title: Python add constant values to argparse parameter
body: <p>I have googled this around but can't seem to find if this question has been asked already. I want to have a value (say "blah") to be always present for a list option, plus additional arguments if they are provided.</p>
  
  <p>e.g.</p>
  
  <pre><code>import argparse
  args = argparse.ArgumentParser()
  args.add_argument("--foo", nargs="+")
  
  args = args.parse_args(["--foo", "bar1", "bar2", "bar3"])
  args.foo.append("blah")
  
  
  print args.foo
  ['bar1', 'bar2', 'bar3', 'blah']
  </code></pre>