title: Python Argparse append with nargs= >1
body: <p>So I am trying to create a parser which can take an arbitrary list of choices (things to plot), with optional parameters specified for each choice.  And have eschewed the idea of sub-subparsers for just using multiple arguments of the form:</p>
  
  <pre><code>--plot foo --xlim 0 10 --ylim -5 5 --plot bar --xlim -5 5 --ylim 0 10 --clim 1e24 1e27
  </code></pre>
  
  <p>But I can't seem to get an append action to work with nargs=2:</p>
  
  <pre><code>&gt;&gt;&gt; import argparse
  &gt;&gt;&gt; parser = argparse.ArgumentParser()
  &gt;&gt;&gt; parser.add_argument('--foo', action='append', nargs=2, type=int)
  _AppendAction(option_strings=['--foo'], dest='foo', nargs=2, const=None, default=None,  type=&lt;type 'int'&gt;, choices=None, help=None, metavar=None)
  &gt;&gt;&gt; parser.parse_args("--foo 1 2 --foo 3 4")
  usage: [-h] [--foo FOO FOO]
  : error: unrecognized arguments: - - f o o   1   2   - - f o o   3 4
  </code></pre>
  
  <p>I was hoping that I would get a list of lists or a list of tuples. Is is my desired behaviour unsupported by argparse or am I just doing something wrong in my implementation?</p>