title: Display help message with Python argparse when script is called without any arguments
body: <p>Assume I have a program that uses <code>argparse</code> to process command line arguments/options. The following will print the 'help' message:</p>
  <pre class="lang-none prettyprint-override"><code>./myprogram -h
  </code></pre>
  <p>or:</p>
  <pre class="lang-none prettyprint-override"><code>./myprogram --help
  </code></pre>
  <p>But, if I run the script without any arguments whatsoever, it doesn't do anything. What I want it to do is to display the usage message when it is called with no arguments. How is that done?</p>