The following snippet demonstrates the problem. If a subparser flag has a default set, argparse will override the existing value in the provided 'namespace' if the flag does not appear (e.g., if the default is used):

    import argparse
    parser = argparse.ArgumentParser()
    sub = parser.add_subparsers()
    example_subparser = sub.add_parser("example")
    example_subparser.add_argument("--flag", default=10)
    print(parser.parse_args(["example"], argparse.Namespace(flag=20)))

This should return 'Namespace(flag=20)' because 'flag' already exists in the namespace, but instead it returns 'Namespace(flag=10)'. This intended behavior is described and demonstrated in the second example here: https://docs.python.org/3/library/argparse.html#default

Lib's behavior is correct for the non-subparser cause.