file", help = "file to inspire book", default = '')
  </code></pre>
  
  <p>Will this do it?</p>