#!/usr/bin/env python

import argparse
import os
import sys


def get_python_version():
    return '.'.join(str(i) for i in sys.version_info[:3])


def main():
    parser = argparse.ArgumentParser(description='Print Python version')
    parser.add_argument('--check-prefix', action='store_const', const=True,
                        default=False,
                        help='check version matches $PYTHON_VERSION prefix')
    args = parser.parse_args()

    version = get_python_version()

    if args.check_prefix:
        expected_version = os.environ.get('PYTHON_VERSION')
        if expected_version and not version.startswith(expected_version):
            message = ("Version {version!r} must starts with"
                       " {expected_version!r}\n").format(
                        version=version,
                        expected_version=expected_version)
            sys.stderr.write(message)
            sys.exit(1)

    sys.stdout.write(version + '\n')


if __name__ == '__main__':
    main()