title: argparse update choices of an argument
body: <p>Using argparse, is there any way to update the "choices" option of an argument  after it was added to the parser ? <a href="https://docs.python.org/3/library/argparse.html#choices" rel="nofollow noreferrer">Argparse documentation</a> doesn't yield much about updating the choices</p>
  
  <pre><code>import argparse
  
  parser = argparse.ArgumentParser()
  
  choices_list = ['A', 'B']
  parser.add_argument('arg1', choices=choices_list)
  
  # The list of choices now changes 
  choices_list = ['A', 'C', 'D']
  
  # Some code to update 'arg1' choices option ?
  parser.???
  </code></pre>
  
  <p>I tried using 'parser.add_argument' with the new 'choices_list', but it creates duplicate arguments. </p>
  
  <p>Using Python 3.7</p>