Example for extend_const addition to argparse stdlib

extend_const.py

import argparse

class ExtendConstAction(argparse._AppendConstAction):
    # This class is bad because it references private argparse functions/classes.
    # Don't use it in production! It only exists here for demonstration purposes.
    def __call__(self, parser, namespace, values, option_string=None):
        # Copied almost verbatim from _AppendConstAction.__call__
        # https://github.com/python/cpython/blob/39652cd8bdf7c82b7c6055089a4ed90ee546a448/Lib/argparse.py#L1028
        items = getattr(namespace, self.dest, None)
        items = argparse._copy_items(items)
        items.extend(self.const) # This is the only line changed from super().__call__
        setattr(namespace, self.dest, items)

def example(to_parse):
    parser = argparse.ArgumentParser()
    parser.add_argument("--filter-by", action="append")
    parser.add_argument("--by-foo", action="append_const", dest="filter_by", const="foo")
    parser.add_argument("--by-bar", action="append_const", dest="filter_by", const="bar")

    # This is the action I am proposing be added
    # parser.add_argument("--by-both", action="extend_const", dest="filter_by", const=["foo", "bar"])
    parser.add_argument("--by-both", action=ExtendConstAction, dest="filter_by", const=["foo", "bar"])

    return parser.parse_args(to_parse)

example("--filter-by foo --filter-by bar".split())
example("--by-foo".split())
example("--by-bar".split())
example("--by-foo --by-bar".split())
example("--by-both".split())

Output

$ python example.py
--filter-by foo --filter-by bar: Namespace(filter_by=['foo', 'bar'])
--by-foo: Namespace(filter_by=['foo'])
--by-bar: Namespace(filter_by=['bar'])
--by-foo --by-bar: Namespace(filter_by=['foo', 'bar'])
--by-both: Namespace(filter_by=['foo', 'bar'])