One way to make the original example from Victor work, is to use the following action class:

  class ConvertChoices(argparse.Action):                                                                                                                     
      """                                                                                                                                                    
      Argparse action that interprets the `choices` argument as a dict                                                    
      mapping the user-specified choices values to the resulting option                                                   
      values.                                                                                                                                                
      """                                                                                                                                                    
      def __init__(self, *args, choices, **kwargs):                                                                       
          super().__init__(*args, choices=choices.keys(), **kwargs)                                                       
          self.mapping = choices                                                                                                                             
                                                                                                                                                             
      def __call__(self, parser, namespace, value, option_string=None):                                                   
          setattr(namespace, self.dest, self.mapping[value])