title: argparse: make one argument default when no arguments are called
body: <p>What's the best way to set a group argument as the default when no arguments are called.</p>
  
  <pre><code>parser = argparse.ArgumentParser()
  group = parser.add_mutually_exclusive_group()
  group.add_argument("--a", action="store_true") #call when no arguments are provided
  group.add_argument("--b", action="store_true")
  group.add_argument("--c", action="store_true")
  </code></pre>
  
  <p>Let's call my program argparse_ex.py. I want <code>argparse.py</code> (with no arguments) and <code>argparse.py --a</code> to return the same output.</p>