title: How to define a mutually exclusive group of two positional arguments?
body: <p>I would like to use <code>argparse</code> to make some code to be used in the following two ways:</p>
  
  <pre><code>./tester.py all
  ./tester.py name someprocess
  </code></pre>
  
  <p>i.e. either <code>all</code> is specified OR <code>name</code> with some additional string.  </p>
  
  <p>I have tried to implement as follows:</p>
  
  <pre><code>import argparse
  parser = argparse.ArgumentParser()
  group = parser.add_mutually_exclusive_group()
  group.add_argument('all', action='store_true', \
          help = "Stops all processes")
  group.add_argument('name', \
          help = "Stops the named process")
  
  print parser.parse_args()
  </code></pre>
  
  <p>which gives me an error </p>
  
  <pre><code>ValueError: mutually exclusive arguments must be optional
  </code></pre>
  
  <p>Any idea how to do it right? I also would like to avoid sub parsers in this case.</p>