title: Replacing argument in argparse
body: <p>I have a base class I cannot modify (code may have other errors, but please ignore those)</p>
  
  <pre><code>class BaseClass(object):
      def __init__(self):
          self.parser = argparse.ArgumentParser()
          self.parser.add_argument("arg1", choices=("a", "b"))
  </code></pre>
  
  <p>What I want is to override arg1 as follows</p>
  
  <pre><code>class DerivedClass(BaseClass):
      def __init__(self):
          BaseClass.__init__(self)
          self.parser.add_argument("arg1", choices=("a", "c", "d"))
  </code></pre>