steps to reproduce:

    import argparse
    import sys
    
    parser = argparse.ArgumentParser(prog=sys.argv[0], add_help=False)
    parser.add_argument('-a', action='store_true')
    parsed_args, unknown_args = parser.parse_known_args(sys.argv[1:])
    print(parsed_args)
    print(unknown_args)


Expected result:

$ python arparse_test.py -ab
Namespace(a=True)
['b']


Actual result:

$ python arparse_test.py -ab
usage: arparse_test.py [-a]
arparse_test.py: error: argument -a: ignored explicit argument 'b'