title: Default sub-command, or handling no sub-command with argparse
body: <p>How can I have a default <a href="http://docs.python.org/py3k/library/argparse.html#sub-commands" rel="noreferrer">sub-command</a>, or handle the case where no sub-command is given using <a href="http://docs.python.org/py3k/library/argparse.html" rel="noreferrer"><code>argparse</code></a>?</p>
  
  <pre><code>import argparse
  
  a = argparse.ArgumentParser()
  b = a.add_subparsers()
  b.add_parser('hi')
  a.parse_args()
  </code></pre>
  
  <p>Here I'd like a command to be selected, or the arguments to be handled based only on the next highest level of parser (in this case the top-level parser).</p>
  
  <pre>joiner@X:~/src> python3 default_subcommand.py
  usage: default_subcommand.py [-h] {hi} ...
  default_subcommand.py: error: too few arguments</pre>