def build_arg_parser():
    """
    Build an argument parser using argparse. Use it when python version is 2.7 or later.

    """
    parser = argparse.ArgumentParser(description="Smatch calculator -- arguments")
    parser.add_argument('-f', nargs=2, required=True, type=argparse.FileType('r'),
                        help='Two files containing AMR pairs. AMRs in each file are separated by a single blank line')
    parser.add_argument('-r', type=int, default=4, help='Restart number (Default:4)')
    parser.add_argument('--significant', type=int, default=2, help='significant digits to output (default: 2)')
    parser.add_argument('-v', action='store_true', help='Verbose output (Default:false)')
    parser.add_argument('--vv', action='store_true', help='Very Verbose output (Default:false)')
    parser.add_argument('--ms', action='store_true', default=False,
                        help='Output multiple scores (one AMR pair a score)'
                             'instead of a single document-level smatch score (Default: false)')
    parser.add_argument('--pr', action='store_true', default=False,
                        help="Output precision and recall as well as the f-score. Default: false")
    parser.add_argument('--justinstance', action='store_true', default=False,
                        help="just pay attention to matching instances")
    parser.add_argument('--justattribute', action='store_true', default=False,
                        help="just pay attention to matching attributes")
    parser.add_argument('--justrelation', action='store_true', default=False,
                        help="just pay attention to matching relations")

    return parser