title: Argparse: defaults from file
body: <p>I have a Python script which takes a lot of arguments.
  I currently use a <code>configuration.ini</code> file (read using <code>configparser</code>), but would like to allow the user to override specific arguments using command line.
  If I'd only have had two arguments I'd have used something like:</p>
  
  <pre><code>if not arg1:
      arg1 = config[section]['arg1']
  </code></pre>
  
  <p>But I don't want to do that for 30 arguments.
  Any easy way to take optional arguments from cmd line, and default to the <code>config</code> file?</p>