How to set argument_default=argparse.SUPPRESS for subparsers?

I don’t want to get the unset options in the argument namespace. But argparse.SUPPRESS seems not pass on to subparsers.
The following code print(args) got the result. Since sub_arg2 and sub_args3 is not set, how to get ride of them in the namespace?

Namespace(sub_arg1=’1′, sub_arg2=[], sub_arg3=None, subparser=’sub1′)

import argparse

parser = argparse.ArgumentParser(argument_default=argparse.SUPPRESS)
# parser.add_argument('main_arg1')
# parser.add_argument('main_arg2', nargs='*')
subparsers = parser.add_subparsers(dest='subparser')
sparser = subparsers.add_parser('sub1')
sparser.add_argument('sub_arg1')
sparser.add_argument('sub_arg2', nargs='*')
sparser.add_argument('sub_arg3', nargs='?')

args = parser.parse_args('sub1 1'.split())
print(args)

>Solution :

add_parser takes any arguments the ArgumentParser constructor takes:

sparser = subparsers.add_parser('sub1', argument_default=argparse.SUPPRESS)