title: How to pass in undefined parameters when use subparsers
body: <p>When I use the subparsers, the subparers are optional parameters, and I have to choose one of them. Now, I want to implement the ability to pass in a default option when an option parameter that is not defined in the subparse is passed in, for example, the<code>add_codition</code>.</p>
  <pre><code>parser = argparse.ArgumentParser(prog='PROG')
  parser.add_argument('add_condition', nargs='*')
  
  subparsers = parser.add_subparsers(help='sub-command help', dest=&quot;character&quot;)
  subparsers.required = False
  
  # create the parser for the &quot;a&quot; command
  parser_a = subparsers.add_parser('a', help='a help')
  parser_a.add_argument('--bar', choices='ABC', help='bar help')
  
  # create the parser for the &quot;b&quot; command
  parser_b = subparsers.add_parser('b', help='b help')
  parser_b.add_argument('--baz', choices='XYZ', help='baz help')
  </code></pre>
  <p>However, when I pass in options that a subparser and b parser don't have, e.g. <code>key=value</code>. The <code>key=value</code>should theoretically be passed in <code>add_codition</code>.</p>
  <p>The program result always prompts: <code>error: argument character: invalid choice: 'key' (choose from 'a', 'b')</code></p>
  <p>The python version is 3.6.9.</p>