title: Python Argparse strict variable name
body: <p>I try to understand a functionality (or bug ?) of python's argparse.</p>
  
  <p>Here, my simple code:</p>
  
  <pre><code>import argparse
  
  parser = argparse.ArgumentParser(usage="%(prog)s [--start]", add_help=False)
  parser.add_argument("--start", help="Start prog", action="store_true")
  
  arguments  = parser.parse_args()
  start_fpc  = arguments.start
  
  print arguments
  </code></pre>
  
  <p>When I execute this script, both <code>start</code> and <code>star</code> arguments are accepted:</p>
  
  <pre><code>[ rsenet  2015-08-28 12:39:50] /tmp
  $ python test.py --star
  Namespace(start=True)
  
  [ rsenet  2015-08-28 13:59:16] /tmp
  $ python test.py --start
  Namespace(start=True)
  </code></pre>
  
  <p>Any idea why? If yes, is it possible to disable this function?</p>