parser = argparse.ArgumentParser(
    'Calculate something with a quantity commonly called lambda.')
parser.add_argument('-l', '--lambda', help=
    'Defines the quantity called lambda', type=float, dest='lambda_',
    metavar='LAMBDA')
parser.print_help()
args = parser.parse_args(['--lambda', '4.2'])
args.lambda_
Python: How to escape 'lambda'