argparse.Action like "store", but with an optional modifier

optional_option.py

import argparse
import sys
from enum import Enum

class Viewport(Enum):
    All              = "all"
    WorkingDirectory = "here"
    OwnedByMe        = "mine"

    # NOTE The property decorator doesn't work on default and choices,
    # below; probably due to some Enum metaprogramming magic. As such,
    # we have to make do with methods.

    @classmethod
    def default(cls):
        default, *_ = cls
        return default

    @classmethod
    def choices(cls):
        return [*cls]

class StoreOptionalModifier(argparse.Action):
    def __call__(self, _parser:argparse.ArgumentParser, namespace:argparse.Namespace, values:Viewport, _option_string:str):
        setattr(namespace, self.dest, values or Viewport.default())

p = argparse.ArgumentParser()
p.add_argument("--view", dest="view", metavar="WHAT",
               nargs="?",
               action=StoreOptionalModifier,
               type=Viewport,
               choices=Viewport.choices(),
               default=argparse.SUPPRESS,
               help="show the recoverable files from WHAT")

if __name__ == "__main__":
    print(p.parse_args(sys.argv[1:]))