allow certain arguments to short circuit required positional arguments and other argparse errors

blargparse.py

#!/usr/bin/env python

# I had a script which I wanted to have a special option that short-circuited the normal
# argument parsing error handling behavior so that it could be run without thenormal
# required arguments.  This option would work similar to how `--help` or `--version`
# works, except that the parser would return a valid args object in its presence
# rather than exiting the program.

import argparse

class Parser(argparse.ArgumentParser):
    has_errors = False

    def error(self, message):
        self.has_errors = True
        self.error_message = message

    def handle_error(self):
        if self.has_errors:
            super(Parser, self).error(self.error_message)
            
parser = Parser(description='description')
parser.add_argument('a', type=int, help='a')
parser.add_argument('b', type=int, help='b')
parser.add_argument('--verbose', '-v', action='count', help='verbose')
parser.add_argument('--option', '-o', action='store_true', help='verbose')
# when running with -b/--bootstrap, we don't need the regular arguments
parser.add_argument('--bootstrap', '-b', action='store_true', help='bootstrap this script')

args = parser.parse_args()
if parser.has_errors and not args.bootstrap:
    parser.handle_error()