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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _parse_args() -> argparse.Namespace: """Helper function to create the command-line argument parser for faaspact_verifier. Return the parsed arguments as a Namespace object if successful. Exits the program if unsuccessful or if the help message is printed. """

    description = ('Run pact verifier tests against a faas microservice.')
    parser = argparse.ArgumentParser(description=description)

    parser.add_argument('--host',
                        default=os.environ.get('PACT_BROKER_HOST'),
                        help='Pact broker host. (default=env.PACT_BROKER_HOST)')

    parser.add_argument('--username',
                        default=os.environ.get('PACT_BROKER_USERNAME'),
                        help='Pact broker username. (default=env.PACT_BROKER_USERNAME)')

    parser.add_argument('--password',
                        default=os.environ.get('PACT_BROKER_PASSWORD'),
                        help='Pact broker password. (default=env.PACT_BROKER_PASSWORD)')

    parser.add_argument('-f', '--faasport-module',
                        default=os.environ.get('FAASPORT_MODULE'),
                        help='Python path to faasport module. (default=env.FAASPORT_MODULE)')

    parser.add_argument('-p', '--provider',
                        default=os.environ.get('PACT_PROVIDER'),
                        help='Name of the provider. (default=env.PACT_PROVIDER)')

    parser.add_argument('--publish-results',
                        action='store_true',
                        default=False,
                        help='If true, publish verification results to the broker.')

    parser.add_argument('--provider-version',
                        required=False,
                        help='The version of the provider. Defaults to the current git SHA.')

    parser.add_argument('--github-pr',
                        required=False,
                        help=('Url of associated github PR. If PR body has line '
                              '"feature-pacts: x y ...", those feature-pacts will be added to '
                              '--failon tags.'))

    parser.add_argument('--failon',
                        action='append',
                        default=['master'],
                        help=('If verification fails for a pact with one of these tags, this script'
                              ' will fail.'))

    args = parser.parse_args()

    if not args.provider_version:
        args.provider_version = _current_commit_sha()

    if not args.host:
        raise parser.error('Missing host')

    if not args.username:
        raise parser.error('Missing username')

    if not args.password:
        raise parser.error('Missing password')

    if not args.faasport_module:
        raise parser.error('Missing faasport_module')

    if not args.provider:
        raise parser.error('Missing provider')

    return args